IBM Xenix 2.00 serial port problem

[abl]

I have an application running on an IBM PC/AT running IBM Xenix 2.00  that
uses  high  speed  serial  transfer  (9600 baud) to communicate with other
machines.  The protocol used is similar to Kermit in that all data is sent
in packets and retries are performed if a transfer fails.

The problem is that Xenix does not get a lot of the characters sent to it.
Around  1% are lost when the machine is unloaded and much more if there is
another process with a high level of interrupt driven  input/output  (e.g.
other tty activity or network activity).

I would imagine that this is an old and solved problem for most of you  so
any  advice  would be appreciated.  What prompted this request now is that
we have recently received a copy of SCO Xenix 2.2.1 and the  documentation
includes  some  comments  about  the 8250a serial chip used by most serial
ports used  on  PC/ATs.  It  claims  that  there  are  problems  with  the
interrupt  handling  on  this chip and that they where fixed on the 8250b.
Our IBM and AST advantage serial ports use the NS16450N chip - is this the
same as the 8250a and has it the same problems?

Other questions while I am here:

1) Do serial cards like the Digiboard COM /4, /8, /4i or /8i solve all
   known serial problems and if so where can I get one in Europe?

2) Does SCO Xenix solve problems associated with the 'no  more  processes'
   messages  that  IBM  Xenix  2.00 sometimes gives when two or more large
   programs are running?  We think that these problems are due to bugs  in
   IBM  Xenix 2.00 memory management - it is not a problem with the number
   of processes. We also have problems with 'no more processes' when using
   programs which communicate via shared memory.

Thanks in advance,

Alastair Bland, SPS, CERN, Switzerland.

abl at cernvax.cern or abl at lepxs.cern