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If you receive faulty answers (Timeout Errors, Transaction ID Errors, Write Errors etc.), it may
be, that the polling cycle was defined to fast. This causes the non answered polling requests or
even a reboot of the CS121 through the integrated Watchdog, because the system is
overloaded. Further on it can come to delayed answers during the MODBUS over IP or rather
RS485 polling due to traffic into the bus or network, because the CS121 is a multi device,
which has to handle several tasks at the same time.
NOTE:
Please define a
response timeout
of at least 2000ms (at a fast MODBUS over IP or
RS485 network/bus or rather higher accordingly, e.g. 4000ms at slow connections). The
timeout has to be increased until the errors stay out.
E.2.
Available Modbus Function Codes
Implemented MODBUS functions in the CS121 M units:
01H
Read Coils
02H
Read Discrete Inputs
03H
Read Holding Registers
04H
Read Input Registers
05H
Write Single Coil
Which functions are supported depends on the connected UPS. For standard UPS only
functions 03H and 04H are available. In this case the CS121 M makes no difference between
function 03H and 04H. The baud rate is adjustable up to 38400 Baud.
Note:
Please note that the MODBUS adapter client allows a timeout of 40ms at
9600 baud for one value.
E.3.
Exception Codes
Except for broadcast messages, when a master device sends a query to a slave device it
expects a normal response. One of four possible events can occur from the master's query:
If the slave device receives the query without a communication error, and can handle the query
normally, it returns a normal response.
If the slave does not receive the query due to a communication error, no response is returned.
The master program will eventually process a timeout condition for the query.
If the slave receives the query, but detects a communication error parity, LRC, or CRC, no
response is returned. The master program will eventually process a timeout condition for the
query.
