Yokogawa 212 Heat Calculator - Manual (Page 37)

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
Communications 35