IBM System/23 Datamaster: Difference between revisions
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=== Video Subsystem ===
[[File:Datamaster_Video_Connector.png|260px|thumb|left|alt=Datamaster video connector|Datamaster video connector]]
The video subsystem is almost copied verbatim from the Intel 8275 Datasheet. It consists in a 8275 variant plus a 8257 DMA device. The 4178629 made by Intel is the CRTC chip and it behaves exactly the same way as stock 8275s. Compatibility with stock 8275 has been tested with good results, as well as the Soviet KR580VG75, which is also compatible. The Intel 8276 is not compatible without a ROM hack that modifies test 05h in order to deactivate the light pen test.
The video connector present in the motherboard presents at least two supply voltages (+5V and +12V). The other video signals are strictly TTL compatible. Synchronism signals are separate, vertical sync is negative and horizontal sync is positive. The display has three degrees of monochrome tones: full off, normal and highlighted (full on). This is achieved with an intensity signal being carried separately from the video signal.
The Datamaster has two pixel clocks: 18.432 MHz for the American units and 18.2 MHz for the European machines. Its timings makes them incompatible with the later and more common MDA. While European units have both oscillators and the selection jumper, the Americans only have an oscillator and lack the jumper, although its space on board is present.
There have been unofficial attempts to obtain the picture without having to recur to the original monitors. For instance, a composite video addon was first created, followed by a much simpler and overall better RGB-Scart adapter. Finally, the computer has been given support by the RGBtoHDMI project, therefore enabling Datamasters with broken or no monitor at all to be able to output image.
=== Diagnostics port ===
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