Radio Shack 26-1145 User manual

TRS-80
RS-232-C
Interface
Catalog Number 26-1 145
—"\
Radio /hack
TRS-80
MICRO
COMPUTER
SYSTEM
^J
INTRODUCTION and Discussion of Digital Signals
INSTALLING and Checkout
THEORY OF OPERATION
TERMinal Program and Listing
DECWRITER Application Program
Required Equipment:
TRS-80, Level II with 16K RAM
Expansion Interface with
RS-232-C Installed
CUSTOM MANUFACTURED IN THE USA FOR RADIO SHACK J^ ADIVISION OF TANDY CORPORATION

12 V+12 VGND
RS-232-C SCHEMATIC DIAGRAM

To Our Customers
What is an Interface?
It's ageneralized means of communication between your TRS-80 and
some external device, providing the necessary conventions regarding
data-identification, transmission rates, send-receive sequences,
error-checking techniques, etc. However, an Interface does not
provide the programming necessary to use any particular TRS-80/
external device system.
For example, having the Interface installed does not automatically
enable you to send BASIC programs from one TRS-80 to another; to
output to aline printer via the Interface; etc. Such applications require
"driver programs" which must be custom-designed for the equipment
you intend to use.
In this manual you'll find listings for two such "driver programs". One
allows your TRS-80 to function as aterminal, the other allows your
TRS-80 to output to apopular serial interface line printer, the
DECWRITER.
The Radio Shack RS-232-C Interface is designed to meet the EIA
standards. However, we cannot guarantee that it will work with all
so-called "RS-232-C compatible" devices. Nor do we commit
ourselves to provide engineering and programming support for such
applications, or other special custom-use situations.
We do, however, guarantee that our Interface will function correctly
with all our own RS-232-C equipment.
©Copyright 1978, Radio Shack, ADivision of Tandy Corporation,
Fort Worth, TX. 76102 U.S.A.

Introduction
The term RS-232-C refers to aspecific EIA(Electronics Industries
Association) standard which defines awidely accepted method for
interfacing data terminal equipment with data communications
equipment. The RS-232-C Interface is by far the most universally
used standard for interfacing data processing equipment. Most video
terminals, modems, card readers, line printers, mini-microcomputers,
etc., utilize the RS-232-C standard for data interchange between
devices.
The addition of the RS-232-C to the TRS-80 Expansion Interface
opens up awhole new world of compatibility. The RS-232-C
Interface is designed to mount inside the TRS-80 Expansion
Interface. For example, the external equipment could be aserial
line printer, aRadio Shack Telephone Interface, or avideo terminal,
etc. The point is this, now you can interface with any equipment
which is RS-232-C compatible.
Amost useful application program of the RS-232-C Interface will
permit you to connect your TRS-80 via aTelephone Interface to
time sharing computer system. This TERM program is supplied
(on cassette tape) with the RS-232-C Interface.
Many TRS-80 owners would like to be able to use their serial printers
with the LPRINT command in LEVEL II BASIC. With the RS-232-C
Interface installed in the Expansion Interface, you can do that. A
programming example and listing for this application is provided in
this Manual.

Transmission of Digital Data
The transfer of digital data over relatively long distances is generally
accomplished by sending data in serial form using asingle twisted
wire pair to connect the transmitting and receiving devices. One of
two general transmission techniques is commonly used, asynchronous
or synchronous. The transmission technique used in the Radio Shack
system is asynchronous-bit-seriai. Since we don't use the synchronous
technique, we'll not mention it again. Asynchronous transmission
does not require asynchronizing clock to be transmitted with the
data and, the characters need not be contiguous. This means that
gaps of varying lengths may be present between transmission of
individual characters.
The bits which comprise adata character (generally from five to eight
bits in length) and synchronizing start and stop elements are added
to each character as shown in Figure 1.The start element is asingle
logic zero (0) data bit that is added to the front of each character.
The stop element is alogic one (1) that is assed to the end of each
character. The stop element is maintained until the start element of
the next character is transmitted. There is no upper limit to the
length of the stop element. However, there is alower limit that
depends on system characteristics. Typical lower limits are 1.0, 1.42
or 2.0 data-bit intervals (although most modem systems use 1.0 or
2.0 stop bits). The negative-going transition of the start element
defines the location of the data bits in the character being
transmitted. Aclock source at the receiver is reset by this
transition and is used to locate the center of each data bit.
There are several good reasons for using the asynchronous data
transmission system. Aclock signal does not need to be transmitted
with the data, thus, equipment is simpler. Also, the characters
don't need to be sent all at one time; they can be transmitted as
they become available. This is particularly useful when transmitting
data from manual-entry input devices (e.g. akeyboard). The major
disadvantage of asynchronous transmission is that it requires a
significant portion of the communications bandwidth for start
and stop elements.
STOP ELEMENT STOP ELEMENT
,1/
^T^zzO^— L-^D===rrL—
START ELEMENT ONE 8BIT CHARACTER START ELEMENT ONE 8BIT CHARACTER
(11001000) (00100000)
FIGURE 1. ASYNCHRONOUS DATA

