SWTPC PR-40 User manual

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SWTPC PR-40 Alphanumeric Printer Assembly Instructions
The SWTPC PR-40 Printer kit is a 5 X 7 dot matrix impact printer similar in
operation to the well known Centronics printers. It prints the 64 character upper
case ASCII set with 40 characters/line at a rate of 75 lines/minute on standard 3
7/8" wide rolls of adding machine paper. One complete line is printed at a time
from an internal forty character line buffer memory. Printing takes place either
on receipt of a carriage return or automatically whenever the line buffer memory
is filled.
The printer can accept character data as fast as one character per
microsecond or as slow as you wish to send it. The printer's seven parallel data
lines are TTL compatible and may be enabled by a single "data ready" control line
or by separate "data ready" and "data accepted" handshake control lines. This
universal approach makes the printer compatible with all computer and terminal
systems having an eight bit parallel interface; including of course the MITS 8800
and SWTPC 6800 computer systems just to mention a few.
The printer mechanism is attached to a black anodized aluminum chassis with
front trim panel which houses the unit's circuitry including its own 120/240 VAC
50 to 60 Hz power supply. This makes the printer's overall dimensions 9 5/8" wide
X 10 1/2" deep X 8 3/4" high.
Each unit is shipped with one ribbon and one roll of paper. Extra ribbons
are available from us while the standard adding machine paper may be purchased in
office supply stores.
Caution
When handling the print mechanism itself you must make absolutely sure not
to try to rotate the large plastic cylinder that moves the head. If this cylinder
is rotated so the top is turned toward the front of the printer, it may bend the
arm on the roller cam microswitch on the right hand side of the mechanism which
will not only damage the switch but will prevent the printer from operating.
Note
The print mechanism you receive may not have the paper roll holder
installed. If so, it along with the mounting hardware will probably be contained
within a separate bag. The paper roll holder is secured to the back of the print
mechanism with four screws.
Assembling the Unit
The electronics for the printer are constructed on two printed circuit
boards. The larger of the two is the PR-40A and contains all of the logic
necessary to drive the print mechanism. The smaller board is the PR-40B and
contains mostly power supply circuitry. When assembling the printer it is best to
first assemble the PR-40A board, then the PR-40B board, then complete the unit by
installing the chassis components and completing the wiring. The assembly
instructions have been written in this sequence.
Scanned and edited by Michael Holley Nov 25, 2005
Southwest Technical Products Corporation Document Circa 1976

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PR-40A PC Board Assembly
NOTE: Since all of the holes on the PR-40A PC board have been plated thru,
it is only necessary to solder the components from the bottom side of the board,
unless otherwise noted. The plating provides the electrical connection from the
"BOTTOM" to the "TOP" foil of each hole. Unless otherwise noted it is important
that none of the connections be soldered until all of the components of each
group have been installed on the board. This makes it much easier to interchange
components if a mistake is made during assembly. Be sure to use a low wattage
iron (not a gun) with a small tip. Do not use acid core solder or any type of
paste flux. We will not guarantee or repair any kit on which either product has
been used. Use only the solder supplied with the kit or a 60/40 alloy resin core
equivalent. Remember all of the connections are soldered on the bottom side of
the board only. The plated-thru holes provide the electrical connection to the
top foil.
( ) Before installing any parts on the PR-40A circuit board, check both
sides of the board over carefully for incomplete etching and foil
"bridges" or "breaks". It is unlikely that you will find any but should
there be one especially on the "TOP" side of the board it will be very
hard to locate and correct after all of the components have been
installed on the board.
( ) Attach all of the resistors to the board. As with all other components
unless noted, use the parts list and component layout drawing to locate
each part and install from the "TOP" side of the board bending the
leads along the "BOTTOM" side of the board and trimming so that 1/16"
to 1/8" of wire remains. Solder. You should have one 1M ohm resistor
left over.
( ) Install all of the capacitors on the board. Be sure to orient the
electrolytic capacitors correctly. The polarity is indicated on the
component layout drawing. Solder.
( ) Install integrated circuits IC3 thru IC17 on the circuit board. The
integrated circuits must be turned to match the outlines shown on the
component layout drawing. Solder.
( ) Install fuses F1 thru F7 on the circuit board. Solder.
( ) Install the diodes and transistors on the circuit board. Transistors Q1
thru Q7 must be mounted vertically with the bottoms of the transistors
about 3/16" above the top of the board. These components must be turned
to match the outlines shown on the component layout drawing. Solder.
( ) Insert the twelve male PC mount pins into the 12 pin Molex female
connector shell. The pins are inserted from the back or numbered side
of the connector. Orient the shell as shown in the component layout
drawing and mount it in the J1 connector position. Solder.
( ) Insert female receptacle J2 into the PC board making sure to orient it
as shown in the component layout drawing. The small notches on the
inside of the connector must point toward the edge of the board as
shown in the component layout drawing. Solder.
( ) Insert Molex receptacle J3 into the PC board making sure to orient it
as shown in the component layout drawing. The notches on the connector
must point toward the edge of the PC board as shown in the component
layout drawing. Solder.

