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Brüel & Kjaer Vibro │ Instruction
VC-8000 MPS
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21.7 Modbus Wiring
See section 9.1 Modbus Connections and Wiring.
21.8 Common Mistakes
There are a couple of settings on the Modbus configuration that are important to understand.
Serial Modbus Slave Address
If you are using a custom Modbus map – the slave address (Serial Modbus) is taken from the Modbus
map column from the imported spreadsheet.
If you are using a standard Modbus map – the slave address (Serial Modbus) is taken from the SAM
properties screen.
Modbus Register Writes
To reset (or Inhibit, or enable trip multiply) the rack using a Modbus write function, you must enable
“Allow Status Register Writes”. If it is not enabled the rack will return an error message when you try
to write to the register.
Allow Invalid Address
Be careful with the “Allow Invalid Address” setting; normally this should be turned off. However, it can
be handy when the DCS register mapping is not known (i.e. rack replacement scenario).
Normally, if the DCS requests a register that does not exists an error is returned. However, if “Allow
Invalid Address” is checked the VC-8000 will not give an error; it will simply return a data value of
zero for that register.
Register Address is Six Digits (300,001)
Most systems use 5 digits to represent the Modbus register address (i.e. 30,001). The VC-8000 uses
6 digits (i.e. 300,001). Some examples:
How VC-8000 interprets the address field
0x (Read Coils), Address 00,001
101
0x (Read Coils), Address 00,101
10,021
0x (Read Coils), Address 10,021
30,001
0x (Read Coils), Address 30,001
100,042
1x (Read Discrete Inputs), Address 00,042
300,001
3x (Input Register), Address 00,001
407,834
4x (Holding Register), Address 07,834
Table 21-7) Register address field
SAM (Main) Firmware
Older firmware versions (3.x) do not support write functions.
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