B&B Electronics Parallel Port Input/Output Converter PPIO User manual

.PPIO2899 Manual Cover Page
B&B Electronics Mfg Co Inc –707 Dayton Rd -PO Box 1040 -Ottawa IL 61350 -Ph 815-433-5100 -Fax [phone]
Parallel Port Input/Output Converter
Model PPIO
Document No. PPIO2899
B&B Electronics Mfg. Co. Inc.
707 Dayton Road -- P.O. Box 1040 -- Ottawa, IL 61350 USA
Phone [phone] -- General Fax [phone]
Home Page: www.bb-elec.com
Sales e-mail: [e-mail] -- Fax [phone]
Technical Support e-mail: [email protected] -- Fax [phone]
Copyright ?1991-1999 B&B Electronics -- Revised June 1999
Not Recommended for New Installations.
Please contact Technical Support for more information.

PPIO2899 Manual Table of Contents i
B&B Electronics Mfg Co Inc –707 Dayton Rd -PO Box 1040 -Ottawa IL 61350 -Ph 815-433-5100 -Fax [phone]
TABLE OF CONTENTS
INTRODUCTION .............................................................................1
PACKING LIST.................................................................................2
PC PARALLEL PORT DESCRIPTION ...........................................3
PPIO DESCRIPTION & CONNECTION..........................................7
CONTROLLING THE PPIO USING GWBASIC..............................9
CONTROLLING THE PPIO USING PASCAL...............................12
CONTROLLING THE PPIO USING C...........................................18
INTERFACING TO THE PPIO.......................................................27
EXAMPLE USE OF THE PPIO .....................................................29
APPENDIX.....................................................................................33
HEXADECIMAL NUMBERS ..............................................................33
BINARY NUMBERS ........................................................................34

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1
INTRODUCTION
The PPIO allows you to connect your IBM PC (or clone)
computer to the outside world using the computer’s parallel port.
The eight I/O points can be used as either inputs or outputs. As an
output they can control voltages as high as 50 Volts DC and can
handle currents as high as 500 mA DC. As an input they can handle
voltages from 0 to 50 volts with a threshold of 2.5 Volts DC.
CAUTION: Each output of the PPIO can dissipate 1 Watt when
used alone, however, all eight outputs together cannot
dissipate more than 2.25 Watts.
NOTE: The PPIO connects to the parallel port of your computer and
uses most of the available pins on that port. You MUST use a cable
that connects pins 1 through 17 of the DB-25 connector to the PPIO
for it to work properly. To be safe you should use a cable that
connects all 25 pins from connector to connector.
NOTE: The PPIO can only be used with parallel ports that are in
“compatible” or “normal” mode. It will not function properly with
parallel ports in ECP or EPP modes. The mode of the parallel port
can be changed in the BIOS setup by pressing either the F2 key or
DEL key just after the computer begins the boot procedure.
The PPIO comes with sample DOS programs written in
GWBASIC, QuickBASIC, Pascal, and C. These sample programs
can be used "as is" to test the PPIO and to control and display the
status of its I/O pins. Parts of these programs can be used in other
programs to make it easier to interface to the PPIO. Also, by
studying these programs a programmer can learn how to write code
in any language to do a similar job.

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Packing List
Examine the shipping carton and contents for physical damage.
If there is damage, contact B&B Electronics immediately. The
following items should be in the shipping carton:
1) PPIO unit
2) This Instruction Manual
3) PPIO Sample/Test Disk
If any of the above items is missing contact B&B Electronics
immediately.

