trewitt@cascade.Stanford.EDU (Glenn Trewitt) (02/10/89)
Someone asked for information about the pinouts on the miniature DIN-8
serial connectors on Mac II's and SE's. I wrote this up so I wouldn't
forget it a few weeks ago when I was building a Mac<->dot-matrix
printer cable.
Pinouts and cables associated with the modem and printer ports:
>From the Macintosh II Owner's Guide:
Viewing the pins on the end of a connector on a cable:
6 7 8
3 4 5
1 2
Note that pin 4 is actually offset a bit to the left of center. The
Apple manual shows the pinout on the plug on the back of the Mac and is
therefore reversed from this.
The signal assignments are:
1 HSKo Handshake out
2 HSKi Handshake in / external clock
3 TxD- Transmit data -
4 GND Signal ground
5 RxD- Receive data -
6 TxD+ Transmit data +
7 GPi General purpose input
8 RxD+ Receive data +
On the modem port, GPi can be set to be a second external clock.
Note that by making a mirror-image of this pinout, exchanging
(6,8), (3,5), and (1,2) you get a null-modem. (Except for pin 7.)
*** This type of mistake is easy to make, so watch out! ***
The standard Mac Port to modem cable is as follows:
(Mini-8) Mac RS-232 (DB25P)
Port DTE
-----------------------------------------
1 HSKo DTR 20
2 HSKi CTS,CD 5,8
3 TxD- TxD 2
4 GND Sgnd 7
5 RxD- RxD 3
6 TxD+ n/c
7 GPi n/c
8 RxD+ Sgnd 7
Pin RxD+ is grounded to make the differential RS-422 signals
compatible with RS-232 signals. This cable produces a Data
Terminal Equipment (DTE) interface which is intended to be
connected to a Data Communications Equipment (DCE) interface.
DTE interfaces are usually on a DB25P connector (with pins).
DCE interfaces are usually on a DB25S connector (with sockets).
[ Note about RS232 signal names: The signals named TxD (transmit data)
and RxD (receive data) are from the point of view of a Data Terminal
Equipment (DTE) device. For a DTE, data will come out of TxD. For a
DCE, however, data will go into TxD. It's very confusing. If in doubt
about which direction data should be going, get out a voltmeter and
measure from pin 7 (ground) to the pin in question. You will measure
a voltage larger than +3 V or less than -3 V if a signal is being
driven on the pin. If the voltage is near ground or high impedence, it
should be receiving a signal. Better than that is a breakout box,
preferably one with two-color LEDs that can indicate >+3V, <-3V, or
floating. ]
To make a null-modem version of this cable (e.g. for connecting
a printer), just exchange the input and output signals:
(Mini-8) Mac RS-232 (DB25S)
Port DCE
-----------------------------------------
1 HSKo CTS,CD 5,8
2 HSKi DTR 20
3 TxD- RxD 3
4 GND Sgnd 7
5 RxD- TxD 2
6 TxD+ n/c
7 GPi n/c
8 RxD+ Sgnd 7
WARNING:
I have seen DIN-8 to DIN-8 cables in two varieties. One
connects pin 1 to pin 1, 2 to 2, etc., and the other that
exchanges (1,2), (3,5), and (6,8), making a null modem. This
is a deplorable situation, but itUs Apple's fault for not setting
a standard. I prefer to use the straight-through cables, since
other devices, such as A-B printer switches, use the same scheme.
Having a null modem appear out of nowhere is very confusing.
--
Glenn Trewitt, Center for Integrated Systems, Stanford University
{ucbvax,decvax}!decwrl!miasma.stanford.edu!trewitt USENet
trewitt@miasma.stanford.edu Interneturlichs@smurf.ira.uka.de (Matthias Urlichs) (02/14/89)
In comp.sys.mac trewitt@cascade.ARPA (Glenn Trewitt) writes:
<
< The signal assignments are:
< 1 HSKo Handshake out
< 2 HSKi Handshake in / external clock
< 3 TxD- Transmit data -
< 4 GND Signal ground
< 5 RxD- Receive data -
< 6 TxD+ Transmit data +
< 7 GPi General purpose input
< 8 RxD+ Receive data +
< On the modem port, GPi can be set to be a second external clock.
<
< WARNING:
< I have seen DIN-8 to DIN-8 cables in two varieties. One
< connects pin 1 to pin 1, 2 to 2, etc., and the other that
< exchanges (1,2), (3,5), and (6,8), making a null modem. This
< is a deplorable situation, but itUs Apple's fault for not setting
< a standard. I prefer to use the straight-through cables, since
< other devices, such as A-B printer switches, use the same scheme.
< Having a null modem appear out of nowhere is very confusing.
I disagree.
There _is_ a standard: Data go from pin 3/6 of the female into 3/6 of the
male side of the connection. Data go from pin 5/8 of the male into pin
5/8 of the female side. That's it.
And if you adhere to that convention on every DIN-8 connection there is,
you'll never have trouble no matter what you connect to whatever else.
If anyone manufacturers swap this, I'd be very angry with them if I were Apple.
On the DB-25 side, things are different. There one really has to look
carefully at the in/out side; I've seen lots of devices which act as DTE but
feature a DCE connector (or vice versa).
Or, since you can't hurt RS-232 signals no matter how wrong you connect them
to each other (well, almost), simply try it one way, and if that doesn't work,
try it the other way.
If that doesn't work either, set the correct baud rate and try again. :-)
< --
< Glenn Trewitt, Center for Integrated Systems, Stanford University
< {ucbvax,decvax}!decwrl!miasma.stanford.edu!trewitt USENet
< trewitt@miasma.stanford.edu Internet
Disclaimer:
If this is all wrong, it's not my fault: You were reading this upside down.
--
Matthias Urlichs -- Humboldtstrasse 7 -- 7500 Karlsruhe 1 -- FRG
urlichs@smurf.ira.uka.de -- ++49+721-621127@PTT