Radio Shack 8MB hard disk for Model II

Eric Smith spacewar at
Fri Dec 4 04:45:31 CST 2020

On Mon, Nov 30, 2020 at 6:22 PM dwight via cctalk <cctalk at>

> Is there a terminator on both ends. If not it is a static 15 ma.

There's a 220/330 terminator to +5V at both ends. The thevenin equivalent
of one of these terminators  is 132 ohms to +3V. A driver trying to pull
this to 0V is going to have to provide 22.7 mA for a single terminator, but
since it's doubly terminated, it will be 45.5 mA.

They're only using 24mA rated drivers, which would be within spec if it
wasn't doubly-terminated.

SCSI uses double termination, but it also specifies minimum 48 mA drivers.

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