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A3000 SCSI

The A3000's internal SCSI controller has a few minor flaws that can be problematic.

A. The Western Digital 33C93 SCSI chip itself: revision 04 of this chip has some bugs that usually show up when a CD-ROM or tape drive is attached, and revision 08 fixes them.

B. The DB25 used as an external SCSI connector on the A3000 can cause problems. Use only short, high-quality SCSI cables attached to this connector, or run 50-pin ribbon cable from the internal connector.

C. Termination. Various A3000s seem to have come with no terminators, soldered-in SIPs, or even sockets. Check the motherboard controller termination, and follow the guidelines laid out in the Termination section above.

D. Many A3000s had a manufacturing flaw which resulted in terminator power not being supplied at the external SCSI connector. The easiest way to test this is with an external terminator with an LED indicator. Otherwise, you'll need to check pin 25 of the A3000 DB25 SCSI connector for +5V (the shield around the connector provides an easy ground test point). Before testing, you should disconnect all other SCSI devices, both internal and external, because one of them may also supply termpower and confuse the results. If no voltage is supplied on pin 25, diode D800 (or D801, this may vary depending on motherboard revision) is reversed inside the A3000. Unsolder and replace it (this should be a 1N34 type, although a 1N5817 should work and might be more suitable). The motherboard silk screen is likely to be wrong as well, so ignore it.



Last modified: 2021/11/24 22:42