212-M-V2.1
34 Communications
4.3 RS485 WITH MODBUS RTU PROTOCOL
Modbus RTU is an industry standard protocol which allows the Model 212 to
be easily interfaced to DCS or PLC, or to computers running supervisor
software systems.
The Model 212 serves as a slave station in communications and includes the
RS485 hardware and implementation of the standard protocol according to
Modicon Modbus Protocol (RTU-mode) and as detailed in PI-MBUS-300 Rev F.
Implementation specific details are as follows.
4.3.1 Hardware
The Modbus connection is available on terminals 24/25 and links 4 and 5 must
be connected as described at the beginning of this chapter.
4.3.2 Protocol
The protocol uses asynchronous serial bit transmission in half duplex mode
with 1 start bit, 8 data bits and (1 parity bit + 1 stop bit) or 2 stop bits
depending on whether the parity check has been programmed to "NONE" or
not. The baud rate is programmable in range from 300 to 9600 Baud. The
parity check can be programmed to Even, Odd or None. Link service classes
SEND/CONFIRM and REQUEST/RESPOND are provided utilising frame formats
according to PI-MBUS-300.
In RTU mode, messages start with a silent interval of at least 3.5 character
time.
Following the last transmitted character, a similar interval of at least 3.5
character times marks the end of the message. A new message can begin
after this interval.
The entire message frame must be transmitted as a continuous stream. A
typical message frame is shown below:
ADDRESS FUNCTION DATA CRC CHECK
1 byte 1 byte N bytes 2 bytes
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