Speaking of compatibility, we had a 360/50 many ages ago,
and it was "permitted" to put the tape controller on the
multiplexer channel. This prevented long tape operations
from locking up the disk controller on the selector
channel. The model /50 only had one selector. But, if
there was too much traffic on the multiplexer, it could
cause data late issues. We tried to read some of those
tapes on our PDP-11, and the controller couldn't do it. I
got some Magna-See, and found that there were little gaps in
the data records.
These were quite obvious visually, so at least several
character spaces. Apparently, IBM's tape controller had a
2-character FIFO and would only start processing the end of
record after a LONG gap of at least 5-10 characters. Our
Datum controllers immediately looked for a CRC after a
2-char gap, and got a CRC/LRCC error when it hit those
little gaps. This would be on 800 BPI NRZI tapes, of course.
Jon
