In-Situ Aqua TROLL 400 Modbus and SDI-12 Reference Guide - Manual (Page 56)

• ASCII format is required for any kind of wireless serial communications because it
eliminates the message timing requirements needed for RTU mode. Message timing can
be erratic over a wireless link.
• IP formatted messages can be used when the messages are transported using a
secondary transport protocol such as TCP/IP. In this case the secondary transport
protocol ensures that all of the packet bytes are transported correctly. Additionally, this
format provides for packet sequence numbering because the secondary transport layer
eliminates the synchronous Master/Slave nature of the serial RTU/ASCII formats.
8.3 Protocol Overview
Modbus provides message structures to read and write data to/from a device. The protocol also
provides for the extension of the protocol to permit customization of the message structures.
The instrument manufacturer adheres to the standard read/write message structures in its
implementations. Modbus does not provide as part of the standard, a suitable file transfer
message structure. For this function, the manufacturer has used a protocol extension to satisfy
the downloading of data files from the device.
8.4 General Message Formats
The general message format consists of a device address, a function code, a data payload and
a message validity checksum. The message format for each of the three modes changes as
described below.
8.5 RTU Message Format
The RTU message format allows the transmission of bytes of data encompassing the full range
of values per byte 0-255. There are no characters indicating the start or end of the packet. The
end of packet is signaled by a time delay equivalent to a 3 byte transfer time on the
communication link without any data transmitted.
8.5.1 Computer (Master) Message Format
Device Address Function Code Data Payload CRC
• Device Address: 1 byte field with a value ranging from 1 to 247. Broadcast address is 0.
• Function Code: 1 byte field with a value range 1-127 representing the standard or
extended function code. The function code tells the device what operation is to take
place (i.e. Read/Write).
• Data Payload: 0-N bytes with information required to complete the requested function
code operation.
• CRC: 2 bytes 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.
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