I have a very old CPU card, maybe 1980's. It has following chips on it and most probably these are used for communication with the rest of the system.

SN55188 Quad Line Drivers

SNJ55189 Quad Line Receivers

I cannot power-up the board. But I need to figure out the communication bus type used in it. Looking at the above IC's can I safely say that:

  1. It's not using TTL signals on bus.
  2. It's not differential 485 bus.
  3. It's using differential 422 bus.
  • 5
    \$\begingroup\$ For a start, SN55188 are RS232 drivers, not RS485. You may get a better idea of the communications interface by looking at how it interfaces to the outside world. Is this by terminal blocks or other connectors? How are they labelled? \$\endgroup\$
    – Steve G
    Commented Sep 18, 2019 at 8:39

1 Answer 1


I need to figure out the communication bus type used in it.

As commented, those ICs are only used for RS-232 (they cannot be used for RS-422 or RS-485).

Your CPU card might have other interfaces too, of course, but RS-232 is the answer about what interface type those ICs are used for.

Here is an example - an old IBM PC-compatible serial card, showing these ICs next to the RS-232 interface connector on the right-side of the board:

  • RS-232 quad line driver (marked MC1488, equivalent to the TI SN55188)

  • RS-232 quad line receiver (marked MC1489, equivalent to the TI SN55189)

    This specific board has two quad line receiver ICs (i.e. each containing 4 individual line receivers) so it has more than 4 incoming signals wired to the connector. If the "Ring Indicator" (RI) signal is supported, there are 5 typical input signals on an RS-232 interface.

There are also empty sockets for an additional line driver & line receiver IC, if the second 8250 UART IC (U2) is populated, to provide a second RS-232 port via the pin header marked J2.

photo of IBM PC-compatible serial card

(Image source - Wikipedia, photographer)

  • 1
    \$\begingroup\$ J2 has 10 pins, that's enough to include all the signals supported on PC serial ports(five in, three out, and two grounds), that inclues ring indication. being an ISA card it pre-dates ATX so wake on ring is, of-course, out of the question. \$\endgroup\$ Commented Sep 18, 2019 at 9:49
  • 2
    \$\begingroup\$ @Jasen - Hi, you're right, of course. I remember some dual-port serial cards which only supported Ring Indicator (RI) on the primary port. J2 might have an unused pin because, for example, 10-pin headers were easy to get. If J2 supports RI, then that signal must come through one of the unused line receivers in the two already-populated 1489 ICs, since there are only 4 line receivers (i.e. enough for 4 signals, which wouldn't include RI on J2) in the optional 1489 socket, next to the optional 8250 (U2). I'll edit to remove my hypothesis about RI on J2. \$\endgroup\$
    – SamGibson
    Commented Sep 18, 2019 at 10:11
  • 1
    \$\begingroup\$ yeah, that makes sense. \$\endgroup\$ Commented Sep 18, 2019 at 10:15

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