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( ) Insert Molex receptacle J4 into the PC board making sure to orient it
as shown in the component layout drawing. The notches on the connector
must point toward the edge of the PC board as shown in the component
layout drawing. Solder.
NOTE: MOS integrated circuits are susceptible to damage by static
electricity. Although some degree of protection is provided internally within the
integrated circuits, their cost demands the utmost in care. Before opening and/or
installing any MOS integrated circuits you should ground your body and all
metallic tools coming into contact with the leads, thru a 1 M ohm 1/4 watt
resistor (supplied with the kit). The ground must be an "earth" ground such as a
water pipe, and not the circuit board ground. As for the connection to your body,
attach a clip lead to your watch or metal ID bracelet. Make absolutely sure you
have the 1 Meg ohm resistor connected between you and the "earth" ground,
otherwise you will be creating a dangerous shock hazard. Avoid touching the leads
of the integrated circuits any more than necessary when installing them, even if
you are grounded. On those MOS IC's being soldered in place the tip of the
soldering iron should be grounded as well (separately from your body ground)
either with or without a 1 Meg ohm resistor. Most soldering irons having a three
prong line cord plug already have a grounded tip. Static electricity should be an
important consideration in cold, dry environments. It is less of a problem when
it is warm and humid.
( ) Install MOS integrated circuits ICI and IC2 following the precautions
given in the preceding section. As each is installed, make sure it is
down firmly against the board before soldering all of its leads. Be
very careful to install each in its correct position. Do not bend the
leads on the back side of the board. Doing so makes it very difficult
to remove the integrated circuits should replacement ever be necessary.
The "dot" on the end of the package is used for orientation purposes
and must match with that shown on the component layout drawing for each
of the IC's.
( ) Working from the "TOP" side of the circuit board, fill in all of the
feedthru's with molten solder. The feed-thru's are those unused holes
on the board whose internal plating connects the "TOP" and "BOTTOM"
circuit connections. Filling these feed-thru's with molten solder
guarantees the integrity of the connections and increases the current
handling capability.
( ) Now that all of the components have been installed on the board, double
check to make sure all have been installed correctly in their proper
location.
( ) Check very carefully to make sure that all connections have been
soldered. It is very easy to miss some connections when soldering which
can really cause some hard to find problems later during check out.
Also look for solder "bridges" and "cold" solder joints which are
another common problem.
This completes the assembly of the PR-40A circuit board.

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PR-40B PC Board Assembly
( ) Attach all of the resistors to the board excluding resistor R6. As with
all other components unless noted, use the parts list and component
layout drawing to locate each part and install from the "TOP" side of
the board bending the leads along the "BOTTOM" side of the board and
trimming so that 1/16" to 1/8" of wire remains. Solder.
( ) Install all of the capacitors on the board excluding electrolytic
capacitor C4. Be sure to orient the electrolytic capacitors correctly.
The polarity is indicated on the component layout drawing. Note the two
set of holes for C3 to accommodate two different size capacitors - use
the correct set depending on your capacitor. Solder.
( ) Install transistor Q1 on the PC board. The transistor must be turned to
match the outline on the component layout drawing. Solder.
( ) Install transistor Q2 on the PC board. The transistor must be oriented
so that its flat side is against the circuit board and should be
secured with a #4-40 X 1/4" screw.
( ) Install all of the diodes on the PC board making sure to orient each as
shown in the component layout drawing. Solder.
( ) Install all of the male PC type pins in female connector shells J1 and
J2 from the numbered side of the connector. Then install each connector
in its proper location oriented as shown in the component layout
drawing. Solder.
( ) Now that all of the components have been installed on the board, double
check to make sure all have been installed correctly in their proper
location.
( ) Check very carefully to make sure that all connections have been
soldered. It is very easy to miss some connections when soldering which
can really cause some hard to find problems later during checkout. Also
look for solder "bridges" and "cold" solder joints which are another
common problem.
This completes the assembly of the PR-40B circuit board.
Chassis Assembly
( ) Some of the print mechanisms we are supplying have two mounting holes
on the bottom while others have three. Install the appropriate number
of mounting screws thru the proper holes on the chassis by running a
number 6 - 32 X 1/2" screw thru the chassis from the inside followed by
a lockwasher and #6 - 32 nut. The printer mechanism itself will be
attached to these screws at a later time.
( ) Snap the large plastic bushing into the chassis from the outside of the
chassis.
( ) Attach fuseholder F1 to the chassis using a #6 - 32 X 3/8" screw,
lockwasher and nut. Orient the fuseholder as shown in the wiring
diagrams.
( ) Scrape away the anodizing around the mounting hole for TS-1 on the
inside of the PR-40 chassis. Scraping away the anodizing insures
electrical contact between the terminal strip and the chassis. Attach
lug strip TS-1 to the chassis using a #6 - 32 X 1/4" screw, lockwashers
and nut. Sandwich one lockwasher between the terminal strip and the
chassis. Orient the lug strip as shown in the wiring diagram.

