Date: Wed, 15 Jul 87 22:36:06 MEZ
From: Bernhard Nebel
Subject: A 2nd RS232 port
Comment: Extracted from digest info-atari16 87-283
HOW TO GET A SECOND RS232 PORT FOR YOUR ATARI
It might happen to you that you need a second RS232 port for your
ATARI ST. Perhaps you have a printer with a serial port and a
telephon modem, as I do. The first idea might be to go to your dealer
expecting that he has an extension card for a few bucks. He hasn't,
and you get curious about whether the ATARI can be regarded as a real
computer. Anyway, you need a solution and don't want to trade your
ATARI for one of those clones and are even more upset about the idea
of exchanging the connectors any time you use another peripheral. This
can be the point when you consider looking more seriously into the
hardware detecting that there is this MIDI port you never spent any
thought about. It is just there for those people who cannot play piano
properly and compensate for this by buying a lot of electronic parts
hoping that they can improve their music by it :-). You may even have
a different opinion about them. You know, however, for sure that *you*
won't use this port. Therefore it seems to be a good idea to use it
for your own purposes.
The above describes precisely my thoughts leading to the modification
of my ATARI described below. It's just an easy, quick and cheap
hardware hack -- it doesn't meet the RS232 standards -- but it works!
I used a RS232 receiver to receive *and* send the signals, because
this was the chip I found somewhere in my old computer (a 6800 !!)
where it served the same purpose. However, note that nowadays
RS232 tranceivers are available which should be used instead.
Note that the description below applies *only* to the ATARI 520 ST and
things may be different for the 1040. Additionally, the description
can be, of course, incorrect. Therefore you should verify that the
modifications described below are correct by consulting the proper
sources. In any way, modifications according to the description below
carried out by you are on your own risk. In particular, you should be
aware that the warranty for your computer will expire after opening
1) A COMMUNICATION PORT
The first thing you have to decide is what communication port you like to
use. I chose the MIDI IN port for three reasons:
- I won't use MIDI after the modification.
- Connectors for this communication port are easily available and cheap.
- Three of the five pins aren't used.
The drawback of this solution is that you might easily confuse the
MIDI IN port with the MIDI OUT port when plugging in your connector
which can result in blowing up your peripheral.
I use the following pin assignment:
Pin 1 = RS232 OUT
Pin 2 = GND
Pin 3 = RS232 IN
Note that pin 4 & 5 are already used.
Pin layout of the communication port seen from inside:
NOTE: In a lot of sources including my ATARI manual I found a different pin
layout which, however, seems to be wrong.
SOLDERING: Pin 2 of MIDI IN has to be connected with pin 2 of MIDI OUT.
NOTE: These pins are only accessible from the bottom side of the board.
2) REWIRING THE MIDI ACIA
The wiring of the MIDI ACIA has to be changed. We have to rewire the
Tx-Data and Rx-Data pins, because we want to use a different
driver. Additionally, the Tx-Clk and Rx-Clk must be connected to a
different clock, because a fixed baud rate of 7812.5 is probably not
appropriate. In the ATARI 520 ST the 6850 sitting next to the sound
chip -- the YM 2149 -- is the right chip.
CUTTING: Pin 2 (Rx Data), pin 3 (Rx Clk), pin 4 (Tx Clk) and pin 6 (Tx Data).
3) A NEW BAUD RATE GENERATOR
Timer A of the MFP 68091 is not used and can be employed as a baud
rate generator for our ACIA. A tiny assembler program setting the
correct baud rate is given below. I usually run this program at boot
time from my AUTO folder. A more sophisticated solution could be to
write an accessory to set the baud rate.
SOLDERING: Connect pin 3 (Rx Clk) and pin 4 (Tx Clk) of the MIDI ACIA with
pin 13 (Timer A Out) of the MFP.
4) A RS232 DRIVER
As I said above, I used a RS232 receiver (SN 75189) for both
transmitting and receiving. That means that the outgoing signal does
*not* conform to the RS232 voltage level specification. However, this
cheap kludge always worked.
SOLDERING: - Mount a 75189 (RS232 receiver) on a small circuit board.
- Connect pin 14 (Vcc) of the 75189 with Vcc, e.g., with pin 14
of a 7404.
- Connect pin 7 (Gnd) of the 75189 with Gnd, e.g., with pin 7
of a 7404.
- Connect pin 13 (4A) of the 75189 with pin 3 (RS232 In)
of the MIDI IN port .
- Connect pin 11 (4Y) of the 75189 with pin 2 (Rx Data) of
the MIDI ACIA .
- Connect a resistor (1K Ohm, probably 10K works as well) with
pin 11 (4Y) of the 75189 and Vcc.
- Connect pin 3 (1Y) of the 75189 with pin 1 (RS232 Out) of
the MIDI IN port.
- Connect a resistor (1K Ohm) with pin 3 (1Y) of the 75189
- Connect pin 1 (1A) of the 75189 with pin 6 (Tx Data) of
the MIDI ACIA.
- Finally, you should install the circuit board somewhere on
These are all hardware modifications in order to get a second RS232
port. There is, however, some software necessary before you actually
can use the new port. First, the baud rate has to be programmed into
timer A. A small assembly program is given below. Second, the ACIA
registers have to be set up to the proper values. At boot time they
get set to 8-Bit, no-parity. Third, you have to write programs which
actually use the port. I, for my part, changed GEM-KERMIT and my
TERMINAL accessory in order to work with the MIDI port.
SETTING THE BAUD RATE:
* Initialize Timer A as baud rate generator for MIDI ACIA
* Bernhard Nebel 14.12.86
* valid values for data and control are:
* baud control data
* 19200 1 1
* 9600 1 2
* 4800 1 4
* 3600 1 5
* 2400 1 8
* 2000 1 10
* 1800 1 11
* 1200 1 16
* 600 1 32
* 300 1 64
* 200 1 96
* 150 1 128
* 134 1 143
* 110 1 175
* 75 2 64
* 50 2 96
gemdos equ 1
bios equ 13
xbios equ 14
Pterm0 equ $00
Cconws equ $09
rsconf equ 15
setprt equ 33
xbtimer equ 31
modem .equ *
move.l #-1,-(sp) vector
move.w #64,-(sp) data
move.w #1,-(sp) control
move.w #0,-(sp) timer = A
mess dc.b "MIDI port initialized (300 baud, 8-Bit, Nosync).",$0D,$0A,0
Have fun - Bernhard
} MAILER STANFORD 7/15/87
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