Ryan,
I just finished upgrading several PM2's for a client, and after
completing the project I had a couple of PM2's that acted very
strange. Yesterday I got into one box with a voltmeter and logic
probe and found three bad serial ports. In my case the three
bad ports were sufficient to cause the PM2 to intermittently fail
(several panics and reboots per day).
On one of the serial ports, I had the same type of problem as noted
above. I traced it to the 26LS32 IC (very small chip), which is a
Quad Differential Line Receiver. For serial port S0, the Carrier
Detect would be on IC U42 pin #9. Pin #9 is the input from the
RS232 connector and pin #11 is the output that heads directly over
to the 16C552 Dual Asychronous Communications IC (pin #29, input).
This is an older PM2 with a motherboard stamped as 400-1220 REV A.
I forced the RS232 pin #8 (Carrier Detect) high and then low, and
IC U42 failed to relay the proper logic levels to the 16C552 IC,
indicating a bad U42. At one point, I accidently slipped with the
logic probe shorting two unknown pin numbers together. IC U42
started to work normal, which is likely to be very unreliable
over time.
Since this particular PM2's S0 port was good until one day when we
seemed to have a lot of static electricity, I am fairly certain
that static electricity caused the S0 port to fail. Rebooting the
box did not help. The electronic design of this portion of the PM2
does not have any form of electrical protection against external
static electricity, so I'm guessing that one of us accidently did
in the serial port IC by not following good discharge practices.
This particular PM2 had two other serial ports that were bad from
years past. One serial port was bad due to an internal short in
IC U67 (26LS30, Quad Single Ended RS-423 Line Driver), and another
serial port had IC U62 (26LS32, Quad Differential Line Receiver)
with an internal short. One or more of these bad IC's were
causing internal system interrupts and other improper conditions
as seen with the logic probe, which resulted in the PM2 ComOS
v3.3.3 rolling over (e.g., Panic) and rebooting intermittently.
Doubtfull this had any relationship to ComOS version levels.
Since there are no published mechanisms (other than RMA'ing) to
view/test the condition of all RS232 input/output leads, there
really is no convenient way to test the usability/reliability of
each serial port pin without using a logic probe or similar
equipment on several of the seven active RS232 interface pins.
Based on this elementary analysis from this particular PM2, I'd
suspect its fair to assume that defective serial ports (regardless
of the cause) are highly likely to reduce the overall reliability
of any PM2; and, the greater the number of defective serial ports,
the greater the probability is of having a very unstable PM2.
Since the Carrier Detect lead status is displayed in the
"show s0" screen, for your particular problem you should be able
to use a break-out box with a portable PC (or whatever) to force
the CD pin high/low to determine its current functionality. Be
sure all other RS232 pins are presented with appropriate signals
before forcing CD, as the Dual Asych Comm IC may not present the
proper signals for the 'show s0' screen without them. Since the
PM2 relies on the CD lead to indicate the start of a login
session (as seen by USERNAME in the display), its probably fair
to assume that IC U42 is bad.
I would also spend a little time verifying that appropriate
grounding practices are in place to reduce the probability of
static electricity damage. Hope that helps...
Rich
adar0@routers.com