touch@dsl.cis.upenn.edu (Joe Touch) (07/01/90)
I'm having a problem with my mac serial port. The port works fine when connected to a modem - I connected as follows: RS422(8-pin) RS232 (modem) GND 4 --+----------------------- GND 7 RD+ 8 --+ RD- 5 -------------------------- RD 3 TD- 3 -------------------------- TD 2 The problem occurred when trying to connect to a PC. I found that the MAC sent to the PC fine, sending 1mA, while the PC tried to send 6mA to the MAC, and nothing got transmitted. I tried to connect the MAC to itself (i.e. connect TD- to RD-), and found that the signal was 21mA, and nothing gets transmitted. THe same thing on the PC works fine (connect TD to RD), and 1mA is used. My question has to do with the current used by the RS422 - do I need a resistor on the RD- line to prevent its drawing too much current from the RS232? I know the RS232 is SUPPOSED to be level-sensitive, but the current drawn is pulling the levels too low for the PC driver. As a last resort, I can build an isolation buffer between the two, but that seems extraordinary. Joe