Owners manual

Modbus Protocol User Guide 23
6: Troubleshooting and Technical Support
Using the IAP Device Server firmware is normally easy. However, if a problem
occurs, it is difficult to troubleshoot without an in-depth knowledge of Modbus and the
system dynamics of polling.
Some general guidelines for troubleshooting:
 Start polling slowly and increase speed gradually.
 Cabling is the most common problem with device networking. If you have
created a custom cable, make sure your pinout is correct.
 Ideally, you should have the ability to watch the serial line communications.
Most host applications do a poor job of explaining errors. In many situations,
the host application declares “No response” when in fact, the device did
respond, and the application did not understand the response.
How fast can I poll?
First, remember that you still have the serial link in there and therefore cannot expect
to poll any faster than you could by a direct serial link. In fact, since you are adding a
number of queuing systems between your application and device, you may even lose
a bit of performance. For example, some download tests showed remote download
by Modbus/TCP bridged to Modbus/RTU ran about 20 percent slower than direct
download by Modbus/RTU.
Above all, remember that the serial speed (or baud rate) consumes the largest
amount of time (see the table below). Suppose you issue a Modbus poll for 125
registers. This requires a 255-byte response, which at 19.2kbps requires over 133
msec just to physically shift across the wire, while at 300 baud it takes nearly 10
seconds!