The rate at which asynchronous data is transmitted is defined as
the baud rate. Baud rate is the inverse of the time duration of the
shortest signal element. Normally, this is one data bit interval.
The baud rate is equal to the bit rate if one stop bit is used; but for
systems which use more than one stop bit, the baud rate does not
equal the bit rate.
Asynchronous transmission over asimple twisted wire pair can be
accomplished at moderately high baud rates (10K baud or higher,
depending on the length of wire, type of drivers, etc.). Transmission
over the telephone network is generally limited to approximately
2K baud and amodem is required to convert the data pulses to
tones that can be transmitted through the telephone network.
Radio Shack's Telephone Interface is the ideal modem for this
RS-232-C Interface.
Signal Conventions
The E.I. A. RS-232-C electrical specification defines voltage levels
and corresponding logic conventions associated with data and control
information transmitted between equipment. For data interchange,
the signal is considered in the marking condition when the voltage
measured at the interface point is more negative than —3 Volts(with
respect to signal ground). The signal is considered in the spacing
condition when the voltage is more positive than +3 Volts(with
respect to signal ground). The marking condition corresponds
to alogic one (1)and the space condition corresponds to alogic
zero (0). For timing and control interchange circuits, the function
is considered to be"on"when the voltage on the interchange circuit
is more positive than +3 Volts(with respect to signal ground);
and is considered to be"off"when the voltage is more negative than
—3 Volts(with respect to signal ground). The "on"condition
corresponds to alogic zero (0) and the"off"condition corresponds
to alogic one (1). Table 1summarizes this information.
NOTATION INTERCHANGE VOLTAGE
Negative Positive
Binary State
Signal Condition
Function
1
Marking
OFF Spacing
ON
TABLE 1. ON/OFF CONDITION

Pin Designations and Signal Descriptions
The mechanical specification of the RS-232-C requires a25-pin
connector (called aDB-25). Table 2specifies the pin assignments
and signal descriptions as they apply to the Radio Shack RS-232-C
Interface.
Pin Number Abbreviation Description
1PGND Protective Ground
2TD Transmit Data
3RD Receive Data
4RTS Request-to-Send
5CTS Clear-to-Send
6DSR Data Set Ready
7SGND Signal Ground
8CD Carrier Detect
20 DTR Data Terminal Ready
22 Rl |Ring Indicator
TABLE 2. PEN DESIGNATIONS AND SIGNAL DESCRIPTION
Protective Ground: This must be bonded to the chassis or equipment
frame. It may also be connected to Signal Ground.
Transmit Data: Direction-to data communication equipment. Signals
on this circuit are generated by the data terminal equipment for
transmission of data to remote equipment. This signal should be
held in the marking condition during intervals between characters
and at all times when no data is being transmitted.
Received Data: Direction-from data communication equipment.
Signals on this circuit are received from remote equipment which
transmits data to the terminal. This signal should be held in the
marking condition during intervals between characters and at all
times when no data is being received.
Request-to-send: Direction-to data communication equipment.
This signal is required by the terminal equipment to control the
direction of data transmission by the data communication equipment.
On one-way or duplex channels, the "on" condition maintains the
data communication equipment in the transmit mode. The "off"
condition maintains the data communication equipment in the
non-transmit mode.
On ahalf duplex channel, the "on" condition maintains the data
communication equipment in the transmit mode and inhibits the
receive mode. The "off" condition maintains the data communication
equipment in the receive mode.