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( ) Attach transformer T1 to the chassis using four #8-32 X 3/8" screws,
flatwashers and nuts. Run the screws thru the outside of the chassis,
place a flatwasher between the inside of the chassis and the
transformer leg, place another flatwasher on the top of the leg and
secure with a #8-32 nut and lockwasher. Orient the transformer so the
side with the green secondary leads is toward the inside of the
chassis.
( ) Snap the eight PC board standoffs into their respective holes from the
inside of the chassis.
( ) Attach the two large capacitor clamps to the chassis using #6-32 X 1/4"
finishing screws, lockwashers and nuts. Orient the clamps as shown in
the wiring diagrams.
( ) Snap PR40B's electrolytic capacitor C4 into place in the two capacitor
clamps on the front of the chassis. Be sure to orient the capacitor as
shown in the wiring diagrams. The negative terminal of the capacitor
must be nearer the bottom of the chassis.
( ) Attach toggle switch S1 to the chassis using the supplied finishing
washer and nut. Orient the switch as shown in the wiring diagram.
( ) Crimp the strain relief onto the line cord at a point about six inches
from the end of the cord using a pair of pliers. While compressing the
strain relief insert the short end of the line cord and the strain
relief into the hole provided on the back of the chassis and release.
Attaching Connectors to the Print Mechanism
The print mechanism is supplied with two connectors installed, one for the
print head and one for the printer motor. The connector for the print head is a
fifteen pin AMP connector and will mate to a connector located on the PR-40A
board. The connector for the printer motor is a nine pin AMP connector and must
be cut off right at the connector so that another connector may be installed in
its place.
( ) Cut off the nine pin motor connector for the print mechanism right at
the connector.
( ) You should note that the print mechanism motor is divided into two
distinctive halves. The inner half has one black and one white wire
coming out of it and the outer half also has one black and one white
wire coming out of it. Attach and solder a female connector pin to the
end of each of the four wires. Insert the wires into the respective
pins of a six pin Molex male connector as listed below:
MOTOR WIRE CONNECTOR PIN #
inside white 1
outside white 3
outside black 4
inside black 6
The word inside refers to being nearest the body of the printer while
outside is farthest away. The wires are each electrically dependent and
must be connected exactly as listed in the preceding table for the
printer motor to run correctly.

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( ) Attach and solder three 16" pieces of #24 (light) gauge hookup to each
of the three switch-terminals on the roller cam microswitch located on
the right hand side of the printer mechanism. On the opposite end of
the upper lug's wire, attach a male pin. Attach a female pin to the
unconnected end of, the remaining two lug wires. Solder.
( ) Insert each of the pins into the specified pin location of a six pin
Molex male plug using the table below. The pins must be installed from
the back or numbered side of the connector:
ROLLER CAM SWITCH LUG CONNECTOR PIN #
upper 3
middle 1
lower 2
( ) Complete wiring steps 1 thru 4 of the wiring table if you will be
operating the printer on 120 VAC. For 240 VAC, 50 Hz operation, skip
wiring steps 2 and 3 and instead connect together and insulate
transformer T1's brown and black-red wires. Then connect T1's remaining
black and brown-white wires just as described in the wiring table steps
1 and 4.
( ) Complete wiring steps 5 thru 10 of the wiring table and then snap fuse
Fl into place.
( ) Complete wiring steps 11 thru 34 of the wiring table. A female
connector pin will be attached to one end of each of the wires. Make
absolutely sure each pin is installed correctly into the specified hole
of the correct connector block, since the pins cannot be removed after
installation. The pins must be installed from the back or numbered side
of the connector. Take note that J1 on the PR40B board is a 15 pin male
connector block while J1 on the PR40A board is a 12 pin male connector
block.
( ) Check to make sure that all electrical connections have been soldered.
Cut off any extra length of the tabs on C4 if necessary.
( ) Snap the two PC boards into the chassis. The larger PR40A board should
be oriented so the upright transistors are adjacent the power
transformer, while the smaller PR40B board should be oriented so the 15
pin connector is near the power transformer.
( ) Plug each of the J1 connectors onto their respective mates.
( ) Set the printer chassis upside down (components exposed) and set the
print mechanism itself right next to it. Connect the connector from the
print mechanism's motor wires to the PR40B's J2 connector block.
( ) Attach the connector from the print mechanism's roller cam microswitch
wires to the PR40A's J3 connector block.
Installing the Ribbon and Paper Roll
The paper roll is installed by pressing the plastic roll holder thru the
center of the paper and placing the spool on the printer so the paper pulls from
the inside back of the printer. This allows the paper to contact the paper guides
at all times thus centering the paper for printing. Feed the paper thru the
guides adjacent the paper roll and advance the platten.
The ribbon is installed by first orienting the spools so the ribbon is fed
and spooled from the outside of the spools and then snapping the spools into

