Intelligent Technologies Modbus to QCPort Adapter Manual
January 2006
MN05002002E (C)
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Page 43
Pass-Through using Registers
Modbus pass-through messages can be transmitted by writing to the Modbus Pass-
Through special function registers. The first register in this series (offset 0) specifies the
number of bytes in the Modbus message (CRC should not be included), and the second
register in the series is the start of the message. The Modbus CRC must not be included
as part of the message, as the Modbus Adapter calculates and appends the CRC to the
message before transmission. Due to the fact that the message length register triggers
the Modbus pass-through message transmission, the Modbus frame registers (register
1..n) should be written first, followed by the message length. All of the registers can be
written at the same time if the “write multiple registers” command is used. Once the
Modbus Adapter has queued the message for transmission, the message length will be
set back to zero. Because of this feature, the message length should always be read
before any data is written to the Modbus pass-through special function register area. If
the message length is a non-zero number, this will indicate that a Modbus pass-through
transmission is still occupying the register space and has not yet been queued for
transmission.
Table 42 Pass-Through Register Information
Description Modbus
Register
Size
(Reg)
Usage Read/Write
Modbus Wormhole Tx
Channel A
12395
251
Modbus wormhole transmit
area A
R/W
Modbus Wormhole Rx
Channel A
12646
251
Modbus wormhole Receive
area A
R/W
Modbus Wormhole Tx
Channel B
12897
251
Modbus wormhole transmit
area B
R/W
Modbus Wormhole Rx
Channel B
13148
251
Modbus wormhole Receive
area B
R/W
Table 43 Modbus Ethernet Pass-Through Receive Message Format
Byte
CHA CHB Data
0
12395
12897
Modbus Message Byte Count
1
Unused
2
12396 12898 To
Address
3
Function
Code
4
12397 12989 Data
5
Data
6
12398 12990 Data
7
Data
n
Last
data
byte
If the Modbus wormhole transmission invokes a response on the destination port, the
received response will be placed in the Modbus Wormhole receive register area. The
format of the receive registers is identical to the format of the transmit registers except
that the Message length register will be replaced by a sequence number register
indicating the sequence # of the received message.
