IBM System/23 Datamaster: Difference between revisions

| ROM || 128KB-272KB

The CPU selected to serve as the core of the system was the Intel 8085. However, as the component references were ofuscated identifying the CPU was not a straightforward task. It was hinted that the component with reference 4178015 could be the part. To confirm the true identity of the integrated circuit, an exchange with a legit 8085 was done: the remarked part went into a [[Alphatronic P2|P2]] CPU board and the P2 CPU went in place of the unknown component. The test went successfully and the cross reference was proved as fact.<ref name=":0">Own research</ref>

RAM in the Datamaster is completely contained in one to two boards complementary to the motherboard and are attached through a card-edge connector of 36 pins each. This facilitates the removal and placement of memory cards into the system and grants the possibility of fast repair and extension of the computer. There are two memory slots, and at the same time official memory boards were produced in two sizes: 32KB and 64KB respectively. This led to five possible official configurations.<ref name=":1">SY34-0171-0 IBM 5322 Computer Service Manual, page 36</ref>

The memory boards contain one or two TMS4132 arrays of 9 memories each. Having nine units per bank allows them to have a data byte width of data plus a ninth bit for parity. Both because of the rareness of the format of the TMS4132 and the replacement of the official reference led for years to believe that IBM soldered the memories in piggyback to cope with failure rates when in reality it was a feat of Texas instruments to sell them directly soldered after manufacture of both dies. This kind of memory contains a 16KB bank per package and both packages have a slight difference of the pinout to not have conflict with /CAS and /RAS.<ref name=":0" />

|+ Official RAM configurations<ref name=":1" />

The System/23 has a dedicated diagnostics port<ref>SY34-0171-0 IBM 5322 Computer Service Manual, page 37</ref>. 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.<ref name=":0" />

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.<ref name=":0" />

For most cases it is not necessary having the probe, as the same values that are outputted through the service port are also written in the video memory, displaying the passed tests and the errors encountered. However, as the same tests that check the video display controller are the same routines that initialize it, all the tests made before the initialization cannot be seen in the screen until passed. In case some of them failed the screen would not be initialized and therefore the results wouln't be seen. For this reason the probe is still needed.<ref name=":0" />

The power supply of the computer is a heavy block which consists in a linear supply that outputs +5V, +12V, +24V, -5V and -12V<ref>SY34-0171-0 IBM 5322 Computer Service Manual, page 30</ref>. Its connector is a 12-pin molex structured in three columns and four rows. Other than the voltages, it also provides with a "power good" signal as posterior IBM computers would, such as the IBM PC. The first IBM PC prototype used the same connector and possibly the same pinout. Adapting an ATX supply to feed the main board is possible and relatively easy to do (actually, it is recommendable when testing the system in a bench). When the connector transitioned to the PC it was linealized but maintained all contacts present in the Datamaster power supply connector, including the key pin (N.C.). The change might have been motivated to reduce friction when attaching/deattaching the cord.<ref name=":0" />

The System/23 Datamaster implements a set of self-test routines identified as "PID-1200". With them it tests the CPU, the memory and the different peripherals of the computer. Usually the results of the tests are written to the screen at start-up but if an error occurs before the initialization of the screen a probe is needed. The tests are identified by an hexadecimal value and can be presented unstyled, underlined or inverted. In the case of being underlined it means that the feature tested wasn't found by the system, whereas if it is inverted it has been detected but was tested and found faulty. If the text is left unstyled, it means the test passed. According to the Diagnostics user guide, theThe test routines and their tested areas are the following<ref>6841631 System/23 Diagnostic User Guide, page 87</ref>:

Note that in case a ROM is unselected or unpowered it will mark its test as "missing". ROMs 0E and 0F aren't present on the board and therefore its tests will always result with a "missing" status.<ref name=":0" />