FieldServer FS-8700-66 - Manual (Page 20)

FS-8700-66 GE-SNP Driver Manual Page 20 of 28
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Appendix B. Troubleshooting

Every 2
nd
message Times Out A possible cause is that the poll frequency is so low that the time
between polls is greater than the Max Link Idle Time (of the PLC). If the time is exceeded the connection is
closed. Thus when the next poll is received by the PLC it ignores it and hence it times out in the
FieldServer. After timing out, the driver re-establishes the connection for the next poll.

Very slow Comms Rate A timer known as the T1 timer is the minimum time that must pass before a
response to a message can be sent. Both ends of a SNP connection have their own T1 timer (hard coded
in the driver). When a connection is made the timers are exchanged and the larger of the two is used.
The drivers T1 timer is set to a very small number. SNP message #6 indicates the size of the T1 timer. You
may need to configure the PLC to improve communications.

Multidrop RS-485 networks. SNP protocol specifies that there may only be one Master on a
network at a time. When using nameless Nodes (Node_ID=0) there may be only one PLC on the
connection.