OZONE ANALYZER BMT 964 Manual, Rev. 04/2021
53
Please note: It is not possible to use the RS-232 and the MODBUS interface at the same time.
When Modbus is disconnected, RS-232 is available again. Shield should connect to connector
housing, not pin 3.
Set Communication Parameters
Communication parameters can be set with the program BMT 964 Link, or, if the bridge be-
tween pins 1 and 2 of the MODBUS connector is detected, also on the front panel menu (Set
I/O -> MODBUS Parameters). The following can be set:
Link speed:
2400 Baud
4800 Baud
9600 Baud (default)
19200 Baud
38400 Baud
Parity:
None (default)
Odd
Even
MODBUS address can be set between 1 and 247. Default address is 203. These parameters are
stored in non-volatile memory.
MODBUS Operation
A MODBUS message contains an address, a function code, a register address, user data, and
a CRC error checking field. When a MODBUS slave receives a query with its address, it first per-
forms error checking (CRC and Parity check). It will provide an answer if error checks were suc-
cessful. If not the query will be ignored.
The instrument will then check query data, and, if invalid data is detected, will respond with a
MODBUS Exception. An exception has two fields that differentiates it from a normal response:
The high bit of the function code is set (function code + 80h), and the data field contains the
exception code.
The following exception codes are used:
Exception Code Definition Description
1 Illegal Function This function code is not supported
2 Illegal Data Address Start register is not available
3 Illegal Data Value Quantity of requested registers not supported
4 Slave Device Failure An illegal parameter was sent in the query
Note: The MODBUS standard defines exception code 4 as: "An unrecoverable error occurred
while the slave attempted to perform the requested action." In this implementation of the proto-
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