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place so the ribbon is routed inside the tensioner and looped around the guides
on both sides of the platten. The ribbon is self-reversing so it doesn't matter
which spool is placed where. After installing the ribbon, be sure that each
ribbon spool will turn freely on its respective shaft. If a spool is excessively
tight on a shaft sanding the shaft slightly with sandpaper should help. If the
binding is being caused by the two springs on the spindle that hold the spool in
place a small washer can be placed between the bottom of the spindle and the
mechanism that it is attached to. This will allow the spool to ride lower on the
spindle.
You are now in a position to test and set the line length and print
intensity adjustments on the printer. By having everything exposed as you do now,
adjustments and troubleshooting will be considerably easier. Once the unit has
been calibrated and thoroughly tested, as described later in the instructions,
the print mechanism can be disconnected and reinstalled and secured on the top of
the printer chassis using #6-32 nuts and lockwashers. All of the wires for the
print mechanism are then run thru the large bushing on the top of the printer
chassis. The I/0 cable feeding the printer is also run out of this large bushing
so that the bottom of the printer may be closed with the perforated cover
supplied with the kit. The cover is secured with #6-32 screws and tinnerman nuts
which are also supplied with the kit. The four rubber feet should be attached to
the outside of the cover about 1" in from each corner.
Wiring the Printer's Input Data Connector
The actual ASCII data fed to the printer is input thru connector J4 on the
PR-40A board. In order to test and set the calibration controls on the printer,
it will be necessary to load a diagnostic program into your computer system to
feed data to the printer. The program should feed the printer continual,
printable ASCII data. This will force the printer to print full 40 character
lines which are necessary to set the line length trimmer resistor, R17 for
symmetrical margins. Trimmer resistor R25 sets the print intensity and may be
adjusted when printing any line length.
If you are using the PR-40 printer with the SWTPC 6800 Computer System you
should use the PRNTST-1 diagnostic contained within this instruction set,
otherwise you will need to write a program for your own specific computer system
which performs similar functions. You may even feed the printer directly from an
ASCII keyboard if you can live with not being able to read the line until a
carriage return is entered. The keyboard must output a negative going
"keypressed" strobe for direct compatibility with the PR-40 Printer.
The data connections to J4 on the PR-40A board should be made as follows:
J4 Pin # Data Connection
1 ground
2 "data accepted" output - from printer
3 "data ready" from computer
4 ground
5 ASCII bit 5
6 ASCII bit 6
7 no connection
8 ASCII bit 3
9 ASCII bit 4

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10 ASCII BIT 0
11 ASCII bit 1
12 ASCII bit 2
If you will be using the PR-40 Printer with the SWTPC 6800 Computer System,
you will need to drive it with the output half of an MP-L option on the SWTPC
6800 Computer System. Connections should be made between the two as shown in the
PR-40/SWTPC 6800 Interconnection drawing. When writing programs to drive the PR-
40 printer with the SWTPC 6800 Computer System, it will be necessary to first
configure the 6820 peripheral interface adaptor integrated circuit on the MP-L
option of the SWTPC 6800 as in the diagnostic and then use an output character
routine similar to that used in the PRNTST-1 diagnostic.
Checkout and Adjustment
For the initial testing of your PR-40 disconnect the printer solenoids by
pulling loose J2 on the PR40A board if you have the solenoids connected. In order
to checkout the printer you should use an analog type DC voltmeter having a 0-10V
scale. (Most digital meters give false readings for this particular application).
Connect the negative (-) end of your meter to the PR40's chassis or PC board
ground and hook a 4.7K ohm 1/2 watt resistor (supplied) between the + and -
terminals of your meter. Turn on the power and connect the positive meter probe
(with the 4.7K resistor attached) to the collector of Q1 on the PR-40A board (the
metal part of the case with the mounting hole in it). The voltage should read
very low and go up as you initiate your PRNTST diagnostic program. Also note that
the print motor should start up and move the head although no printing will
occur. If the voltage does not rise recheck your program. The voltage should rise
to anywhere between 1 and 6 volts depending on the setting of the density
control. Check each of the seven transistors for this voltage. If the voltage
goes beyond about 6 volts recheck your circuit board for mistakes - DO NOT HOOK
UP THE SOLENOIDS UNTIL THE PROBLEM IS CORRECTED.
After you have verified that all voltages look OK switch the printer off
and connect J2. Re-load your diagnostic and switch the printer on. Running your
diagnostic should activate the printing mechanism and printing should start. You
can now adjust the line length and print density. Adjust R25 for the print
intensity most desirable. The line width can then be adjusted by R17. Note that
if the width is extended too far the characters on the right edge will be much
narrower than normal. After the line width has been adjusted the paper can be
centered by moving the two white plastic guides on the round bar underneath the
platen at the rear of the mechanism. If necessary, the entire paper carrier can
be moved slightly by loosening the four screws holding it to the main mechanism
and sliding it to one side or the other as necessary.
The Printer Mechanics
The entire design is based on a remarkably simple and reliable print
mechanism. The printed characters are formed by moving the print head
horizontally across the paper while selectively energizing solenoid driven print
wires on the head which strike an inked ribbon and imprint dots on standard
adding machine paper. All seven of these solenoid driven print wires converge at
the tip of the print head in a vertical line which is perpendicular to the
horizontal direction of movement of the print head. By selectively firing the
print wires, 5 dot wide X 7 dot high characters are printed as the print head
moves across the paper. A one dot time spacing is left for separation between the
printed characters.

