GE 90-30 PLC Series Installation And Hardware Manual page 423

SNP Multidrop Troubleshooting
If you are having trouble connecting to a PLC or module over the multidrop system, check the
following:
GFK-0356Q Appendix I SNP Multidrop
Is there a problem with all PLCs or only one? Try connecting to other PLCs over the
multidrop system. If you cannot connect to any, check for a common problem such as a
defective cable. If you can connect to all but one PLC, use the direct connection method
described in the next paragraph. Also, if you only have a problem with the last PLC on the
multidrop link, the last section of cable may have a problem. Or, perhaps you can connect to
all PLCs up to a certain point, but none beyond that point. This would also strongly suggest
that there is a problem in a section of cable.
SNP ID may be incorrect. You may not be able to connect because you are specifying the
wrong SNP ID. If you are not sure of a PLC's correct SNP ID and would like to check it, you
can connect your programmer directly to the PLC's programmer port and read its SNP ID from
the software's ASSIGN PLC ID screen, (as described in "Assigning an SNP ID to a PLC with
Logicmaster," above). Make sure you change the programming software's connection method
to Direct for this test. When set for Direct connection, the software will communicate with a
directly connected PLC without regard to its SNP ID.
Communications settings may not match. If the PLC's serial port communication settings
and the programming software's communication settings do not match, they will not be able to
communicate. These settings include such things as BAUD Rate, Parity, Stop Bits, etc. If you
suspect this to be the problem, try connecting directly to the PLC as described above in "SNP
ID may be incorrect." If you cannot connect directly, there may be a communication settings
mismatch. If so, try setting the programming software to its default communication settings.
Multidrop may not be selected as the connection method. The default connection method
in the programming software is Direct, which requires that you be connected directly to a PLC
or module's programmer port. If this default setting is not changed to Multidrop, you will not
be able to connect to a selected SNP ID over a multidrop system.
You may have a hardware problem. Inspect the multidrop cable; it may be wired
incorrectly, damaged, or disconnected. A wire may be loose on one of the connectors. Also
check the status of the PLC you are trying to connect to. It may not be powered up; it may be
stopped; or it may have some other problem. Eliminate the PLC itself as a possible problem
by connecting your programmer directly to the PLC's programmer port. You should be able to
communicate with a PLC using this direct connection even if the programming software is
configured for Multidrop, as long as the SNP IDs match.
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