Commands
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02
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Reading:
2. The identification system sends a response to the MODBUS master. If an error occurs, the
response contains an exception code.
7.3.4
General notes on creating the control program
TCP connection:
Many MODBUS masters enable the use of transactions accompanied by a TCP link
connection and disconnection. As outlined in the MODBUS specification, we recommend
maintaining as opposed to terminating the connection following a transaction.
Transaction timeout:
The timeout time heavily influences the load on the network you are using. 1000 ms can be
accepted as a guide value.
Transaction cycle time:
The repeat rate must be higher than the number of data carriers read per time unit (per
channel). A correspondingly short cycle time must be selected. The FIFO load register can be
monitored to determine whether the refresh rate of the controller is sufficiently high.
Cyclic reading and writing of registers:
Writing:
An identical identification command can only be executed again in succession if the first four
telegram bytes change. The toggle bit can be used for this. In this way, the controller can
transfer a register set several times during a cyclic data exchange without issuing a second
identification command unintentionally.
All identification commands contain a channel number. This is ignored when MODBUS/TCP is
used. The channel is defined using the register address only. The ident channel is still included
in the telegram when the register is read.
Reading:
The identification system only transfers a response to an identification command once. Note
therefore that the controller evaluates each telegram transferred via the bus. When using
enhanced
commands, the following procedure for distinguishing between old and new data is
recommended because the controller CPU cycle is usually quicker than the bus cycle:
1. Check whether a telegram is available: Telegram length > 0?
2. Is the reply counter for this telegram different to the previous one?
3. If the status set to 0?
Can all three cases be answered with "YES", is a new, valid answer available?
Start address
Writing start address
Number of registers to be read:
Maximum 125d
Table 7.14
Parameters required for a request
Note!
