Figure 2.
INTRODUCTION
The purpose of this article is to show you how, with the
addition of a few inexpensive components you can convert a
WC2050 modern board into a general purpose RS232 serial
interface.
In October of last year [editors note, this would be 1985], Anchor Automation auctioned off several thousand completely assembled WC2050 Modem boards as electronic surplus. Fortunately for Timex users, these boards were discovered by Timex support companies like Zebra Systems, Clifford Associates, Sunset Electronics, and RMG and are being resold to Timex users at bargain basement prices.
BLOCK DIAGRAM
Now take a look at Figure 2. It shows a block diagram of the
modem board. The address decoding crystal oscilator
frequency divider and 8251 serial I/O, and power supply
portions provide nearly all we need for an RS232 interface
As a complete modem, the input and output serial data
available at the points marked A and B, would be connected
to the modems analog circuitry and translated between tone
frequencies and voltage levels. For an RS-232 interface we
need to redirect these signals to DC voltage level translators.
RS-232 PARTS
Next take a look at Figure 3. It shows a schematic of the
recommended RS-232 adaptor circuit. The parts list is
contained in Figure 4. Notice that there are only three IC’s.
A MC1489 Line Receiver is used to buffer The incoming
signal; a MC1488 Line Driver is used to buffer the outgoing
signals; and an Intersil ICL7662 switching regulator is used to
develop the required negative supply voltage from the
modem's +9 Volt supply. An alternative to using the
switching regulator would be to just use a second 9 VDC A/C
adaptor. In our prototype we used a 9-Pin Male Atari
Joystick style connector for our RS232 output, but you can
directly wire in a cable or another choice of connector.
INSTALLATION PROCEDURE
Before installing the R$-232 adaptor board, be sure you have
a working modem board in front of you. It is not important
that the modem's analog section be working as long as the
digital portion of the board is working. Of course if the entire
modem does work, the digital portion will work.
Build up the circuit shown in figure 3. The parts are easy to find except for the Intersil regulator (see below). An optional etched and drilled PC board it available to simplify building the circuit but any small protocard such from radio shack will do fine. as the ones available
CONSTRUCTION HINTS
The RS-232 adaptor board requires 9 signals from the modem
board:
OUTPUT SIGNALS: RTS, TX, DTR
INPUT SIGNALS: RX, CTS, DSR
POWER SIGNALS: GROUND, +5 Volts, +9 Volts
Eight of these signals are available at the pin, of the 8251 serial I/O chip. The only signal not present on the chip is the +9 volt power, which can be picked up on one side ol’ the modem's DATA Light emitting Diode (LED). Our prototype b:ard (shown in Figure 1), takes advantage of this and uses a pin-and-socket arrangement to make the 8 connections at the IC. First a 28-pin IC socket is soldered right on top of the 8251 Then pins are soldered onto the RS-232 PC Board so that the board can plug in right on top of the IC. The ninth signal required is made by attaching a single wire between the RS-232 board and the +9 volt power where the LED is. The positive side of this LED is the lead closest to the corner of tbe rnodem board.
TRACE CUTS
You must make three trace cuts on the modem PC board.
This is to disconnect the three modem input-signals that go
from the modem's analog section to the 8251 chip. These
signals are replaced with those coming from your RS-232
adaptor board. The traces to cut are shown in Figure 5.
SOFTWARE & TESTING
If you want want to test out your RS-232 by itself you can do
a simple wrap around test by temporarily connecting CTS to
RTS and TX to RX. A sample TS2068 software driver for
the modem is shown in figure 6. Under this test arrangement,
whatever you send out will be wrapped around and received
back.
Figures and Diagrams
Figure 2.
Figure 3.
Figure 4.
FIGURE 4. WC2050-TO-RS232 PARTS LIST
IC1 MC1489 QUAD LINE RECEIVER
IC2 MC1488 QUAD LINE DRIVER
J1,J2 WIRE JUMPERS, 0.3 INCH
J3 WIRE JUMPER, 4.5 INCH
A1 CONNECTOR, DB-9 MALE “JOYSTICK TYPE”
A2 DUAL ROW WIRE-WRAP HEADER PINS
R1 RESISTOR, 1K 1/4W
PC89-C PRINTED CIRCUIT BOARD
C3 0.1UF 25V CER. DISC. (SUPPLY BYPASS)
----POWER SECTION VERSION ONE -
IC3 INTERSIL ICL7662 POWER CONVERTER IC
C1 10UF 16V ALUM. ELECTROLYTIC (CHARGE PUMP)
C2 10UF 16V ALUM. ELECTROLYTIC (OUTPUT)
----POWER SECTION VERSION TWO
2.5MM OR 3.5MM JACK (NEGATIVE POWER)
AC POWER ADAPTOR
The following RS-232 Adaptor items are available
from Zebra Systems:
Etched & drilled PC board with project instructions, price $5.95.
Intersil ICL7662 Power Converter IC & Specs. ,$5.0O
Complete kit for adaptor including PC Board, all 3
IC’s, Capacitors, Connectors and instruction, $19.95
Add $3.00 for Postage & Handling, quantity discounts
are available.
Figure 5.
Figure 6. Sample TS2068 BASIC Driver for wrap around test at 1200 baud. Change 78 in line 14 to 79 for 300 baud.
10 OUT 119,0: REM send Zero to clear UART chip
20 OUT 119,0
12 OUT 119,0
13 OUT 119,64: REM UART reset
14 OUT 119,78: REM 1200b, 8 bit NO parity
15 OUT 119,55: REM Enale Xmit and Recieve
20 REM Each byte sent OUT 115 will arrive on the Transmit line of the RS-232 port.
25 REM Example:
30 PRINT "This is a test."
40 LPRINT "This is a test."
50 LET a$="This is a test.": GO SUB 1000
60 PRINT "End of test."
70 LPRINT "End of test."
80 LET a$="End of Test." : GO SUB 1000
90 STOP
1000 LET i=LEN a$: IF i=0 then GO TO 1050
1010 FOR x=1 to i
1020 OUT 115, CODE a$(x):PAUSE 4
1030 PRINT CHR$ IN 115;
1040 NEXT x
1050 RETURN