short blocks on SI 9700 ctlr with STC tridensity transport

[dunigan at ornl-msr.arpa]

Dump and tar to STC tri-density tape drive via SI 9700 controller
on 4.2 unix (through ut.c driver) is losing a byte on some writes,
some blocks are only 10239 rather than all being 10240.  Seems
to be a function of the start-stop nature of tar and dump, a simple
tape-certify program writing from memory does not exhibit the
problem.  Happens at 1600 and 6250, and if you re-write the tape
the short blocks occur at different places -- but re-reading
always reports the same sequence  (reading on another tape
subsystem reports the short blocks)

  hints or suggestions?

tom
  dunigan at ornl-msr.arpa