🚀 go-pugleaf

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
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