Intelligent Technologies Modbus to QCPort Adapter Manual
January 2006
MN05002002E (C)
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Page 41
Pass-Through On Ethernet using port 2000 and 2001
It is possible to connect Modbus RTU slaves on QCPort Channel A and Channel B while
QCPort devices are connected and scanning. This Pass-Through feature will slow the
scan rate for that channel so it is recommended that if this feature is used, that all the
Modbus devices be connected to a channel that is not scanning QCPort devices.
The format for Port 2000 and 2001 will be the same as the format used for standard
Modbus TCP messaging (Modbus frame pre-pended with a MBAP header). The
difference between Port 502 Modbus messages and Port 2000 and 2001 Modbus
messages will be with routing, the port 2000/2001 Modbus messages will be stripped of
the TCP frame and sent to the appropriate channel. A Modbus CRC will be automatically
added to the end of the message, and the message will be transmitted on the
corresponding port. If a response is received on the serial port before the receive
message timeout, the response will be formatted as a Modbus TCP message, and sent
back to the Modbus master. If no serial receive message is received before the timeout
period, an ACK will be sent back to the Master indicating a receipt of message.
Notice
Port 2000 and 2001 use the unit ID from the MBAP header when constructing the
serial Modbus frame.
With in the control program, connect to Port 2000 for Channel A or Port 2001 for Channel
B. To read/set a register of a specific device, send the message to the Modbus Device
ID to read/set and construct the functions and data just as if communicating directly to
Modbus slave device. The data in the response field will constructed just as if the
Modbus master was directly communicating to a Modbus slave. The message is
constructed low byte high byte, but gets sent on the wire low high byte low byte, refer to
the example below.
Table 40 Modbus Ethernet Pass-Through Transmit Message Format
Address
xx HEX
Function Code
xx HEX
Data
Starting address HI
xx HEX
Starting address LO
xx HEX
Data HI
xx HEX
Data LO
xx HEX