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This method of printing characters is not new but the method of moving this
wire impact print head is unique. Rather than using dual motors, clutches timing
bars and the other hardware usually associated with this type of print head, this
printer rotates a long cylinder just beneath the print head. The length of the
cylinder itself is a little longer than the head's printing width on the paper.
The cylinder has a uniform single cyclic zig-zag track formed on its outer
circumference, running from the left side of the cylinder to the right side then
back to the left again. A small projection on the bottom of the print head rides
in this track so that as the cylinder rotates the print head moves back and forth
from left to right. This technique moves the print head across the paper at a
constant velocity except for the extreme ends, where nothing is printed anyway.
This approach greatly simplifies the electronics needed to drive the printer
since no head positioning circuitry is necessary. The cylinder itself is turned
by an AC motor on the lower right hand side of the print mechanism. A small
ribbed nylon belt interconnecting the two rides on gear teeth of both the motor
and cylinder. Also attached to the right side of the cylinder is a cam that
actuates a roller arm micro-switch riding on the cam. This is how the printer's
electronic circuitry senses the "start of line" position of the print head. On
the left side of the cylinder is an eccentric driven pawl arm that advances the
paper one line for each revolution of the cylinder which is the same as one cycle
of the print head.
Let's go thru a cyclic operation of a printed line where we will first
assume the head is in rest position just left of center. When a line print
command is initiated by the control circuitry, the motor starts and the head
begins to move from the center position toward the far left side of the printer
where the head reverses direction. This non-print dead zone gives the motor,
cylinder, and print head time to attain full speed. As the head begins its
movement from left to right, the cam actuated micro-switch opens telling the
electronic circuitry to start outputting character forming solenoid driving
pulses. Somewhere before the print head reaches the far right hand edge of the
paper the solenoid pulses will cease while the head continues to move. When the
head reaches the right end of its travel, it will reverse direction and begin to
move back toward the center of the printer. During this return movement, the pawl
arm will rotate the platen one line for the line feed. The motor is then turned
off just to the left of center where it started originally. Character data is not
accepted by the printer's circuitry during an actual print cycle, however feeding
continual print data from a computer to the printer may take place so fast that
the print motor may never appear to stop between repeatedly printed lines
although it actually does.
The operation of printing ribbon used on the unit is also amazingly simple.
A ratchet technique not only advances the ribbon incrementally for each cycle of
the print head but automatically reverses it when it reaches the end of one of
the two spools. This means you need only change the ribbon when the printing
become too light for easy legibility.
The Electronics
The electronic circuitry driving the fore mentioned print mechanism can
vary from nothing but motor and solenoid drivers constantly serviced by the
microcomputer to a fully self-contained hardware control unit with memory needing
only 7 bit parallel ASCII data and a "data ready" strobe control line from the
computer. This printer system fits into the latter category. The printer has its
own 40 character FIFO (first in - first out) memory allowing the computer to send
character data at whatever speed it wishes. The entire line is printed upon
receipt of a carriage return (0D16) or automatically whenever the 40 character

