Telephone line problems will not cause framing errors unless you have
the modem configured to turn off all error detection and compression
mechanisms; or, unless you have the modem misconfigured to have the
DTE (rs232 serial port speed) match or locked to the DCE (telco line)
speed. There is a very slight advantage in PPP throughput to turn off
compression and error detection, but its so slight most folks don't
bother. There's no reason to ever have a modem configured so as to have
the DTE speed locked to the DCE speed when using portmasters.
95 times out of 100 the framing errors result from mismatched serial
port speeds between the modem and the portmaster, and the majority of
those cases are related to the installers lack of knowledge as to what
the modem's *default* DTE serial port speed is on power up. I'm guessing
the majority of the modems sold in the last several years default to
115,200 baud, so if you configure the pm2 to match that, framing errors
should go away or occur very infrequently.
I've not seen it as yet, but its possible the 16C552 IC in the portmaster
(Dual Async Comm Element) has gone south. That IC contains all of the
circuitry for controlling the serial port speed and the detection of
framing errors. Since each 16C552 supports two serial ports in the
portmasters, the probability of one portion of the IC going south and
not the other portion is rather low.
Its more likely the 26LS32 (serial port line receiver) or 26LS30 (serial
port line driver) IC has gone south. Since these two IC's interface
*directly* to the portmaster's serial port pins, it is more susceptible
to damage due to static discharges and other external influences
(eg, over voltage spikes, incorrect cable wiring, installing/deinstalling
serial port cables).
The easiest way for you to test the PM2 is to configure the serial port
speed to something like 9600 baud and attach an external PC to that
serial port. Test it, and if data transfer occurs as normal, then
bump the serial port speeds up to the next higher rate. Keep doing that
until you reach the point where framing errors occur. If that occurs
at speeds like 115,200 baud and below, the PM2 serial port is
defective for one reason or another. If it never occurs, then there
is some external reason like bad cabling, modem, dirty serial port
contacts causing the problem.
I have seen several PM2's produce framing errors, and when the serial
port connector is sprayed with a contact cleaner, all errors disappear.
Radio Shack sells a contact cleaner that has a small amount of an
oil base to it that works well. The non-oil based cleaner will work,
however the errors will return within days or weeks. The non-oil
based cleaner will leave your finger white if you spray it in the store.
Sometimes the oil-based cleaner will be sold as Tuner Cleaner.
Regardless of the quality of the serial port connector used in the
portmaster, all metals tend to develop some small amount of corrosion
(that cannot be seen) over time, so cleaning those rs232 connectors
once a year should be included in your preventative maintenance
schedule.
Hope that helps...
Rich
adar0@routers.com