Ref: 13680018
Title: How to Troubleshoot Serial Links and Modems
Date: 10/26/90

Copyright 3Com Corporation, 1991.  All rights reserved.

In synchronous line communications, clocking (pins 15 and 17) is
all-important.  The clocks are associated with transmitted data.
Synchronous data WILL NOT MOVE without a clock source!

Before beginning to troubleshoot a serial link, make sure you have a clock
source at both ends.  If the remote modem is not configured either for
internal clocking or loopback clocking (derive clock from the modem at the
other end) you will not get synchronization.


Before you begin to troubleshoot a serial link, check these items:

  *  Make sure pins 15 and 17 are in your cable at both ends.

  *  Make sure there is a clock source at both ends.  It is possible to have
just one modem set for internal clock, and the remote modem set for loopback
clock, but it is much more common to have internal clocking set on both
modems.

  *  As you conduct the tests, write down the test and the results.  Even
better, draw a picture as shown below for each test and write down the
results.  You will instantly gain the phone company's respect if you provide
them with logical, clear-cut troubleshooting information.  For example,

Internal clock to send
    -------- TX(transmit)                 (receive)RX ---------
   |        |========================================|         |
BR-| modem  |  Telco circuit is 4 wires 2 TX, 2 RX   |  modem  |-BR/3000
   |________|========================================|_________|
             RX                                    TX
                                              Internal clock to send, or
                                              loopback clock to send


To troubleshoot a serial link, follow these steps:

1.  Test individual modems.

a.  Put the local modem in ALB (Analog Loopback) and ST or TP (self test or
test pattern).  Check results with the modem manual to make sure they are
correct.

b.  Put the remote modem in ALB (Analog Loopback) and ST or TP (self test or
test pattern).  This sends a test signal from within the modem to the
telephone line interface and loops it back to confirm that the modem is
operating correctly.  Sample diagram:

     ST/TP>>>>>>>>>>\
                     |ALB (tests local modem)
    indicators<<<<<</
    (test results)

c.  When you are sure both modems are operating correctly, turn off the
ALB and ST/TP on both modems.


2.  Test the data link (phone line).

a.  Put the remote modem in DL (Digital Loopback), either by calling
someone at the remote site and have them push the DL button (or otherwise
invoke DL depending on that modem's operation), or invoke RDL (Remote Digital
Loopback).  Modems supporting RDL let you push the RDL button locally to
instruct the remote modem to go into Digital Loopback.  If the modem has
the RDL feature but remote loopback cannot be invoked, you probably have a
bad line--confirm it by having someone manually invoke DL at the remote site.

b.  After the remote modem is in Digital Loopback, press the ST or TP button
again on the local modem.  The local modem will send a test pattern over the
phone line.  The remote modem will digitally loop it back after it passes
through the modem circuitry, but before it goes into the Netbuilder.  The
pattern will come back across the phone line and the local modem indicator
lights will tell you whether or not you have a valid link.  Again, check
results with the modem manual to make sure they are correct.  Generally,
blinking RD (Receive Data) indicates errors, solid RD indicates a good
connection, and no RD indicates that the signal is not returning at all.
Sample diagram:

    ST/TP>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>\
          (tests local modem, phone line, and remote modem)    |DL
    indicators<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<</
    (test results)

Another way to test the data link is to put the local modem in DL and have
the remote modem send the test pattern.


3.  If test 2 fails, determine whether you can send but not receive data,
or receive but not send data.  Have the modems send the test pattern ST or
TP to each other one at a time.  Sample diagram:

    ST/TP>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>indicators
       (tests TX (send) and RX (receive) phone lines individually)
    indicators<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<ST/TP


Interpreting the results:

If the Brouters were not set for internal clocking and there were no modems,
they should not have come up.  If there were modems, then either both modems
were set to internal clock or, one was set to internal and the other to
loopback clock.  If the modems are set right, you may have a bad phone
circuit; the testing outlined above will give you the information to prove to
the phone company that their circuit is bad.  When the phone company tests
circuits, they test from the local phone company switching office to the local
drop at your customer site.  They do not do an end-to-end test, which the
above procedure provides.  These test results will generally get the phone
company to fix the problem.