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PC PARALLEL PORT DESCRIPTION
To understand how the PPIO can be controlled you must
first understand how the parallel port works inside the computer. The
parallel port is designed to connect the computer with the outside
world. It can have up to 12 TTL compatible outputs and up to 9 TTL
compatible inputs. It cannot have both at the same time since some
inputs and outputs share pins. Eight of the outputs and five of the
inputs are dedicated. Four of the lines can be either inputs or
outputs.
The main use for the parallel port is to send data to a
printer. The port uses the eight dedicated outputs for data and the
other lines for handshakes. Under normal printer operation, the
computer will put an eight-bit byte on the eight lines (pins 2-9) and
then use the Strobe output (pin 1) to tell the printer to read the data.
Upon receipt of an Acknowledge (pin 10) from the printer the
computer knows that the eight bit word was received. It can then
send the next word. Other lines are used for busy, off line, etc.
All the control of the parallel port is done through software. If
you look at the hardware you find that the parallel port is connected
directly to the computer bus. This means that we can address these
inputs and outputs in any way we want if we do not use the port to
drive a printer.
Each PC has port addresses where the parallel ports, serial
ports, hard disk, etc. can be addressed. These are located in the
address range from 0000H to 03FFH (the H indicates the use of the
hexadecimal numbering system). Each parallel port has one main
port address for outputting data. The next two addresses above that
address are used for handshaking control. For instance, on a typical
computer a parallel port could be located at address 0378H. The
handshaking lines will then be located at 0379H and 037AH. Table 1
shows the correspondence between the bits of the port and the pins
of the DB-25 female connector.
Referring to Table 1, if you output a 01H (which is 00000001
in binary) to your computer’s port 0378H, then pin 2 of the parallel
port’s DB-25 connector will be HIGH and pins 3 through 9 will be
LOW. To do this using GWBASIC you would use the command:
OUT &H378,&H01. Using this method you can output any pattern
you wish on pins 2 through 9 of the parallel port. To turn ON pins 2,
4, 6, and 8 use: OUT &H378,&H55. 55H is the same as 01010101
binary. This port (0378H) can be read as an input, but mostly it is

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used to read the status of the port. The PPIO does not need to use
this port as an input port.
Again, referring to Table 1, if you input the data on port
0379H, then whatever TTL level pins 15, 13, 12, 10, and 11 are will
show up as bits 3 through 7. The state of bits 0, 1, and 2 will be
unknown since they are not hooked to anything, but they probably
will be HIGH (ONE). Note that the data on pin 11 will be inverted, if
pin 11 is LOW you will get a HIGH on bit 7. The pins with the “bars”
over them are all inverted. This is done by the hardware in the
parallel port over which we have no control. A way to read these pins
using GWBASIC is: A1=INP(&H379). A1 will then have the results of
the input from port address 0379H. Bit 3 of A1 will have the status of
pin 15, bit 4 the status of pin 13, etc.
Note that on the third address (037AH) the pins can be
either inputs or outputs. If you use the GWBASIC command OUT
&H37A,&H01 (00000001 BINARY) then pin 1 will be LOW (because
it is inverted), pins 14 and 17 will be HIGH (they also are inverted),
and pin 16 will be LOW (it is not inverted).

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Table 1
Note 1: X means no connection to any DB-25 pin.
Note 2: Bit 4 of 37AH as an output is used to control the interrupt IRQ7.
When this bit is HIGH, IRQ7 is enabled and when this bit is LOW, IRQ7 is
disabled. As an input, this reads the status of the IRQ7 interrupt to see if it
is enabled or not.
Note 3: The “bars” over the top of some pin numbers indicate that those
signals are inverted (in hardware) from PC's bus to DB-25 pin.
You can also read pins 1, 14, 16, and 17 using the INP(&H37A)
command. For this to work properly, you must first force all the
outputs HIGH. The way the parallel port is wired, if you do not force
the outputs HIGH they will interfere with the inputs. To do this, use
the command OUT &H37A,&H04 (04H is the same as 00000100
binary). Pins 1, 14, and 17 are inverting so a ZERO written to them
forces them HIGH. Pin 16 is non-inverting so the ONE written to it
forces it HIGH. After issuing the OUT command you can then do a
A2=INP(&H37A) that will read the TTL levels on those pins. In this
case the four upper bits (bits 4-7) will be unknown, but will probably
be HIGH.
378H 379H 37AH
BIT Output Input Output Input Output Input
0 2 2 X X_ 1_ 1
1 3 3 X X_ 14 _ 14
2 4 4 X X 16 16
3 5 5 X 15 _ 17 _ 17
4 6 6 X 13 See Note 2 See Note 2
5 7 7 X 12 X X
6 8 8 X 10 X X
7 9 9 X_ 11 X X

