Elgas EDT 101, User Manual

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4
Modbus Communication Protocol
The following text describes implementation of the Modbus communication protocol in the
transducer. In order to assure a complete understanding of the operation of the device, the
user should familiarize himself with the Modbus protocol in detail in the reference
literature [1].
Numeric values are given in decimal representation by default. If hexadecimal representation
is used, suffix “h” or “hex” is added.
4.1 Modbus Protocol Overview
The transducer is a Modbus compatible measurement device. The transducer supports 8-bit
Remote Terminal Unit (RTU) data transmission mode with a subset of the standard commands
used by the most Modbus compatible host controllers and a few proprietary commands.
4.1.1 Physical Communication Layer
Communication is carried over the RS 485 bus at baud rate 38400 bit/s. The transmitted
characters have 1 start bit, 8 data bits and no parity, 1 stop bit. These parameters are not
configurable.
4.1.2 Transactions on Modbus networks
The Modbus protocol uses a master/slave technique to control bus access. There can be one
master and up to 247 slave devices. Only the master can initiate a transaction. Transactions
are either a query/response or broadcast/no response. During the query/response transaction
master sends a query to the single slave which process it and responds. During the
broadcast/no response transaction the query is processed by all slave devices but none of
them responds.
Queries, responses and broadcasts are transmitted as frames with a fixed structure. In RTU
mode, frames start with a silent interval of at least 3.5 character times. The first field then
transmitted is the address field followed by a function field, a data field, and CRC field.
Following the last transmitted character, a silent interval of at least 3.5 character times marks
the end of the frame. A new frame can begin after this interval.
The entire message frame must be transmitted as a continuous stream. If a silent interval of
more than 1.5 character times occurs before completion of the frame, the receiving device
ignores the incomplete message and assumes that the next byte will be the address field of a
new message.
Frame format
Start
3.5 char
times Address
field
1 char Function
field
1 char Data
field
n chars CRC
field
2 chars End
3.5 char
times