info-mac@utcsrgv.UUCP (info-mac) (05/15/84)
Date: Mon, 14 May 84 13:27:29 EDT From: Dave Farber <uw-beaver!farber@udel-ee.arpa> To: info-mac%sumex-aim.csnet@udel-ee.arpa Subject: Query on rs232/422 mac connectors We are getting about 2.5 volts out of our mac on pin 7. Is that normal? I believe the specs for rs232 say > 3 volts
info-mac@utcsrgv.UUCP (info-mac) (05/16/84)
Date: Mon 14 May 84 22:18:34-PDT
From: Joseph I. Pallas <uw-beaver!PALLAS@SU-SCORE.ARPA>
Subject: Re: Query on rs232/422 mac connectors
To: farber@UDEL-EE.ARPA
Cc: info-mac@SUMEX-AIM.ARPA
In-Reply-To: Message from "Dave Farber <farber@udel-ee.arpa>" of Mon 14 May 84 17:39:58-PDT
We are getting about 2.5 volts out of our mac on pin 7. Is that
normal? I believe the specs for rs232 say > 3 volts
It can't be considered abnormal, since (according to the pinout in
<info-mac>macintosh.rs232) pin 7 is an input. The Mac represents
an rs232 Data Terminal, not Data Set. If you're talking to a modem,
pin 7 should go to rs232 pin 5. The connections I used are:
Mac Modem
3 7
5 2
7 5
9 3
Some modems will want to see DTR (pin 20) active also, which can be
faked by tying it to pin 5 (I'm not sure if that's the canonical way
to fake it, though). Other modems will ignore DTR or can be made to
(e.g., USR Password has a switchable option).
Hope that helps!
joe
-------info-mac@utcsrgv.UUCP (info-mac) (05/16/84)
Date: Tue, 15 May 84 15:45:17 EDT From: Bob Rees <uw-beaver!rrees@BBN-UNIX.ARPA> Subject: Re: Query on rs232/422 mac connectors In-Reply-To: Your message of Mon, 14 May 84 13:27:29 EDT To: Dave Farber <farber@udel-ee.arpa> Cc: info-mac@sumex-aim.arpa > We are getting about 2.5 volts out of our mac on pin 7. Is that > normal? I believe the specs for rs232 say > 3 volts Pin 7 (of the Mac serial port connector) is an INPUT pin. It connects to both CTS (for modem-style handshake input) and TRxC (for external clock input) of the 8530 SCC chip. The theory is that applications requiring CTS don't need an external clock, and vice versa. You get to choose the function of pin 7 by setting up the registers in the 8530 chip -- disable CTS interrupt and select external clocking to use TRxC, enable CTS interrupt and select internal clocking to use CTS. Although the 8530 chip can also be programmed to use TRxC as an output pin (clock source), I have been unable to get this mode to work. My suspicion is that pin 7 goes to the input of a line receiver chip whose output goes to the CTS & TRxC pins of the 8530. The line receiver is some sort of TTL buffer/driver with input bias/clamping/hysteresis to provide RS-232/422/423 (as well as TTL-level) input compatibility. The 2.5 volts that you see is just the open-circuit input bias voltage of that line receiver. Hope that this explanation has helped to demystify the Mac's serial ports. - Bob Rees