Clear-to-Send: Direction-from data communication equipment. This
signal is generated by the data communication equipment and
indicates whether or not the data set (modem) is ready to transmit
data. The "on" condition is an indication to the data terminal
equipment that the data set can accept data on the Transmit Data
circuit. The "off condition is an indication to the data terminal
equipment that it should not transfer data to the data set.
Data Set Ready: Direction-from data communication equipment.
This signal indicates the status of the local data set to the data
terminal equipment. The "on" condition of this circuit indicates
that the data communication equipment is not in test, talk or dial
mode and has completed any timing functions required to complete
call establishment (answer tone, etc.). The "off condition will
appear at all other times and indicates that the data terminal should
accept only Ring Indicator signals and ignore all other signals
(appearing on any other interchange circuit).
Data Terminal Ready: Direction-to data communication equipment.
This signal is used to control the switching of the data communi-
cation equipment to the communications channel. The "on"
condition indicates to data communication equipment that it
should connect to the communications channel and that it should
maintain the connection as long as the "on" condition is present.
The "off condition causes the data communication equipment
to be removed from the communications channel following any
in-process transmission of data.
Ring Indicator: Direction-from communication equipment. The
"on" condition of the circuit indicates that aringing signal is being
received on the communications channel. In general, this means
that the data set is being polled and that data communication is
desired by the polling device. The "off condition is held during
the off segment of the ringing cycle (between actual rings) and at all
other times when ringing is not being received.
Carrier Detect (Receive Line Signal Detector): Direction-from data
communication equipment. When "on", this signal indicates that
the data set is receiving acarrier from aremote data set via the
communications channel. The "off condition indicates that no
carrier is being received or that the signal quality is unsuitable for
data demodulation.

Installing The RS-232-C
Note: Some of the first TRS-80 Expansion Interface units were
manufactured with a42-pin connector (required lor the
RS-232-C Interface). If your Expansion Inierface does not
incorporate this connector, you must return it for proper
installation of this connector and Hie RS-232-C. Any at tempi
to install this connector on your own will aulomalically void
all warranly.
If your Expansion Interface incorporates the necessary 42-pin con-
nector, install the RS-232-C Interface as follows:
1
.
Position the TRS-80 Expansion Interface as shown in Figure 2and
remove the Expansion Door.
2. Remove two machine screws and washers from the 42-pin con-
nector.
3. Position the RS-232-C Interface as shown. Line up the 42-pin
connector mounting holes with the screw holds in the RS-232-C
Interface Printed Circuit Board. Fasten with two washers and
screws.
4. Position the RS-232-C as shown below (front view). Remove cover
from the center of the Expansion Interface. Carefully press the
40-pin connector of the cable supplied with the RS-232-C over the
edge connector which will be exposed; the flat cable must extend
downward. Connect the other end of the cable (DB-25) to the
desired Modem or Line Printer, etc.
EXPANSION BOARD
PORT CARD EDGE-J1
EXPANSION INTERFACE
FRONT VIEW OF EXPANSION INTERFACE

EXPANSION DOOR
MACHINE SCREWS *
WASHER
VIEW OF FLOOR
WITH 42 CONTACT
CONNECTOR INSTALLED
TRS-80 EXPANSION INTERFACE
FIGURE 2. RS-232-C INSTALLATION

Checkout
1.Set the RS-232-C Sense Switches (listed in Table 3on Page 13) for
the desired Baud Rate, Parity, etc., as shown in the following
example;
Switch S8 S7 S6 S5 S4 S3 S2 SI
Baud Rate =300 CLOSED CLOSED OPEN
One stop bit and
parity enabled CLOSED CLOSED
Word length =7bits
of data plus one
parity bit
CLOSED OPEN
Even parity OPEN
Also, set TERM/COMM Switch to TERM (assuming you are
connecting to aModem).
2. Load the cassette tape via the Level 11 BASIC "System" command
using "TERM" as the label. Type aslash symbol (/) and press
13^13;! to activate the program (which is located at 5000-Hex).
3. Correct activation of the TERM tape will be indicated by a clear-
ing of the Video Display Screen; the cursor will be at the upper
left corner of the Video Display screen.
Note: If you connect together pins 2and 3of the 25-pin connector
tCDB-25) on the RS-232-C cable] ,the keyboard output will be
"echoed" back to the Video Display Screen, indicating correct
operation of the Interface and TERM program.
4. Replace the Expansion Door.