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line buffer has been filled. All control characters with the exception of a
carriage return are ignored by the printer. They are not stored in the FIFO line
memory since they cannot be printed anyway. Repeated line feeds are initiated by
sending repeated carriage return control commands. Since the printer prints upper
case ASCII characters only, all lower case characters sent to the printer are
transposed to their upper case equivalent before printing. The printer's line
buffer memory is automatically cleared by a hardware power-up reset circuit when
printer power is first applied. The printer's motor is triac controlled and is
powered by a 120 VAC secondary on the power supply's power transformer. This not
only provides power line isolation but allows the entire unit to be run on either
120 VAC or European 240 VAC power systems since the power transformer has two
primary windings which may be either parallel or series connected.
The seven ASCII parallel data input lines and "data ready" and "data
accepted" control lines are all TTL compatible. The inputs represent a maximum of
two standard TTL loads while the "data accepted" output will drive ten standard
TTL loads. Data is presented to the printer by storing the selected ASCII data on
the seven data input lines and strobing the normally high (logic 1) "data ready"
input line low. This line should go low (logic 0) for at least 1 microsecond and
when it does the normally high "data accepted" will also go low. The character is
not actually loaded until the "data ready" input is returned to its normally high
state. The "data accepted" line will then normally return high as well,
indicating that the character has been loaded. However, when loading the 40th
character on a print line or a carriage return command this "data accepted" line
will not return high until the character data has been printed and the printer
memory is ready for more data. The printer will ignore all data sent to it while
the "data accepted" line is low. So you will usually want to make sure the "data
accepted" output line is high before sending the printer data to be printed.
If you are careful not to output data faster than one character per
microsecond and allow a minimum one second delay before sending data after
sending a carriage return or the 40th character of each line then you may avoid
using the "data accepted" line altogether. However, using the "data accepted"
line will give your system the fastest possible print speed.
How It Works
All ASCII character data is presented to the FIFO memory, IM thru hex
inverter/buffer IC12. NAND gate IC14A makes any necessary conversions from lower
to upper case characters. 8 input NAND gate IC13 monitors the incoming ASCII data
in search of a carriage return (0D16 or 0000 11012) control command. If it
decodes a carriage return it will prepare normally high control flip-flop IC8A to
go low on the falling edge of the "data ready" (input strobe) control signal. NOR
gate IC15A decodes all control characters and along with IC15B inhibits any
control characters from being loaded into the FIFO memory. NAND gate IC9A is
responsible for generating the "data accepted" output whenever data has been
accepted by the FIFO memory or acknowledged by the control character decode
logic. It also inhibits the "data accepted" output while the printer is in the
process of printing a line.
Control flip-flop IC8A's Q output is high when the printer is in the idle
state. A FIFO full signal decoded by NOR gate IC15D or a decoded carriage return
command by IC13 resets this flip-flop low which turns the printer's motor on thru
transistors Q1 and Q2 located on the power supply board which starts a line print
sequence.
When the print head advances to the "start of line" position, the roller
arm microswitch changes states which flips RS latch NAND gate IC11D low which

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allows free running oscillator IC10 to feed divide by 2 flip-flop, IC8B. This
divide by 2 feeds both the dot counter, IC4, and solenoid timer IC6. This
solenoid timer is used to set the "ON" time of the printer's solenoids. If set
too long, the solenoids will overheat, if set too short, the printed lines will
be too light. An "ON" time duration of 400 microseconds is the best compromise
setting. Decade counter IC4 has its B and C outputs tied back to its zero reset
inputs forcing it to become a modulo 6 counter. As the counter progresses from 0
to 4 inclusive, the selected character lines are decoded by the ROM, IC2 using
the ASCII data fed to it by FIFO memory, IC1, and fed onto the solenoids thru
buffers IC3 and IC7 and Darlington drivers 01 and Q7. Counter state 5 is decoded
by the ROM as a blank for the one dot space necessary to separate the printed
characters. In the latter phase of the five count the FIFO is shifted, the
counter resets, and the sequence repeats. When the FIFO finally empties, the ROM
is disabled and blanks the output for the rest of the print cycle. When the still
moving print head reaches its normal "rest" position, the roller arm microswitch
again changes states. This time RS latch NAND gate IC11C flips low setting
control flip-flop IC8A back to normally high state.
Timer IC5 is a power up clear circuit which empties the FIFO memory and
helps prevent random firing of the print solenoids during power up.
Note
When turning the printer off it is best to use the toggle switch on the
front of the printer chassis. This toggle switch not only interrupts the 120
VAC being delivered to the power transformer on the printer but also
disables the 40 VDC line feeding the solenoids. It is necessary to break this
line while the DC voltages in the system are bleeding down in order to prevent
the print solenoids from randomly firing and printing a vertical line on the
paper.
In Case of Problems
In case of problems, the best procedure is to remove power and recheck all
assembly steps. Since you are probably feeding the printer with a computer
system, don't rule out a possibility of a problem either in the interconnection
between the two or in the program driving the printer.
As far as the printer itself is concerned, check the voltages on the PR-40B
board to make sure the DC voltages are within 10% of their specified ratings as
shown on the PR-40B schematic. Other than this the only thing you can do is go
thru the PR-40A board components one by one until you can pin down the source of
the problem.
If you still cannot find the problem or do not wish to service the unit
yourself, repair services are available from us at a cost of $20.00 labor plus
parts.

