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.
(Image source - Wikipedia, photographer)