COMM/TERM SWITCH
TERM COMM
W^?H?^^3!WW?
RS-232-C INTERFACE BOARD
\
DIP SWITCHES S1-S8
IH§1
FIGURE 3. RS-232-C SWITCHES

Theory of Operation
The Radio Shack RS-232 Interface is aversatile programmable
interface which will permit you to interface with almost any
equipment which is RS-232-C compatible. There are essentially two
different categories of RS-232-C equipment, namely data terminal
equipment and data-comm (data communications) equipment.
The Radio Shack RS-232-C Interface is configured from the data
terminal's point of view, for interfacing with data-comm equipment.
You can control the configuration of the Interface two ways, by
sense switch selection or by direct selection using software control.
Universal Asynchronous Receiver-Transmitter
Figure 4is ablock diagram of the RS-232-C Interface. The heart of.
the Interface is the UART (Universal Asynchronous Receiver-
Transmitter). This MOS LSI device (TR1602A) contains most of the
hardware needed to receive and transmit serial data. The UART is an
industry standard, general purpose, programmable device for
interfacing an asynchronous serial data channel to the parallel data
structure of amicroprocessor. The transmitter section of the
UART converts parallel data from the microprocessor into a
serial word which contains the data along with start, parity and
stop bits. The receiver section converts aserial word (with start,
data, parity and stop bits) into parallel data, and verifies proper data
transmission by checking parity and receipt of avalid stop bit.
The operation of the UART can be programmed as follows:
The word length can be 5, 6, 7or 8bits; parity generation and
checking can be inhibited, the parity may be even or odd and
the number of stop bits may be either one or two, with
one-and-one-half when transmitting a5-bit code.
The UART contains several internal registers which will permit you
to configure the serial data channel, monitor the UART status, load
data to be transmitted, and read data that is received on the serial
data channel. Later we will discuss the UART registers in more
detail.
10

XTAL
CPU BUS
V^i ii yS
BRG
±_JL RS-232-C 25 PIN SERIAL
CONNECTOR
UART
cMODEM
STATUS
REGISTER
LEVEL
SHIFTERS
HANDSHAKE
LATCH
TTL TO EIA
LEVEL
SHIFTERS
^.
s.
EIA TO TTL ^~
LEVEL V
SHIFTERS ^_
+5VIN DC/DC
CONVERTER 1
VOLTAGE
DIVIDER
+12 VOUT
—12 VOUT
-5 VOUT
FIGURE 4. RS-232-C BLOCK DIAGRAM
11

Baud Rate Generator
Directly above the UART in Figure 4is ablock labeled BRG (Baud
Rate Generator). This section of the interface outputs two clock
signals essential to the proper operation of the UART. The two
clock signals (TRANSMIT FREQ &RECEIVE FREQ) determine the
transmit and receive baud rates of the serial channel. A5.0688 MHz
crystal is used as atiming reference for the BRG. The BRG can be
programmed to divide this reference frequency down to the frequen-
cies needed by the UART. For both transmit and receive operations,
the UART requires aclock at 16 times the baud rate desired. The
BRG transmit and receive frequency outputs can be independently
programmed by loading the appropriate constants into its internal
registers. This feature of the BRG makes it possible to transmit and
receive data at different baud rates.
Sense Switches
Directly below the UART in Figure 4is ablock labeled SENSE
SWITCHES. This section is made up of eight single-pole single-throw
switches (S1-S8) which you use to select the baud rate, bits per
word, parity (odd or even), stop bits (one, one-and-one-half, or
two), and enable or disable parity generation. If you want, you can
ignore these switches and under software control, directly configure
the Interface for non-standard baud rates, different receive and
transmit frequencies, etc. Table 3summarizes the operation of the
sense switches.
Handshake Latch
The Radio Shack RS-232-C Interface can control (with proper soft-
ware) the logic state of two of the control signals on the Interface
(Request-To-Send and Data Terminal Ready). This is accomplished
by loading the Handshake Latch with the appropriate bit pattern.
Four of the interface signals can be sensed by the CPU (Clear-To-
Send. Data Set Ready, Carrier Detect, and Ring Indicator) by
reading the Modem Status Register, shown in Figure 4. The
Handshake Latch and Modem Status Register, with the appropriate
software, allows ahandshake dialog to occur between the CPU
and the equipment connected to the RS-232-C Interface.
12