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There are three port addresses that are normally used on a PC
for parallel ports: 3BCH, 378H, and 278H. When your PC is reset or
turned on these three addresses are scanned by the BIOS in the
order shown above. The first one that the BIOS finds with a parallel
port installed is assigned the logical name LPT1. The second, LPT2,
etc. You can connect the PPIO to a port located at any of these
three addresses. During power up the computer will assign that port
a logical name but we will ignore it and communicate to the port
directly.
If you are using a base address of 0278H the I/O port addresses
in Table 1 would change to 0278H, 0279H, and 027AH. With a base
address of 03BCH the addresses are 03BCH, 03BDH, and 03BEH.
Check your computer manual to find out which address your parallel
port has. It is also possible to purchase a special parallel port from
B&B Electronics that can be set at any address in the I/O port
address space from 0000H to 03FFH.
The above parallel port information is true for the vast majority of
the PC compatible ports. However, a few computer manufacturers
have chosen to make their parallel ports non-standard. On some
battery powered computers pins are disabled to save power. Some
ports may also have extra “direction control” bits. If you have
problems where one or more bits are always on or off you should
check your owner's manual. You may have to enable the port or set
the “direction bit” correctly to get the port to work with the PPIO. If a
pin is missing you may have to install a different parallel port card to
get the PPIO to work properly.

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PPIO DESCRIPTION & CONNECTION
The PPIO makes use of the eight output pins (pins 2 through 9)
at address 378H to drive its eight outputs. It uses the upper four bits
of address 379H (pins 13, 12, 10, and 11) for the upper four input
bits of the PPIO. It uses the lower four input bits of address 37AH
(pins 1, 14, 16, and 17) for the lower four input bits of the PPIO. This
assumes you are using the same port addresses as in Table 1. In
this way the PPIO can have eight inputs or eight outputs. Refer to
Figure 1 for the PPIO schematic.
Each pin of the PPIO is bi-directional. It can be used as either an
input or an output. Referring to Figure 1 you will see that PPIO bit 0
can be driven by pin 2 of the parallel port or it can be read by pin 1 of
the parallel port. The drivers are open collector Darlington
transistors that can sink up to 500 mA and are protected by “kick
back” diodes that are connected to the positive power supply. These
outputs can handle voltages as high as 50 Volts DC. If bit 0 of port
0378H is HIGH, pin 2 of the parallel port is HIGH and the PPIO
output pin will be LOW and can sink 500 ma. If bit 0 goes LOW, pin
2 will go LOW and the PPIO transistor will go OFF.
To use PPIO I/O bit 0 as an input you first set bit 0 of port 378H
LOW to turn OFF the driver transistor. From then on, if you force the
PPIO bit 0 to ground, a LOW will show up on pin 1 of the DB-25
connector. If you look at Table 1 you will note that pin 1 is inverting
in the computer parallel port. This means that a LOW on pin 1 will
show up as a HIGH in the computer. This is called negative true
logic. The PPIO receivers are set up as inverters or non-inverters to
compensate for the inverting and non-inverting inputs of the parallel
port.
All you have to remember is that if you force one of the PPIO I/O
bits to ground, it will be a ONE when you read it in the computer. If
you leave the PPIO I/O pin open or force it HIGH (above 2.5 volts), it
will be a ZERO when you read it in the computer.
PPIO I/O bits 0 through 3 are connected to bits 0 through 3 of
port 037AH and PPIO I/O bits 4 through 7 are connected to bits 4
through 7 of port 0379H. (This assumes that you are using a parallel
port at 0378H in your computer.) A LOW on any PPIO I/O pin will
show up as a HIGH on the corresponding bit in the computer.

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Figure 1

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CONTROLLING THE PPIO USING GWBASIC
Refer to the fragment of GWBASIC code in Figure 2 to see how
to input the bits and make one 8-bit word.
100 OUT &H37A,&H04: REM This disables the 37AH Outputs
120 REM inside of the computer
140 REM so that we can use
160 REM 37AH as an input port.
180 REM It also disables
200 REM the IRQ7 interrupt.
.
.
300 A1=INP(&H37A) AND &H0F: REM Input the
320 REM lower 4 bits
340 REM and mask OFF
360 REM (force to 0)
380 REM the upper 4 bits.
400 A2=INP(&H379) AND &HF0: REM Input the high 4 bits
420 REM and mask OFF
440 REM the lower 4 bits.
460 IB=A1 OR A2 : REM This combines them
480 REM into one Input Byte.
.
. Figure 2
The above assumes that you are using a parallel port located at
0378H. If you are using a different port you will have to replace the
hex addresses shown with the proper ones for your port. Refer also
to the PPIO.BAS program on the supplied disk for an example of
how to use GWBASIC to interface with the PPIO.
To output bits from the computer to the PPIO interface (still
using GWBASIC and the above 0378H example) use the following
line:
500 OUT &H378,OB
Where OB is the byte you want to output. If, for instance, you want
to turn ON (force LOW) PPIO bit 0, you must turn ON (force HIGH)
bit 1 of the variable OB. This can be done by ORing OB with &H01.
If you want to turn OFF (force HIGH or open) the same PPIO pin you
must turn OFF (force LOW) bit 1 of OB. Do this by ANDing OB with
NOT &H01. Use the data Table 2 to handle all eight PPIO bits.

