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Stand: 2002-06-10
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Date: Tue, 5 Dec 89 16:36:44 EST
From: zben@umd5.umd.edu (Ben Cranston)
Subject: Serial port document (long)
MIT EE claims it is benign but confusing. Caveat Solderor...
This document contains notes on the Macintosh serial port and its use, with concentration on hardware interface issues.
*** DANGER WARNING WILL ROBINSON!!! ***
The DB-25 on the back of a Macintosh is NOT a serial port! It is a SCSI
parallel port. Any attempt to use this connector as a serial port will NOT
function correctly and may cause damage to the Macintosh and/or the equipment
being connected.
The two serial ports of a Macintosh are mini-Din-8 connectors which are labeled with a telephone (the "modem port") and a printer ("printer port"). This is the pinout of the serial connectors. We are looking at the back of the Macintosh (or alternatively at the BACK of a male plug):
Macintosh Plus Serial Connectors (Mini-DIN-8)
/------###------\ 1 HSKo Output Handshake
/ ### \ (Zilog 8530 DTR pin)
/ \ 2 HSKi / Clock Input Handshake or extern clk
/ [|] [|] [|] \ (Depending on 8530 mode)
/ 8 7 6 \ 3 TxD- Transmit data (minus)
| |
| | 4 Ground Signal ground
| === === === |
| 5 4 3 | 5 RxD- Receive data (minus)
| |
| | 6 TxD+ Transmit data (plus)
\----+ === === +----/
\###| 2 1 |###/ 7 N/C (no connection)
\##| |##/
\| |/ 8 RxD+ Receive data (plus)
\------###------/
###
Note this is a RS-422 interface so the signals come in a balanced pair, a positive (plus) and a negative (minus), for each data signal. As we shall see below, there is an easy method for matching this to RS-232.
We buy the mini-Din-8 connectors at our local electronics surplus store. They cost just under four dollars each, but are not quite as nice as the Apple molded plugs (for example, they don't have the nice orienting-D shape). We are now carefully removing the pins from the connector, soldering the wires to the pin, then replacing the pin in the connector body. We fan out the end of the (stranded) wire into a little umbrella around the head of the pin, then we solder all around. A "third hand" reduces this task from impossible to merely tedious.
On the original 128K and the 512K upgrade machines (which have a DB-9 connector instead of the mini-Din-8) the Output Handshake line was held in a "marking" condition by hardware (a small resistor to the appropriate power supply rail). On later Macintoshes there are logic and a line driver for this line. This change introduces the following incompatabilities:
1. SOME of the older terminal programs don't have the code to explicitly drive HSKo high.
2. SOME terminal programs drop HSKo when they close down.
3. SOME modems require DTR and will drop carrier if DTR goes away.
If the cable design given below, mapping HSKo to DTR, is used, there are two recognized pathological conditions which can happen:
A. Cannot use modem at all, because of 1 and 3 together.
B. Modem drops out when switching between terminal programs, 2 and 3 together.
Of course, some people consider B a feature, in that it will hang up the phone when you switch off the computer. Personally, I hang up the phone when I am done and I like to switch from terminal program to terminal program. If one of the above conditions happen, there are only three alternatives.
I. If at ALL possible, set your modem up to IGNORE DTR and stay connected. Look for a DIP switch for this. I personally made this choice.
II. Use only terminal programs which "properly" drive HSKo. You get to operationally define "properly" :-)
III. Drive DTR from DSR at the modem end of the cable, as described below.
DIN-8 MALE DB-25 MALE
GROUND 4 O--+--------------------O 7 GROUND
RECV DATA + 8 O--+
RECV DATA - 5 O-----------------------O 3 RD (Receive Data)
XMIT DATA - 3 O-----------------------O 2 TD (Transmit Data)
HANDSHAKE OUT 1 O--+
HANDSHAKE IN 2 O--+--------------------O 20 DTR (Data Terminal Ready)
Note that in RS-232 the data signals are inverted (marking is minus) while the control signals are not (marking is plus). Thus the transmit data minus signal from the Mac is just right for driving the modem. Leave the transmit data plus signal disconnected. If you ground this you will short out a driver, and it will probably get hot. Similarly the receive data signal from the modem/DCE is inverted, so it can drive the Mac's receive data minus line, but in this case the receive data plus line is grounded to prevent any extraneous signals from being induced into the circuit.