12
Wiring Table - PR-40 Alphanumeric Printer
WIRE FROM TO
STEP LENGTH GAUGE PART TERMINAL SOLDER PART TERMINAL SOLDER
1 full - T1 Black - TS-1 C No
2 full - T1 Brown - TS-1 No
3 full - T1 Blk-Red - TS-1 A No
4 full - T1 Brn-Whit. - TS-1 A No
5 - - line cord A - TS-1 A Yes
6 - - line cord B - Fl A Yes
7 12 1/4" heavy TS-1 C Yes S1 C Yes
8 11 1/2" heavy Fl B Yes S1 D Yes
9 PR-40B's R6 C4 (+) No C4 (-) No
10 4 1/2" heavy C4 (+) No S1 B Yes
11 full - T1 Red - PR-40B J1 pin 1 Yes
12 18" heavy C4 (-) Yes PR-40B J1 pin 2 Yes
13 full - T1 Red - PR-40B J1 Pin 3 Yes
14 18" heavy C4 (+) Yes PR-40B J1 pin 4 Yes
15 15" heavy Sl A Yes PR-40B J1 pin 6 Yes
16 full - T1 Green - PR-40B J1 pin 7 Yes
17 6' heavy TS-1 B Yes PR-40B J1 pin 8 Yes
18 full - T1 Blue - PR-40B J1 pin 9 Yes
19 full - Tl Green - PR-40B J1 pin 10 Yes
20 full - T1 Grn-Yel - PR-40B J1 pin 11 Yes
21 full - Tl Blue - PR-40B J1 pin 12 Yes
22 full - T1 Grn-Yel - PR-40B J1 pin 13 Yes
23 full - T1 Yellow - PR-40B J1 pin 14 Yes
24 full - T1 Yellow - PR-40B J1 pin 15 Yes
25 8" heavy PR-40B +40 Yes PR-40A Ji pin 1 Yes
26 8" aeavy PR-40B +8 Yes PR-40A J1 pin 3 Yes
27 8" heavy PR-40B +40 Yes PR-40A J1 pin 4 Yes
28 8" heavy PR-40B +8 Yes PR-40A J1 pin 6 Yes
29 8" heavy PR-40B GND 2 Yes PR-40A J1 pin 7 Yes
30 heavy PR-40B G1 Yes PR-40A J1 pin 8 Yes
31 " heavy PR-40B G1 Yes PR-40A J1 pin 9 Yes
32 8" heavy PR-40B GND 2 Yes PR-40A J1 pin 10 Yes
33 8" heavy PR-40B MC Yes PR-40A J1 pin 11 Yes
34 heavy PR-40B -12 Yes PR-40A JI pin 12 Yes

13
Parts List PR-40A Printer Board
Resistors
R1 - R7 2.7K ohm 1/4 watt resistor
R8, R11 - R13, R21 10K ohm 1/4 watt resistor
R9, R10, R15, R16, R22 - R24 1K ohm 1/4 watt resistor
R14 100 ohm 1/4 watt resistor
R17 20K ohm trimmer resistor
R18 39K ohm 1/4 watt resistor
R19, R20 4.7K ohm 1/4 watt resistor
R25 10K ohm trimmer resistor
R26 15K ohm 1/4 watt resistor
R27 3.3M ohm 1/4 watt resistor
R28 1 M ohm 1/4 watt resistor
Capacitors
C1, C11 0.1 mylar capacitor
C2, C7 - C9 0.01 mfd capacitor
C3 0.47 mfd tantalum capacitor
C4, C6, C10, C12 0.1 mfd disc capacitor
C5 0.022 mylar capacitor
C13 220 mfd @10 VDC electrolytic capacitor
Semiconductors
IC1 3351 FIFO memory
IC2 5241 ABL character generator ROM
IC3, IC7 7407 hex O.C. buffer
IC4 7490 decade counter
ICS, IC10 555 timer
IC6 74121 one shot
IC8 7474 dual D flip flop
IC9 7420 dual 4-input NAND gate
IC11, IC14, IC16 7400 quad NAND gate
IC12 7404 hex inverter
IC13 7430 8-input NAND gate
IC15 7402 quad NOR gate
IC17 7805 voltage regulator
Dl - D7 15 volt zener diode 1N5245 or 1N4744
D8 1N4148 silicon diode
Q1 - Q7 RCA125 PNP Darlington transistor
Q8 2N5210 transistor
Misc
Fl - F7 minature fast acting fuse - Littlefuse
#275 01.5 or equiv.

14
Parts List PR-40b Printer Board
Resistors
R1 2.2K ohm 1/4 watt resistor
R2 820 ohm 1/2 watt resistor
R3 100 ohm 1 watt resistor
R4 1K ohm 1/4 watt resistor
R5 120 ohm 1/2 watt resistor
R6 2.2K ohm 1 watt resistor
Capacitors
C1 5000 mfd @10 VDC electrolytic capacitor
C2 470 mfd @25 VDC electrolytic capacitor
C3 1 mfd @400 VDC capacitor
C4 4000 mfd @50 VDC electrolytic capacitor
Semiconductors
D1 - D6 1N5402 silicon rectifier
D7 - D8 1N4003 silicon rectifier
D9 12 volt 1 watt zener, 1N4742 or equiv.
Q1 SS1122 silicon transistor
Q2 T2800B triac
Misc.
S1 DPST toggle switch
F1 2 1/2 amp slo-blo fuse
T1 Power Transformer - 120/240 VAC primary,
32 VAC @2A, 12.5 VAC @1A C.T., 120 VAC
@200 Ma, 24 VAC @100 Ma. C.T. secondaries

