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                       RS-232 INTERFACING TO THE USER PORT
                                  By Doug Reed

(Author's note: this public domain article was first printed in the
El-Shift-Oh newsletter, August, 1985. It is being released to the nets due
to the avid interest in RS-232 modems. Some changes have been made for
completeness.)

The VIC-20, C-64, and C-128 are extremely versatile machines if the user
has╕ the means and determination to use them. The user port can be used as
a parallel output port, or as an RS-232 port. Our scheme is to utilize the
RS-232.

Commodore has, rather than use a specialized communications chip as most
computers use, chosen to control all RS-232 functions using a standard
6522 Versatile Interface Adapter (VIA) for the VIC, or 6526 Complex
Interface Adapter (CIA) for the 64, as well as some complex built-in
software. This is not without limitations, mainly speed, due to the CPU
taking care of so many household tasks, such as screen updating, updating
jiffy clocks, etc. 1200 baud is about the most the C-64 can handle, and
with some programs it doesn't do this so good. Xmodem7.3 has worked about
the best for me (including Vidtex).

To use the RS-232 port, you must use a simple hardware interface between
the C-64 and the RS-232 device. As you may know, TTL uses voltage levels of
approximately O-1 volts to represent a logic O and 3-4.5 volts to represent
a logic 1. Well, RS-232 uses -3 to -15 volts as a '1' and +3 to +15 as a
'O'. And, as you may have figured out by now, ALL of the chips in the VIC
and 64 are TTL compatable, which is a not so marvelous way of saying they're
not RS-232 compatable. If this weren't enough, our friends at the big C=
were somehow able to complicate matters by inverting some of these signals.

So, our problem is to shift the computer's TTL output levels to a standard
RS-232 level. Since the VIC-1O11A RS-232 converter cartridge is set up for
outputs to a modem, this is how this circuit is set up. The original
circuit comes from the August 1982 issue of Compute!. The Compute!
circuit did not take into account the inversion of five signals, but these
are provided here with the 74O4.

This circuit works with a modem as is, and next month I will have an article
to construct a null modem adapter so that you may use a RS-232 printer
with this circuit.

I assume that if you are really interested in this little project that you
already know how to do some electronic construction. The parts will total
around twenty dollars (list), and you probably already have some in your
scrounge box that you can use. The only hard to find part is the connector
between the board and the user port. It is a 12/24 pin connector with
0.156 inch lead spacing. If you go to a surplus electronics store, look
for these numbers: '5O 24A 3O' and/or '251 12 3O 16O'. You can expect to
pay about two bucks on this one. An alternate source is Digi-Key, ph.
800-344-4539, part # C1-12, $2.90 plus $2.00 service charge. This part
will need some surgery as the circuit board will be thinner than the
connector. Try soldering all of the pins on one side of the connector,
then bend the other side's pins down to the circuit board. Now solder
these. You'll also need a DB-25 female circuit board connector of either
the solder or wire-wrap variety, depending on how you go about your
construction. This and all other parts should be available at your local
Radio Shack (tell them they're for your Commodore). I highly recommend
using sockets for the three ICs for two reasons: 1) you can do all your
wiring without damaging the ICs, and 2) if you have an IC go bad, it will
be easier to replace.

                                   Parts List
1 - 7404 (Radio Shack #276-18O2)      1 - 75188 or MC1488 (#276-2520)
1 - 75189 or MC1489 (#276-2521)       3 - 1N4001 diodes (#276-1101)
3 - 47uF, 16V capacitors (#272-1O27)  1 - 1OuF, 16V capacitor (#272-1O25)
4 - 1O kohm, 1/4w resistors(271-1335)
1 - circuit board (I recommend Radio Shack's #276-154 3 voltage board.)
1 - DB25F 25pin 'D' connector - style optional (PC mount, or wire-wrap)

I think that most of the wiring is straight-forward from the diagram.
Some items that should be mentioned:

     1) C1 is the 1O uF capacitor.
     2) C2 - C4 are the 47 uF capacitors.
     3) Ground is connected to:
          74O4  - pin 7
          75188 - pin 7
          75189 - pin 7
     4) +5V is connected to:
          74O4  - pin 14
          75189 - pin 14
     5) +VCC is connected to: 75188 - pin 14
     6) -VCC is connected to: 75188 - pin 1
     7) Connect one side of each of the resistors to +5V. Connect the other
        side individually to the 75189 at pins 2, 5, 9, and 12.

     Check with an ohmmeter that the +VCC, -VCC, +5V, and ground lines are
not shorted to each other before you plug it into your computer. If you
have made no wiring errors, it should work fine with any RS232 modem.
Good luck.



        Addendum and Clarification - 6/26/94


The schematic is not able to totally clarified, due to the limitations in
space - I have tried to recreate the schematic so that it can be easily
understood.

The listing above is the clearest way to describe the powerform connections
to this project - they should be clear enough that any hobbiest technician
should be able to follow.

It is highly recommended that sockets be purchased for each of the IC's.
You may also consider using wire-wrap, rather than solder to assemble this
project.  It makes correction of wiring errors much cleaner.

A case is optional - in fact, I would not consider building a case for this
project; if you feel you need to protect the circuitry, build it so that
your wiring is on the bottom, and then let it rest behind your computer
table in such a way that nothing will be able to fall on it.