TRS-80 Model 4 help requested

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Thu Feb 27 18:20:00 2003

> Unlike IBM Drives, TRS-80 drives are usually hard
> jumpered for specific positions (Not always, but
> internal drives are..)

Not so. Most genuine Radio Shack drives had _all_ the drice select
jumpers fitted. The edge connectors on the cable had contacts removed so
that only the 'right' drive select signal went to each drive. This
applies to both internal and external drives.

One problem with this is that the side select contact was removed also.
People who fitted DS drives had to make up their own cable (and it was a
lot easier to make a cable with all the contacts in place and to remove
the incorrect jumpers from each drive).

> They can be set to positions 0, 1 ,2 or 3. And the
> last drive in the chain needs to be terminated (like
> SCSI Drives).

This is something that worried me when he started swapping drives around.
If you don't have the termination resistor in place, it won't work, and
may even corrupt the disk (don't ask how I found that out...)

>
> For external drives, the drives are set to all
> selects, and pins are pulled from the drive cable to
> determine what position they are in. If you notice on
> IBM Floppy drive cables, there is a twist between the
> A: and B: connector. That twist brings the select from
> the 0 drive to the 1 select pin. All IBM drives are
> set to position 1 (second position).

The 'IBM twist' also allows for sepearate motor-on signals to the 2
drives, so that the motors can be controlled separately. A total of 7
wires are involved in the twist, but 3 of them are ground.

-tony
Received on Thu Feb 27 2003 - 18:20:00 GMT