Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10 5/3/83; site utcsrgv.UUCP Path: utzoo!utcsrgv!info-mac From: info-mac@utcsrgv.UUCP (info-mac) Newsgroups: ont.micro.mac Subject: Re: Query on rs232/422 mac connectors Message-ID: <4303@utcsrgv.UUCP> Date: Wed, 16-May-84 09:31:41 EDT Article-I.D.: utcsrgv.4303 Posted: Wed May 16 09:31:41 1984 Date-Received: Wed, 16-May-84 10:03:17 EDT Sender: peterr@utcsrgv.UUCP Organization: CSRG, University of Toronto Lines: 30 Date: Tue, 15 May 84 15:45:17 EDT From: Bob Rees 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 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