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IBVA2

Macintosh Serial Interface.

The Macintosh uses RS-422. However, Apple built the hardware to
allow easy interoperability with RS-232 devices.

Here's my monospaced ASCII rendering of the female mini-DIN8 jack
on the back of all modern Macintoshes. Note the physical gap
between pins 4 & 5 (for orientation).

.-|-|-. 1 = HskO (handshake out)
/ \ 2 = HskI (handshake in / external clock)
/ 8 7 6 \ 3 = TxD- (transmit data minus)
! 5 4 3 ! 4 = Grd (signal ground)
\ 2 1 / 5 = RxD- (receive data minus)
\ / 6 = TxD+ (transmit data plus)
'-----' 7 = n.c. (no connection)
8 = RxD+ (receive data plus)

[The above drawing should look circular and all the equal signs]
[should be in a vertical line. If they are not, switch to a ]
[monospaced font with tab stops each 8 characters. ]

Macintosh RS-232
mini-DIN8 DB-25P DE-9P
male plug (male) (male) RS-232 usage

1 HskO ----------+- 20 DTR ------------ 4 Data Terminal Ready
'- 4 RTS ------------ 7 Request To Send
2 HskI ------------ 5 CTS ------------ 8 Clear To Send
3 TxD- ------------ 2 TxD ------------ 3 Transmit Data
4 Grd ------------ 7 SG ------------ 5 Signal Ground
5 RxD- ------------ 3 RxD ------------ 2 Received Data
6 TxD+
7 n.c.
8 RxD+ ------------ 7 SG ------------ 5 Signal Ground
shell ------------ 1 FG ------------ shell Frame Ground

The above cable supports hardware handshake (RTS-CTS flow control)
if your Mac software knows how to use it. Alternately (for software
handshake, aka X-ON/X-OFF, only), you can wire as follows:

1 HskO ------------ 20 DTR ------------ 4 Data Terminal Ready
2 HskI ------------ 6 DSR ------------ 6 Data Set Ready

The hardware handshake cable is much more usefull and also works
fine in software handshake environments.

RS232 is a single ended, voltage oriented interface. RS422 is a
differential interface. Once a common signal ground is provided,
the Mac's TxD- line becomes RS232 TxD. It meets the RS232 specs
since Apple wisely chose +5V and -5V as their differential driver
outputs.

In the reverse direction, the Mac's RxD- and RxD+ lines are looking
for differential voltage as a comparative pair. By tying the Mac's
RxD+ line to signal ground, the RxD- line becomes a single ended,
voltage-oriented input line. It can be directly driven by the RS232
RxD line.

Recall that RS232 is non-symmetrical. It has DTE (Data Terminal
Equipment) and DCE (Data Circuit-terminating Equipment) sex ends.
DTE usually uses a male DB-25 or male DE-9 (IBM PC/AT or modern PC
clone). In modern convention, DCE usually uses a female connector.
To work, one end device must be DTE and the other must be DCE.

In RS232, all signal description terms are from the DTE perspective.
So TxD (Transmitted Data) is the data stream going from the terminal
or PC into the modem.

You can plug the "Mac to RS232 DTE" cable described above directly
into a modem. If you are going to plug it into a PC serial port, you
will need a separate null modem.

Or you can redesign the cable, build in the null modem, and have a
"Mac to RS232 DCE" cable. Swap the DB25P male plug for a DB25S female
socket connector. Swap the transmit and receive data lines (pins 2 &
3 on DB25). Swap the handshake lines (Mac 1 to DB25 pin 4 or 20, not
both; and Mac 2 to DB25 pin 5, 6, 8, or all three) as appropriate for
the style of flow control you are using.

It is possible to hand-solder to the little, tightly spaced pins on
a mini-DIN8 male connector. I've done it. As a better alternative,
I recommend buying a premade Mac peripheral cable with mini-DIN8 male
plugs at BOTH ends. Then cut it in half (or as appropriate), use your
ohm meter to figure out the wire color code, then solder on a DB25 or
DE9 connector as you see fit. Much easier (and, as a side benefit,
you get a nice molded plug on the Mac mini-DIN8 end).

Lee
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Jones Computer Communications lee@frumble.claremont.edu
509 Black Hills Dr, Claremont, CA 91711 voice: 909-621-9008
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