View Full Version : Tandon TM-100A

October 24th, 2016, 01:17 PM
I now have my latest project, an SD SBC-200 with 8 slot motherboard (from s100computers) with Versafloppy II and Expandoram II up and running (what a mission!).

I have a Tandon TM-100A drive that I have hooked my setup to. The 100A is a bit of an oddball and is different from the standard 100 version (I believe the 100A's came with the TRS80 model III's).
When I first plugged it in it ran fine, I then tried out a few other drives, and finally settled on using the Tandon. Plugging it back into the system, the motor now runs constantly (where it didn't before). Looking at the drive motherboard I can't see anywhere for setting things like 'drive number', 'head enable load with motor on' etc that are usually included on drives. I did see that the resistor pack (150ohm 6pin SIP) terminator is missing.
The manual states:

The Disk Drive has been provided with the capability of
terminating the input lines listed below:
* Motor On
* Direction Select
* Step
* Write Data
* Write Gate
These lines are terminated through a 150 ohm resistor pack
that is installed in a SIP socket (R51, located adjacent to
J2. In a multiple drive system, only the last drive on the interface
is to be terminated. All other drives on the interface
must have the resistor pack removed.

I am only using one drive, so I assume I need the resistor pack.

So my question is, would not having the resistor pack installed possibly make the motor run constantly. I'm trying to understand why it was working fine at one point, but on second attempt the motor runs non stop. Could it be something to do with these inputs floating?
(On a side note, even with the drive running constantly, it does seem to function correctly)

Not having a resistor pack on hand, I was thinking to try using a standard 150 resistor and connect one end to pin 1, and try the other end in the pin that corresponds to 'motor on', and see if that fixes the problem.

Any advice would be greatly appreciated.


October 24th, 2016, 02:19 PM
If you're driving the TM100A with regular 7438 OC drivers, you really do need a terminating resistor. There's got to be something as a collector load for reliable operation. Otherwise, the signal floats and can be either high or low. You can use a bunch of 150 ohm resistors (the actual value isn't too critical; 1K would work just as well, for example) and stuff the leads into the terminator socket or use a DIP plug with the resistors soldered in.

October 24th, 2016, 02:48 PM
Thanks Chuck(G),
Thats exactly what I needed to know.
I'll solder something up this eve and give it a go.


October 25th, 2016, 01:12 AM
Yep, that did the trick.

Much thanks.

EDIT: I should add that using 150 ohm resistors didn't actually stop the drive spinning!
Using 100 ohm resistors seemed to work perfectly.