IBM System/23 Datamaster: Difference between revisions

=== Diagnostics port ===

 

The System/23 has a dedicated diagnostics port. It is controlled by a single 8255 by using its port B, whereas ports A and C have no relation in function; for this reason it is deduced that port B is working in mode 0, without handshaking of any sort. This fact is confirmed by the eight data lines which conform the data port being directly connected only to port B and the remaining four signals are for supplying power to the probes.

 

After finding the pinout of the diagnostics port a simple probe containing a led per data line was built, and it showed a count when booting. Removing the RAM board(s) caused the count to stop at "00000100" (4 in decimal), which corresponds to a memory check test procedure of PID 1200. With those facts it was deduced that the port was outputting a two-digit hex code which was written just before the start of a test routine and would stop when the error was considered critical.