Yokogawa 212 Heat Calculator - Manual (Page 34)

4.1 METER-BUS
The Model 212 serves as a secondary station in communications and includes the
Meter-bus hardware and protocol according to IEC 870-5 and as detailed in
EN 1434-3 with variable data format for reading out all parameters.
Hardware
The Meter-bus connection is available through galvanically isolated terminals
24/25 (irrespective of polarity) or optical interface (optional). The levels of input
voltage/output current are as detailed in EN 1434-3.
Bus Address
The bus address of the Model 212 is programmable in range between 0 to 250.
Some addresses are reserved according to EN 1434-3 and have a special
meaning:
0 default address for unconfigured meter
251...253 reserved
254 answer regardless of own address
255 broadcast, no response.
Protocol
The protocol uses asynchronous serial bit transmission in half-duplex mode with
1 start bit, 8 data bits, 1 even parity bit, 1 stop bit and programmable baud rate
(300 to 9600 baud). Link service classes S2 (SEND/CONFIRM) and S3
(REQUEST/RESPOND) are provided utilising frame format class FT1.2
according to IEC 870-5.
This protocol incorporates a frame counter bit (FCB) which has to be toggled by
primary station if the last communication was undisturbed. The Model 212 keeps
a copy of the last transmitted CONFIRM or RESPOND frame in order to
retransmit it if the next FCB has not been toggled.
32 Communications