BAUD RATE S6 S7 S8
110 Closed Closed Closed
150 Closed Closed Open
300 Open Closed Closed
600 Open Closed Open
1200 Closed Open Closed
2400 Closed Open Open
4800 Open Open Closed
9600 Open Open Open
PARITY ENABLE S4 STOP BITS S5
Parity Enable
Parity Disabled Closed
Open One Stop Bit
Two Stop Bits
Closed
Open
WORD LENGTH S2 S3
(Excluding
parity bit)
5Bit Word Closed Closed
6Bit Word Closed Open
7Bit Word Open Closed
8Bit Word Open Open
PARITY SELECT
Odd Parity
Even Parity
SI
Closed
Open
TABLE 3. OPERATION OF SENSE SWITCHES
Logic Conventions
The logic internal to the RS-232-C operates with TTL logic levels
(3.5V or more =Logic 1,and 0.8V or less =Logic 0). The logic
convention used for interfacing two RS-232-C devices is ElAlevels
(—3V or less =Logic one, +3V or more =Logic 0). The logic
levels must therefore be converted from one convention to the other
when interfacing two devices. The blocks in Figure 4labeled
EIA To TTL and TTL To E1A provide this level conversion.
13

Port Addresses
The Radio Shack RS-232-C Interface is an 1/O-mapped device which
uses four port addresses (E8H. E9H. EAH and EBH) for communica-
tion with the CPU. I/O-mapped devices in aZ-80 system use the
lower address bits (A0-A7) in conjunction with the IN* and OUT*
signals to address the desired ports. Figure 5shows the timing
associated with this addressing scheme.
SYSTEM CLOCK
A0-A7
IN*
DATA BUS
OUT*
DATA BUS
T2 ITW
PORTADRESS
OUT
J-
-GHh
1.8 Mhz
READ CYCLE
WRITE CYCLE
FIGURE 5. I/O TIMING DIAGRAM
Decode Logic
The block labeled DECODE LOGIC in Figure 4provides eight
low-going strobes that are used by the RS-232-C. These strobes
allow the CPU to select any of the registers and latches in the
RS-232-C Interface for an input or output operation. Table 4
summarizes the I/O port address allocation as implemented in the
RS-232-C Interface and the function performed.
ADDRESS OUT (I/O PORT WRITE) IN (I/O PORT READ)
E8H Master Reset (Any Data) Modem Status Register
E9H Baud Rate Select Configuration Sense
Switches
EAH UART Control Register
and Handshake Latch UART Status Register
EBH Transmit Data Register Received Data Register
TABLE 4. I/O MAPPED MEMORY ALLOCATION
14

Each of the above registers performs aspecific function related to
the RS-232-C Interface. An output to the Master Reset Port (E8H)
resets the UART into aknown state. This operation should be
performed once before attempting to load or read any of the
UART's registers. This operation is performed by executing the
following Z80 instruction:
OUT (0E8H), A;OUTPUT TO MASTER RESET LOCATION
The contents of Aare not important and do not relate in any way to
the result of the operation. An input operation from this port
address (E8H) causes the contents of the Modem Status Register
to be loaded into aregister of the CPU. This is accomplished by
executing the following Z80 instruction:
IN A, (0E8H) ;READ MODEM STATUS REGISTER
Execution of this instruction loads the contents of the Modem
Status Register into A. The information now in Arepresents
information relevant to the status of external equipment connected
to the Interface.
Any output operation to Port E9H loads the Baud Rate Generator
with aconstant which determines the receive and transmit baud
rates of the serial channel. This is accomplished by executing the
following Z80 instructions:
LD A, 22H ;LOAD CONSTANT IN A
OUT (0E9H), H;LOAD BRG WITH CONSTANT
Execution of this sequence results in the serial channel being con-
figured to transmit and receive data at 110 baud. Table 5
summarizes the relationship between the constants loaded into the
BRG and the resultant baud rates selected. The high-order nibble
(D7-D4) loaded into the BRG determines the transmit baud rate,
while the low-order nibble (D3-D0) determines the receive baud rate
of the serial channel.
15

