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From: johnw@med.Stanford.EDU (John Wiederhold)
Newsgroups: comp.sys.ibm.pc.rt
Subject: Simple and Cheap Solution to RT serial problems
Keywords: serial fix
Message-ID: <210@med.Stanford.EDU>
Date: 12 Nov 89 04:31:05 GMT
Sender: johnw@med.stanford.edu (John Wiederhold)
Reply-To: johnw@med.stanford.edu (John Wiederhold)
Organization: Stanford University Medical School
Lines: 39

After getting fed up with "device /dev/ttys0 overflow" errors while
I was running at 9600 baud, I realized that there is a reasonably
simple and inexpensive way to fix this.  I got standard IBM serial
card meant for your basic IBM-PC/XT, and replaced the UART chip on
it.  Once installed in the RT, it shows up as ttyc0.  The card costs
about $25-30 and the chip costs about $15.  Now my RT runs at 19200
baud without one complaint.

The details:
The standard IBM serial card for a PC will work in your RT (at least in
my 6150-125), you just have to install it and go to your /dev directory
and do the appropriate MAKEDEV.  This card will give you slightly 
better performance than the built in serial ports (plus they have
standard 25 connectors).  To increase the performance you can replace
the standard UART chip with a National Semiconductor one that
has FIFO (First In First Out) buffers built in.  The UART is a
forty pin chip (I think its the biggest one on the board).  The
standard one is a 8250 and the one you want to replace it with is a
NS1655AN.  I would suggest getting a forty pin chip socket while
you're at it.  They cost less then a dollar and will prevent any damage
to the chip when you try to solder it in.

Once it's all done and installed you RT should run at 19200 without
problems.

Disclaimer: 
I assume no responsibility for any damage done to your system by this
modification and I can't guarantee that it will work for you.  It's up
to you to determine the suitability of the modification to your needs.
As I said my system is an IBM-RT 6150-125 runing AOS4.3.  The change
greatly improved the reliablity and performance of the serial port.  I
basicly use it for kermit connected to a 19200 baud modem, but I would
soon like to get SLIP working.


Good luck.  Feel free to send any questions or comments to me.

John Wiederhold
Stanford Medical School Information Systems Group