Black Box SCSI parity and termination power

C.S.S. Black Box SCSI parity and termination power modification

CSS Black Box Components

SCSI-2 (or Fast-SCSI) hard disks require a parity bit on the SCSI bus. Some have a jumper to en-/dis-able this feature but others (like many IBM’s) don’t. Most Memory Card Readers also won’t work if that data bit is not provided. The Black Box pre-dates SCSI-2 so it doesn’t carry this signal on the bus but it’s relatively easy to add it afterwards.

SCM PCD-50B – Screenshot 01

All you need to do is adding a 74LS280 (or 74HCT280) IC. There are several ways to do this but the most simple method is to just mount it on top of the existing 74LS640 (or 74HCT640) line driver on the Black Box. It has all the signals you need to connect the parity generator, however, it requires some wiring because not all pins line up. The parity IC outputs both an even and an odd signal. Be sure to use the odd one. The other one can be used to connect a funky I/O LED or otherwise just leave it as is.

CSS Black Box Parity Upgrade

Be aware of GSS branded IC’s (GS74LS280). All 3 I tried didn’t work as expected on both my Black Box’ and Atari MegaSTe. Never had problems with other brands (Motorola, Texas Instruments, etc.)

BB parity – Ugly test

When using a SCSI-2 hard disk, don’t attempt to do a low-level format on it (HDFMT.COM). The firmware of the hard disk should prevent this but you can’t be too sure. If it for some reason succeeds it will destroy important manufacturer data, rendering your disk useless. Just start to create partitions and use DOS or FMTDIR.COM or a-like to do a soft-format.

Another signal the Black Box doesn’t have on it’s SCSI bus is termination power. Some SCSI devices may need this to function properly. Connecting pin #26 of the SCSI bus to a +5V source will solve this. For added safety you may add a fuse in between or do it the ‘official’ way.

Termination power

Usually the parity signal and termination additions can be added to other SCSI systems in pretty much the same way. I modified the ACSI/SCSI controller in my MegaSTe using the same principle and can imagine the SCSI hosts from the old Mac’s and Amiga’s aren’t that different.

Datasheet: SN74LS280
Datasheet: SN74HCT280
Datasheet: SN74LS640/641/642/645