Summary of BRG Programming
Radio Shack software supports the baud rates which have ayes in
the Switch Selectable column of Table 5. You can program
non-standard baud rates by loading the BRG with the appropriate
constants. An input operation from port E9H loads aregister in the
CPU with the bit pattern programmed by the Configuration Sense
Switches. As we noted before, the Sense Switch Block in Figure 4
consists of eight single-pole single-throw switches. When aswitch
is open, it pulls up (+5V) the input of an octal tri-state buffer;
when aswitch is closed, it pulls an input low.
NIBBLE TRANSMIT OR SWITCH
LOADED RECEIVE BAUD RATE lOx CLOCK FREQ. %ERROR SELECTABLE ?
OH 50 0.8 kHz No
1H 75 1.2 kHz No
2H 110 1.76 kHz Yes
3H 134.5 2.1523 kHz 0.016 No
4H 150 2.4 kHz Yes
5H 130 4.8 kHz Yes
6H 600 9.6 kHz Yes
7H 1200 19.2 kHz Yes
8H 1800 28.8 kHz No
9H 2000 32.081 kHz 0.253 No
AH 2400 38.4 kHz Yes
BH 3600 57.6 kHz No.
CH 4800 76.8 kHz Yes
DH 7200 115.2 kHz No
EH 9600 113.6 kHz Yes
FH 19,200 316.8 kHz 3.125 No
TABLE 5. SUMMARY OF BRG PROGRAMMING
The decode logic, in response to an input operation from port E9H,
provides alow-going strobe which is connected to the enable input
of the tri-state buffer. This strobe, when low, allows the buffer to
present the logic state of its inputs to the data bus for loading
into the CPU. This operation is performed by executing the
following instructions:
IN A, (0E9H) ;LOAD SWITCH SELECTIONS
After execution of this instruction, the Aregister on the CPU will
contain the bit pattern which corresponds to the switch settings.
This information must now be interpreted by the software and the
proper sequence of commands issued by the CPU to configure
the Interface.
16

An output to the Port Address EAH loads the UART Control
Register and Handshake Latch. These two functions are separate
and distinct, with each using different bits of the register for
control. The upper five bits (D7 -D3) of this Port Address
determine the word length, stop bits, and parity convention of
the serial channel. The lower three bits (D2 -DO) are latched
control outputs, one of which disables Transmit Data when low,
and the remaining two control Data Terminal Ready and
Request-To-Send. Acertain degree of caution should be used
when outputting to this Port Address to avoid an unwanted
interaction. Table 6summarizes the registers and bit allocations
of each for the RS-232-C Interface.
DATA
BIT MODEM STATUS
REGISTER CONFIGURATION
SENSE SWITCHES
UART CONTROL
REGISTER AND
HANDSHAKE LATCH UART STATUS
REGISTER
D7 Clear to send
Pin 5DB-25 Even Parity Enable
1=even, =odd Even Parity Enable
1=even, =odd Data Received
1=Condition true
D6 Data Set Ready
Pin 6DB-25 Word length Select 1Word length Select 1Transmitter Holding Register
Empty 1=Condition true
D5 Carrier Detect
Pin 8DB-25 Word length Select 2Word length Select 2Overrun Error
1=Condition true
D4 Ring Indicator
Pin 22 DB-25 Stop Bit Select
1=2bits, =1bit
Stop Bit Select
1=2bits, =1bit
Framing Error
1=Condition true
D3 Unused Parity Inhibit
1disables parity
Parity Inhibit
1disables parity
Parity Error
1=Condition true
D2 Unused Baud Rate 3Break
Disables Transmit data
Unused
Dl Receiver Input UART P20 Baud Rate 1Request to send
Pin 4DB-25 Unused
DO Unused Baud Rate 2Data Terminal Ready
Pin 20 DB-25 Unused
IN 0E8H IN 0E9H OUT 0EAH IN 0EAH
TABLE 6. BIT ALLOCATIONS FOR REGISTERS AND LATCHES
17