Note also that we are driving both HSKi and DTR from HSKo so the problems described above can happen. An alternative arrangement would drive these signals from the modem/DCE's source of DSR, like this:
+--O 6 DSR (Data Set Ready)
HANDSHAKE IN 2 O--------------------+--O 20 DTR (Data Terminal Ready)
Some dumb modems might require Request To Send (RTS) which one would wire like this:
+--O 6 DSR (Data Set Ready)
HANDSHAKE IN 2 O--------------------+--O 20 DTR (Data Terminal Ready)
+--O 4 RTS (Request To Send)
Finally, if you have only 3-wire cable and don't need DTR handshake, you can wire each side to be happy like this:
HANDSHAKE OUT 1 O--+ +--O 6 DSR (Data Set Ready)
HANDSHAKE IN 2 O--+ +--O 20 DTR (Data Terminal Ready)
+--O 4 RTS (Request To Send)
DIN-8 MALE DB-25 FEMALE
GROUND 4 O--+--------------------O 7 GROUND
RECV DATA + 8 O--+
RECV DATA - 5 O-----------------------O 2 TD (Transmit Data)
XMIT DATA - 3 O-----------------------O 3 RD (Recieve Data)
HANDSHAKE IN 2 O-----------------------O 20 DTR (Data Terminal Ready)
The same analysis applies with respect to the data signals, except that in this case the transmit and receive are switched around, since one guy's transmit should be the other guy's receive and vice versa. Note receive data plus is grounded while transmit data plus is left disconnected.
For this particular cable we have wired the terminal/DTE's DTR back into the Macintoshes HSKi to implement a hardware handshake. Assume the terminal side is a printer that is being overrun. One of the things these printers can do is drop DTR. By wiring it through to the handshake input we make it possible for the Macintosh software to temporarily pause in sending, until the printer's buffers empty out and the printer reasserts the DTR signal.
Some terminal devices may need to see DSR (Data Set Ready) or CD (Carrier Detect) or CTS (Clear to Send), in which case they may be driven from an appropriate source.
+--O 20 DTR (Data Terminal Ready)
This is probably appropriate +--O 6 DSR (Data Set Ready)
for a communications terminal +--O 8 CD (Carrier Detect)
in which DTR is a totally static
signal and does not move. +--O 4 RTS (Request To Send)
+--O 5 CTS (Clear To Send)
or
+--O 4 RTS (Request To Send)
This is probably appropriate +--O 6 DSR (Data Set Ready)
for a printer that flaps DTR +--O 5 CTS (Clear To Send)
as the buffer fills and empties. +--O 8 CD (Carrier Detect)
The logic is to drive from whichever of DTR or RTS is NOT flapping around as buffers fill and empty or as the terminal transmits and receives...
To connect directly to an IBM PC we believe CD must be asserted. That is, an IBM PC will not accept data unless it also sees the CD signal.
Somebody on comp.sys.mac.hardware asked for cables for a 128K/512K Mac! I didn't know there were any more of those out there!!! :-) Here are the corresponding connections, please use these in conjunction with the analysis and suggestions provided above:
DB-9 MALE DB-25 MALE
GROUND 3 O--+--------------------O 7 GROUND
RECV DATA + 8 O--+
RECV DATA - 9 O-----------------------O 3 RD (Receive Data)
XMIT DATA - 5 O-----------------------O 2 TD (Transmit Data)
+ 12 Volts 6 O--+
HANDSHAKE 7 O--+--------------------O 20 DTR (Data Terminal Ready)
DB-9 MALE DB-25 FEMALE
GROUND 3 O--+--------------------O 7 GROUND
RECV DATA + 8 O--+
RECV DATA - 9 O-----------------------O 2 TD (Transmit Data)
XMIT DATA - 5 O-----------------------O 3 RD (Receive Data)
HANDSHAKE 7 O-----------------------O 20 DTR (Data Terminal Ready)
On the DB-25 pin 1 is the FRAME ground and pin 7 is the SIGNAL ground. Equipment that requires connection to pin 1 is badly designed (IMHO). As a very last resort you might try a 1 to 7 jumper.
As you can imagine from seeing all these alternatives, an RS232 breakout box is real handy, since you can try all these patches without having to warm up a soldering iron. The only other thing I can say is:
IF IT DON'T WORK, DON'T LEAVE IT TURNED ON LONG ENOUGH TO GET HOT!
Communications driver chips are built very ruggedly and will stand an amazing amount of mistreatment for a short period of time. But if you let two drivers fight for an hour one or both of them will burn out...
I've read this over a dozen times to make sure there aren't any totally glaring errors, but I cannot be responsible for anybody's smoked hardware. Let's be careful out there!
Ben Cranston (zben@Trantor.UMD.EDU)
Network Infrastructures Group
Computer Science Center
University of Maryland at College Park
of Ulm
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