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7.1 Communication protocol MODBUS RTU
Transmission service
The master station is the primary station that initiates all information transfers. The
satellite stations are secondary stations that only transmit information when re-
quested.
Transmission speed
The transmission speed can be 1200, 2400, 4800 or 9600 Baud. The transmission is
asynchronous RS485 with one start bit, 8 data bits and one stop bit. Standard trans-
mission speed is 9600 baud.
After changing the transmission rate, the MDW/MDH must be restarted to accept the
change. You can do this in two ways:
1. Disconnect the MDW / MDH from the power supply (pull the fuse briefly)
2. Via button combination, press and hold the buttons
▲ and ▼ together for approx.
8 sec.
Address
The addresses 1 to 255 are reserved for the 255 secondary stations.
Request/ Answer
Public function code 03h - read holding registers
The master sends the public function code 03h (read holding register), start address,
number of the register and the address of the secondary station.
Address range:
0x00
unsigned long manufacturer number
0x02
unsigned long volume
∑
0x04
unsigned long
0x06
unsigned long volume
–
0x08
unsigned long user volume
0x0A
signed long flow
0x0C
Error Code*