
G4 Multi Channel Force Instrument
7-1
7. Communication
General
The G4 Instrument has two serial communication ports, one Ethernet port and an
optional fieldbus module.
The serial communication ports, the Ethernet port and the optional fieldbus
interface module are used for communication with a control unit.
Serial interface
The instrument is equipped with 2 serial communication ports: COM1, COM2.
COM1 is a RS-232 port and COM2 is a RS-485 port
The COM2 serial communication utilises RS-485 for 2-wire or 4-wire. RS-485 is an
interface working with differential voltages, giving a noise resistant transmission in
a network with several units and long distances. The host computer (master) must
have an asynchronous communication port for RS-485, or use a converter
for RS-232 to RS-485 conversion or USB to RS-485.
If 2-wire transmission is used, the control unit must be capable of data flow
direction control or utilise a converter for automatic data flow direction control.
When 4-wire transmission is used, no data flow direction control is needed.
When the RS-232 port is used it’s possible to communicate with one instrument directly
from a PC with a RS-232 port without using a converter.
Modbus RTU Slave
General
All the G4 units connected to the network can listen to what is transmitted in the
network, but only one unit at a time may transmit. A time-sharing principle is
needed to allow communication in both directions (half duplex).
All communication in the network must be initiated by the control unit (master).
When the instrument is working together with a master the instrument units are all
slaves, only allowed to reply to master commands. As the master has addressed a
command message to a specific slave unit, it listens for the reply during a specified
time, before sending next command message.
If the reply from a slave unit fails it may be due to:
•
Mismatch in communication parameters. (baud rate, address, . . )
•
More than one slave unit has been transmitting at the same time.
This can distort the reply message and make it impossible to decode.
See ‘7. Communication
−
Modbus protocol’ section for detailed information of
register numbering, register content definition, commands etc.
More information about Modbus RTU can be found at ‘www.modbus-ida.com’ and
many other places.
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