MegaSTe ACSI/SCSI parity and termination power

MegaSTe ACSI/SCSI parity and termination power

The internal hard disk controller of the MegaSTe doesn’t support SCSI-2, or Fast SCSI, devices. A solution to get those SCSI devices to work isn’t that difficult to accomplish. Like I did on my Atari 8-Bit, it’s just a matter of adding the required parity bit and/or termination power signals yourself.

MegaSTe parity connections

The 74LS280 is a parity generator that outputs the missing data bit when connected to the 8 existing data bit lines. Just feed them to the IC in any order and connect the odd output (not the even) to pin #18 of the SCSI bus.

MegaSTe parity wiring

I’ve put the new IC on top of the ACSI rom with a small piece of tape that sticks on both sides but you can place it about anywhere. Just don’t make the data wires too long. Even slow hardware has it’s limits. Using thinner wires than I did likely results in a better cosmetic overview.

MegaSTe parity upgrade

Termination power can be added by connecting pin #26 of the bus to 5VDC. You may opt to go for the ‘official’ method and add a fuse, diode and capacitor too.

Termination power

Datasheet: SN74LS280
Datasheet: SN74HCT280