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2.4 Troubleshooting
·
Start with a simple network, one master and one slave. With Eastron Digital meter products this is easily
achieved as the network can be left intact whilst individual instruments are disconnected by removing the
RS485 connection from the rear of the instrument.
·
Check that the network is connected together correctly. That is all of the “A’s” are connected together,
and all of the “B’s” are connected together, and also that all of the “Gnd’s” are connected together.
·
Confirm that the data “transmitted” onto the RS485 is not echoed back to the PC on the RS232
lines.(This facility is sometimes a link option within the converter). Many PC based packages seem to not
perform well when they receive an echo of the message they are transmitting. SpecView and PCView
(PC software) with a RS232 to RS485 converter are believed to include this feature.
·
Confirm that the Address of the instrument is the same as the “master” is expecting.
·
If the “network” operates with one instrument but not more than one check that each instrument has a
unique address.
·
Each request for data must be restricted to 40 parameters or less. Violating this requirement will impact
the performance of the instrument and may result in a response time in excess of the specification.
·
Check that the MODBUS Protocol mode (RTU or ASCII) and serial parameters (baud rate, number of
data bits, number of stop bits and parity) are the same for all devices on the network.
·
Check that the “master” is requesting floating-point variables (pairs of registers placed on floating point
boundaries) and is not “splitting” floating point variables.
·
Check that the floating-point byte order expected by the “master” is the same as that used by Eastron
Digital meter products. (PCView and Citect packages can use a number of formats including that
supported by Eastron Digital meter).
·
If possible obtain a second RS232 to RS485 converter and connect it between the RS485 bus and an
additional PC equipped with a software package, which can display the data on the bus. Check for the
existence of valid requests.
3 MODBUS Protocol General Information
Communication on a MODBUS Protocol Network is initiated (started) by a “Master” sending a
query to a “Slave”. The “Slave“, which is constantly monitoring the network for queries
addressed to it, will respond by performing the requested action and sending a response back
to the ”Master”. Only the “Master” can initiate a query.
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