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Table 2
The big advantage of using the ANDing and ORing as shown in
Table 2 is that it makes each PPIO pin independent. If you have, for
instance, PPIO bit 2 ON and all of the rest of the bits OFF then your
variable OB will be equal to 04H (00000100 binary). PPIO bit 2 will
be LOW or ON. If you then want to turn ON bit 3 do the following:
500 OB=OB OR &H08
520 OUT &H378,OB
Bit 3 will go LOW and bit 2 will stay LOW. At that time OB will be
equal to &H0C (00001100 binary). To turn OFF bit 3 do the
following:
700 OB=OB AND NOT &H08
720 OUT &H378,OB
Bit 3 will go HIGH or open and bit 2 will stay LOW. By using the OR
and AND NOT commands in Table 2, bit 3 has been turned ON and
OFF without disturbing any of the other PPIO Bits.
You may also use the PPIO with some pins as inputs and some
pins as outputs. There can be any number of each and in any order.
Remember that before you can use a PPIO pin as an input you must
first force the output driver on that pin OFF by putting a ZERO on
that bit. For example suppose we want to use bits 0, 1, and 2 as
outputs and bits 3, 4, 5, 6, and 7 as inputs. We also want bit 1 to be
ON (or LOW) when we start the program. We need to turn OFF bits
3, 4, 5, 6, and 7 so we can use them as inputs and turn ON the
output bit 1. You should modify Figure 2 to add the following line:
180 OUT &H378,&H02
From then on you can use the first three lines of Table 2 to
control the three outputs and lines 220-320 of Figure 2 to input data.
I/O Bit Force I/O ON
(Low) Force I/O OFF (High)
0OR &H01 AND NOT &H01
1OR &H02 AND NOT &H02
2OR &H04 AND NOT &H04
3OR &H08 AND NOT &H08
4OR &H10 AND NOT &H10
5OR &H20 AND NOT &H20
6OR &H40 AND NOT &H40
7OR &H80 AND NOT &H80

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When you input the 8-bit word using lines 300-460 of Figure 2,
note that you also get the status of the outputs. If some I/O bits are
used by the PPIO as outputs the status of the bits you read will tell
you the status of the PPIO outputs. If PPIO output bit 0 is LOW then
bit 0 of IB (the variable InputByte) will be HIGH (1). This can be used
to check that the PPIO hardware is connected and is working
properly. If, through a fault, a PPIO bit is shorted to ground then,
when you read that bit it will be HIGH no matter what you output to
the PPIO bit. If there are no shorts or inputs that are LOW on the
PPIO pins then any byte you output should come back the same
when you input.
A good way to test for shorts is by outputting a &HAA (10101010
binary) and checking to see that the input is the same. Then output
a &H55 (01010101 binary) and check the input. This tests for both
pin-to-pin shorts and shorts to ground. At the same time this tests
that the PPIO is plugged in properly and you have the correct port
selected. This will not work if any of the PPIO pins are connected to
a device that is grounding or holding a pin LOW. If you have this
problem you will have to ignore these bits when you read the port.
NOTE: If the PPIO is connected properly and the upper four outputs
(4 -7) are functional, but the lower four outputs (0 -3) are not
functional, the parallel port may be in ECP or EPP mode. It must be
in “compatible” or “normal” mode for the PPIO to work properly. The
mode of the parallel port can be changed in the BIOS setup by
pressing either the F2 key or DEL key just after the computer begins
the boot procedure.

