Hamilton Sundstrand Company Fuel Metering Valves XVG - Manual (Page 5)

Follow RS232 and RS485 standards for wiring and biasing of the communication
bus.
6 XVG/eXVG Modbus Communication Default Settings
The default settings for XVG/eXVG Modbus communication are as follows:
- Baud Rate: 19200
- Parity Mode: Even
- Number of Start Bits: 1
- Number of Stop Bits: 1
- Device Address: 1
7 Protocol
Modbus devices communicate using a master/slave technique, in which only one
device (the master) can initiate transactions. The other device (the slave) responds by
supplying the requested data to the master or by taking the action requested. The
master can address individual slaves or can initiate a broadcast message to all slaves.
Slaves will only return a message to requests that are specifically addressed to them.
However, all slaves will perform the action requested by a broadcast message, but
responses are not returned.
XVG/eXVG Modbus conforms to standard Modbus protocol and support Modbus
Remote Terminal Unit (RTU) mode (ASCII mode is not supported). With RS485, it
is possible to have multiple host masters and XVG/eXVG units on the same network.
However, the following protocol requirements must be observed:
- Only one host master can send a request message at a time.
- Each XVG/eXVG unit on the same network must have a unique address.
7.1 General Modbus Frame
Modbus protocol establishes the format for the master request message by placing
into it the device address, a function code defining the requested action, any data to
be sent, and an error–checking field. See Table 1 for a general Modbus frame which
is followed from left to right.
DEVICE ADDRESS
FUNCTION CODE DATA ERROR-CHECK
Table 1: General Modbus Frame
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