In-Situ Aqua TROLL 400 Modbus and SDI-12 Reference Guide - Manual (Page 58)

• Function Code: 2 byte field with a value range 1-127 representing the standard or
extended function code in hex characters.
• Data Payload: 0-N bytes with response data from the device in hex characters. For an
error response, the payload will be a 1 byte value 1-255.
• LRC: 2 bytes represented in hex characters with a value computed mathematically from
the message bytes. This value can be computed on both sides of the link and verified to
ensure that the bits of the message were transmitted and received correctly.
• End Packet: the characters.
8.7 IP Message Format
The IP message format is based on the RTU format but eliminates the CRC because TCP/IP
will ensure that the message bytes are transmitted correctly. The instrument does not support IP
format. However, the protocol can be used through a connected telemetry device.
TCP/IP is an asynchronous protocol. The significance is that the device must send its response
to the address of the sender (computer). This differs from the serial RTU and ASCII formats
where the protocol assumes only a single master device (computer) which does not have an
address. For this reason, the IP format has a different header than the RTU serial formatted
message.
8.7.1 Computer (Master) Message Format
Xac ID Protocol ID Msg Length Device Address Function Code Data Payload
• Xac ID: 2 byte transaction ID to provide for asynchronous messages.
• Protocol ID: 2 byte field – always 0.
• Length: 2 byte field represents number of bytes following.
• Device Address: 1 byte field with a value ranging from 1 to 247. Broadcast address is 0.
• Data Payload: 0-N bytes with information required to complete the requested function
code operation.
8.7.2 Device (Slave) Message Format
Xac ID Protocol ID Msg Length Device Address Function Code Data Payload
• Xac ID: transaction ID echoed from message.
• Protocol ID: 2 byte field – always 0.
• Length: 2 byte field represents number of bytes following.
• Device Address: Echoed from message. No response for broadcast.
• Data Payload: 0-N bytes with information required to complete the requested function
code operation.
8.8 Data Addresses (Registers)
As shown in the section pertaining to message formats, the function code from the Master’s
message tells the Slave device what operation to perform. There are a number of standard
function codes defined by Modbus. These functions typically operate on atomic chunks of data
historically and generically called registers. A register is quite simply a data address. A Modbus
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