Radio Shack 8MB hard disk for Model II - test-drb@ccmp.vtda.org

30 Nov 2020

I had occasion to look at the service manual for the Radio Shack 26-4150
8MB hard disk which was used with the Model II (and potentially could be
used on 12, 16, 16B or 6000). Note that this drive is not compatible with
the Model I, 3, or 4, and the cable wiring between the computer and the
controller (inside the drive box) is entirely different than what was used
on the later 5, 12, 15, 35, and 70 MB drives.  The Model II host adapter
for the 8MB drive thus is only useful with the 8MB drive, and vice versa.
There were later Model II host adapters that could be used with the 12, 15,
35, and 70 MB drives, which were also useful on the Model I, 3, and 4.

Anyhow, I discovered that according to the documentation, the 8 bit data
bus between the host interface and controller uses 8T26 bidirectional
buffers at each end, which are rated to sink up to 24 mA, but the
schematics show that Radio Shack put 220/330 ohm terminators on the data
bus lines at BOTH ends of the cable! That requires the 8T26 to sink as much
as 55 mA, which means that its logic zero output voltage is likely to
exceed its normal rating. At the very least, this will result in reduced
noise immunity.

I don't have an 8MB host interface or drive in hand to confirm, but the
photos I've found online do show the resistor networks on both ends.

The controller (inside the drive box) is a modified version of a WD1000,
configured for use with the 8-inch SA1004 drive, which operates at 4.34
Mbps, NOT 5.00 Mbps like 5 1/4" drives. Aside from that, it has a different
host pinout than a normal WD1000 (or than the later Tandy controllers), and
has extra circuitry for dealing with write protection. Electrically, the
host interface is otherwise the same as the WD1000. Normally the data bus
for the WD1000 would only be single-terminated at the controller end. That
is in fact what Radio Shack did on all of the later host adapters.