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CONTROLLING THE PPIO USING PASCAL
The PPIO disk includes two source code files as an example of
using the PPIO with the Pascal programming language. PPIO.PAS
is the main routine of the example program. PPIOUNIT.PAS
contains the routines for communicating with the PPIO. In the rest of
this section, we assume that you know the Pascal programming
language. Consult your reference manuals if you need help with the
language.
The Pascal Unit, PPIOUNIT.PAS handles communication with
the parallel port. It defines five variables. They are defined as:
Input_Byte : BYTE; { Byte that’s read from PPIO }
Output_Byte : BYTE; { Byte that’s written to PPIO }
Base_Address : WORD; { Base address of Parallel Port }
Status_Address : WORD; { Status Register Address }
Control_Address : WORD; { Control Register Address }
No function outside the Unit can directly access these variables.
Functions outside the Unit call functions within the Unit to modify
and return the value of these variables.
The procedure Set_Start, sets the variable Base_Address,
Status_Address, Control_Address and Output_Byte. It also sets up
the initial state of the parallel port. The function is defined as:
PROCEDURE Set_Start(Address:WORD; Init:BYTE);
BEGIN
Base_Address := Address; {Parallel Port Base Address}
Status_Address := Base_address+1; {Parallel Port Status Address}
Control_Address := Base_Address+2; {Parallel Port Control Address}
PORT[Base_Address]:= Init; {Write Init Value to Output Address}
Output_Byte := Init; {Put Init value In the Output_Byte Var}
END; {Set_Start}
This function must be the first one used, because all other functions
assume that the variables have been set.
The Procedure, Set_IRQ_OFF, tells the parallel port not to
generate any interrupts. This should be called immediately after
Set_Start. It is defined as:

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PROCEDURE Set_IRQ_Off;
BEGIN
PORT[Control_Address] := $04;{Write 4 to disable IRQ}
END; {Set_IRQ_Off}
The function, In_Byte, returns the value of the variable,
Input_Byte. It is defined as:
FUNCTION In_Byte : Byte;
BEGIN
In_Byte := Input_Byte;
End; { In_Byte }
Notice that this does not read the value of the parallel port. The
function Read_Input_Bit reads the port.
The function, Out_Byte, returns the value of the variable
Output_Byte. It is defined as:
FUNCTION Out_Byte : Byte;
BEGIN
Out_Byte := Output_Byte;
End; { Out_Byte }
Notice that this does not read the parallel port. It only returns the
value of Output_Byte.
The function, Read_Input_Bit, returns the status of the specified
input line. The function is defined as:
FUNCTION Read_Input_Bit (Bit_Number:BYTE) : BYTE;
BEGIN
Input_Byte := (PORT[Status_Address] AND $F0)
OR (PORT[Control_Address] AND $0F) ;
IF ( (Input_Byte AND (1 SHL Bit_Number) ) = 0 ) THEN
Read_Input_Bit := 0
ELSE
Read_Input_Bit := 1;
END; {Read_Input_Bit}

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The parallel port’s status register stores the upper nibble (four
bits) of the input line status. Since the smallest value that can be
read from the port is a byte, some work must be done to get only the
upper nibble’s value. We read a byte from the status register and
bitwise AND it with F0h (240 decimal, 11110000 binary). This sets
the lower nibble of the byte to zero.
The parallel port’s control register stores the low nibble of the
input line status. A method, similar to the one used to get the high
nibble, extracts the low nibble. We read a byte from the control
register and bitwise AND it with 0Fh (15 decimal, 00001111 binary).
This sets the upper nibble of the byte to zero.
Now, combine both bytes into one value by bitwise ORing them
together. This value is stored in the variable Input_Byte. At this point,
the upper nibble of Input_Byte is the same as the upper nibble of the
status register and the lower nibble of Input_Byte is the same as the
lower nibble of the control register.
To return the status of a specific line, a test is done to determine
the state of the line’s corresponding bit stored in Input_Byte. To
determine the bit’s state, all other bits of Input_Byte are set to zero.
This is done by bitwise ANDing Input_Byte with the mask value of
the desired line shown in the following table.
Mask Value
LineHex Decimal Binary
001h 100000001
102h 200000010
204h 400000100
308h 800001000
410h 16 00010000
520h 32 00100000
640h 64 01000000
780h 128 10000000

