I made sure to park the drive in the AT before trying it on my test IBM PC motherboard... Tandon drive moved back to cylinder 0 rather shortly after power was applied- which of course is what is supposed to happen.
Yes, I read "
Self-calibration on power-up" in the TM502 manual.
Tandon has a microcontroller dedicated to handling the burst mode introduced on the ST-412
Well, one of many functions that the TM502's CPU performs.
so it will therefore handle any step rates the ST-412 can
The TM502 manual indicates a range from 5 to 200 uS. One Seagate source of ST-412 specs indicates 5 to 500 uS. If the Seagate source is correct, then your statement is incorrect (e.g. a 300 uS rate would work for the ST-412 but not the TM502).
But beyond step pulse rate, there are other aspects of the step signal that also need to be met. The 'buffered seek' timing diagram in the TM502 manual shows two other timing requirements: pulse width, and inter-pulse gap.
Let's look at your situation of the TM502 connected to the Xebec card (the Xebec-made-for-IBM one supplied in the IBM XT), a situation where you cannot see any stepping (apart from that generated by the drive's self-calibration at power on).
I've just done something that I've wanted to do for a while; hook up an oscilloscope to the STEP output of the Xebec-made-for-IBM card. In fact, I looked at all three variations of the card. All three showed the same:
* Step pulse rate (period) = 70 uS
* Step pulse width = 17 uS
* Therefore (no need to measure), inter-pulse gap = 53 uS (70-17)
If we compare that to the 'buffered seek' timing diagram in the TM502 manual, the STEP output of the Xebec-made-for-IBM card meets all three criteria of the diagram.
So, from that, there is no incompatibility from a stepping point of view. You know that your TM502 is good (it works in your AT), and presumeably you know that your Xebec-made-for-IBM card is good. Assuming that the +12V to the TM502 is good, I can only see one thing left: the control cable on the Xebec-made-for-IBM card (bad or incorrectly connected).