15
SWTPC 6800/PR-40 Printer Diagnostic PRNTST-1
This printer diagnostic has been written to test and/or debug the SWTPC PR-
40 printer when it has been interfaced to the SWTPC 6800 Computer System thru one
of the MP-L parallel interface boards. It is assumed that before loading this
program, the rest of the system is functioning normally with no problems. The
program itself uses 6010 words and is loaded within the 128 word RAM used by the
MIKBUG operating system on the MP-A Microprocessor/ System Board. A program may
reside in external RAM memory simultaneously with the diagnostic loaded within
the 128 word RAM, or the diagnostic may be run with no MP-M memory boards
installed on the system at all. The diagnostic may be loaded either from tape or
instruction by instruction using MIKBUG starting from address A014 thru A024 and
then from address A048 thru A072. The address of the MP-L parallel interface
board feeding the printer must be set using MIKBUG R to load the hexadecimal
address of the selected port into memory locations A002 and A003 with the most
significant byte going into A002 and the least significant byte going into A003.
The starting address locations of the interface ports are given below:
Port Address in Hex
I/0 #0 8000
I/0 #1 8004 (reserved for control interface)
I/0 #2 8008
I/0 #3 800C
I/0 #4 8010
I/0 #5 8014
I/0 #6 8018
I/0 #7 801C
Since the program counter is set when the program is initially loaded, the
diagnostic is initiated as described in the "Go to User's Program" section of the
Engineering Note 100. Once initiated, the program can be stopped only by
depressing the "RESET" button. The program may then be re-started after resetting
the program counter to A04A as described in the "Display contents of MPU
registers Function" section of Engineering Note 100.
The diagnostic works by sending out an ASCII carriage return (0D16)
followed by ASCII characters 2116 thru 3F16 followed by a carriage return (0D16),
followed by ASCII characters 4016 thru 5E inclusive, repeating itself until
stopped with the RESET switch.
Each character is output to the printer by first storing the ASCII
character's bits 0 thru 6 on output lines 00 thru 06 of the selected MP-L
Parallel Interface port, then bringing the normally high "Data Ready" line low
via interface output control line C2 for at least one microsecond then returning
the line high again. The data must be held stable on the interface output lines
however until the printer acknowledges data receipt by bringing its normally high
"Data Accepted" line momentarily low and then high again. This latter low to high
transition signifies that the printer has accepted the character. The MP-L
control line C1 fed by this line has been programmed to see the low to high "Data

16
Accepted" transition and responds by allowing the program to output the next
sequential ASCII character.
To set the printer's line width adjustment, you should use Mikbug to change
the data in memory locations A056 and A065 from 0D16 to 0016. This will eliminate
the transmitted carriage returns and force the printer to print a full 40 column
line. The line width control should then be adjusted for symmetrical margins. The
characters will still be printed sequentially but will not be identical from line
to line.
DO NOT operate the printer more than a minute or so while running this
diagnostic. Continuous printing of full length lines with no spacing between
characters overheats the solenoids on the print mechanism which may decrease its
operating life.
If you are using the SWTPC PR-40 printer with the SWTPC 6800 Computer
System, you may use the OUTCHR subroutine listed from address A014 thru A024 in
the PRNTST diagnostic within your own program for outputting characters to the
printer. The index register must be loaded with the starting address of the MP-L
Parallel Interface board feeding the printer. The character to be printed must be
loaded into accumulator A and the contents of accumulator B are destroyed during
the subroutine.
Never install or remove the interface board when the system is powered up.
Doing so is not only hazardous to the equipment, but bypasses the normal power-up
sequence required by the internal registers within the 6820 integrated circuit in
order to guarantee proper operation.
Mikbug is a registered trademark of Motorola Inc.

17
SWTPC 6800/PR-40 Printer Diagnostic PRNTST-1
A014 A7 OUTCHR STA A 0,X
A015 00
A016 C6 LDA B #$37
A017 37
A018 E7 STA B 1,X
A019 01
A01A C6 LDA B #$3F
A01B 3F
A01C E7 STA B 1,X
A01D 01
A0IE 6D LOOP3 TST 1,X
A01F 01
A020 2A BPL LOOP3
A021 FC
A022 E6 LDA B 0,X
A023 00
A024 39 RTS
A048 A0 MSB Program Counter
A049 4A LSB Program Counter
A04A FE START LDX PARADR
A04B A0
A04C 02
A04D C6 LDA B #$FF
A04E FF
A04F E7 STA B 0,X
A050 00
A051 C6 LDA B #$3F
A052 3F
A053 E7 STA B 1,X
A054 01
A055 86 FSTLIN LDA A #$0D
A056 0D
A057 8D BSR OUTCHR
A058 BB
A059 86 LDA A #$20
A05A 20
A05B 4C LOOP1 INC A
A05C 81 CMP A #$40
A05D 40
A05E 27 BEQ NXTLIN
A05F 04
A060 8D BSR OUTCHR
A061 B2
A062 20 BRA LOOP1

18
A063 F7
A064 86 NXTLIN LDA A #$0D
A065 0D
A066 8D BSR OUTCHR
A067 AC
A068 86 LDA A #$3F
A069 3F
A06A 4C LOOP2 INC A A
A04B 81 CMP A #$60
A06C 60
A06D 27 BEQ FSTLIN
A06E E6
A06F 8D BSR OUTCHR
A070 A3
A071 20 BRA LOOP2
A072 F7