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Notice that in the binary representation of the mask value, line
zero’s mask has bit number zero set, and line one’s mask has bit
number one set, etc. So, instead of retrieving the mask value from a
table, the value is calculated by shifting 00000001b (1 decimal, 01
hexadecimal) right the same number of times as the desired line
number. Once the mask value is calculated, it is bitwise ANDed with
Input_Byte. If the resulting value is non-zero, the line is ON and a
boolean TRUE is returned, otherwise a boolean FALSE is returned.
For example:
Line_Number = 3
Status Register Value = 01010101b
Control Register Value = 10101010b
01010101b (Status Register)
AND 11110000b (F0h mask)
01010000b
10101010b (Control Register)
AND 00001111b (0Fh mask)
00001010b——>OR 00001010b
01011010b
00000001b (1 decimal)
shift-right 3(Line_Number)= AND 00001000b(bit mask)
00001000b
The function, Output_Bit, returns the status of the selected
output line. The function is defined as:
FUNCTION Output_Bit (Bit_Number:BYTE) : BYTE;
BEGIN
If ( ( Output_Byte AND (1 SHL Bit_Number ) ) = 0 THEN
Output_Bit : = 0
ELSE
Output_Bit : = 1;
END; {Output_Bit}

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As with the function, In_Byte, the mask value is calculated by
shifting 00000001b right the same number of times as the desired
line number. Once the mask value is calculated, it is bitwise ANDed
with Output_Byte. If the resulting value is non-zero, the line is ON
and a boolean TRUE is returned, otherwise a boolean FALSE is
returned. Notice that this function gets its value from Output_Byte
that is set when we use the function, Set_Output_Bit. The value is
not read from the port.
For example:
Line_Number = 3
Output_Byte = 10101010b
00000001b
shift-right 3 (Line_Number) = 00001000b
AND 10101010b (Output_Byte)
00001000b
The function, Set_Output_Bit, sets the selected output line ON of
OFF. The function is defined as:
PROCEDURE Set_Output_Bit ( Bit_Number,Output:BYTE ) ;
BEGIN
Output_Byte := ( (Output_Byte AND ( ( 1 SHL Bit_Number)
XOR $FF) ) OR (Output SHL Bit_Number) ) ;
Port [Base_Address] : = Output_Byte;
END; {Set_Output_Bit}
The variable, Output_Byte, stores the status of the output lines.
When a bit in Output_Byte is set to a one, its corresponding output
line is ON, when it is set to a zero, the line is OFF. To set a bit to the
specified status, the bit must first be cleared. To do this, 00000001b
is shifted right for the desired line number and bitwise exclusive
ORed with FFh (255 decimal, 11111111 binary) to produce a mask
value. This mask value has all bits set to one except for the desired
line. It is bitwise ANDed with Output_Byte to clear the desired bit.
Now, having cleared the bit, it can be set to the specified state.
Since the state is either a zero or one, we can shift it right for the
desired line number to get another mask value. This value is then
bitwise ORed with the cleared value to obtain the new value of
Output_Byte. The final step is to write Output_Byte to the parallel
port.

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For example:
Output_Byte = 01010101b
Line_Number = 3
Status = 1
00000001b (1 decimal)
shift-right 3 (Line_Number) = 00001000b
XOR 11111111b
11110111b (mask 1)
AND 01010101b (Output_Byte)
00000001b (Status) 01010101b
shift-right 3 (Line_Number) = OR 00001000b (mask 2)
01011101b (New Output_Byte)
For an example of an application that uses these functions, look
at the source code in PPIO.PAS.

18 PPIO2899 Manual
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CONTROLLING THE PPIO USING C
The PPIO disk includes three source code files as an example
of using the PPIO with the C programming language. PPIO.C is the
main routine of the example program. PPIOFUNC.C contains the
routines for communicating with the PPIO. PPIOFUNC.H contains
the function prototypes and definitions for use by PPIOFUNC.H and
PPIO.C. In the rest of this section, we assume that you know the C
programming language. Consult your reference manuals if you need
help with the language.
PPIOFUNC.H makes some definitions used in the rest of this
section.
#define Status_Address Base_Address+1
#define Control_Address Base_Address+2
typedef enum { false=0, true=1 } boolean;
The module, PPIOFUNC.C, handles communication with the
parallel port. It defines three variables that all functions inside the
module can access. They are defined as:
unsigned char Input_Byte;
unsigned char Output_Byte;
unsigned int Base_Address;
No function outside the module can directly access these
variables. Functions outside the module call functions within the
module to modify and return the value of these variables.
The function, Set_Start(), sets the variables Base_Address and
Output_Byte. It also sets up the initial state of the parallel port. The
function is defined as:
void Set_Start(unsigned int Address, unsigned char Init)
{
Base_Address = Address;
Output_Byte = Init;
outport(Base_Address, Output_Byte);
}
This function must be the first one used, because all other functions
assume that variable, Base_Address, has been set.