Both machines booting
What I tried
The other good ST-251 into the 5160 on its own controller card. Different card from the 5271’s, so a different format, but the same process through DEBUG. FDISK, FORMAT, DOS 5.
What I measured
Both machines boot from A: and from C:. On the 5271 I loaded DOS and typed around with the Inboard in. If the Inboard didn’t work, the machine wouldn’t.
What broke or surprised me
Nothing.
Next
The Inboard’s DOS utility. The alignment tool. Eventually the 3270 side.