touch@dsl.cis.upenn.edu (Joe Touch) (07/04/90)
I'm trying to have the MAC talk to itself - an RS232 loopback, connecting send to receive. I was having problems interfacing the MAC to something else RS232, so I'm trying to back up and figure out what's what. First, the MAC RS232 works fine connected to a modem. Several different modems, in fact, and other types of serial lines. But the MAC won't talk to itself. I measured the follwing with a breakout box - and a digital meter. I was not intending to measure the AC signals, just the DC for now. (i.e. I'm not typing when I measure...) The MAC RS422 has been wired as an RS232 (and works) as follows: RS422 RS232 GND GND RD+ GND RD- RD TD- TD MAC DC open circuit (unconnected) send -4.7 V receive .427 V (should be nominally zero, but isn't) MODEM DC open circuit send -9.2 V receive .07 V (nominally zero) MODEM loopback (send to receive) -8.62 V / 1.4mA (current equivalent to 6.2K Ohm terminator) MAC loopback -3.14 V / 21mA (current is way too high - like a 150 Ohm term.) MAC-MODEM mac-to-modem -4.57 V / 0.6mA (like a 7.6K Ohm term) modem-to-mac -2.22 V / 13.7mA (current is way too high - 162 Ohm) I'm only trying the loopback here. The MAC has been happy for quite a while with only send, receive, and ground out to a modem, and RD+ connected to ground to accomodate the RS232 single-ended signal, as all wiring diagrams for the MAC specify. Any help you could provide would be appreciated. Thanks. Joe