Practical Industrial Data Networks:Design, Installation and ...

136 Practical Industrial Data Networks: Design, Installation and TroubleshootingTerminating stations• Install a 150-ohm resistor at the first and last station connectors on the trunkline.• Use a one inch length of heatshrink to protect resistor and connect betweenterminals 1 and 3.9.2.3 Data Highway Plus network diagnosticsThere are literally hundreds of errors; but there are a few important ones worthhighlighting in this section.Many of them are the result of excessive noise on the network and can be corrected byexamining the actual wiring and removing the source of noise, if possible. If not (forexample, due to the data highway running parallel to a power cable in a cable tray),consideration will have to be given to the use of fiber cabling as a replacement for thecopper-based cable.A few errors (which are identified in the diagnostics registers on the interface module)worth mentioning are:1. ACK Time out. This indicates where a sender has been transmittingpackets and has not received any acknowledgement responses. This isoften due to long cable or the presence of reflections and low level noise.2. Contention. This occurs on noisy or over length cables. A similar remedyto the one above.3. False poll. This indicates the number of times this station has tried torelinquish bus mastership and the station that was expected to take overdidn’t do so. This is often the case on a noisy highway.4. Transmitted messages and received messages. While these are not errors,they are worth examining (and recording) to see the level of activity(number of packets sent and received) against the level of errors.5. Data pin allocation. Not all Data Highway Plus equipment use the same pinconfiguration on the 15-pin connector. With certain types of Allen BradleyPLC the allocation of pins 6 and 8 are transposed.