Communications Options User’s Guide
1
Chapter 1. Modbus Communications
1. Modbus Communications
1.1
Introduction
Your flowmeter hardware and software (
GC3E.MBS
)
have been modified to provide improved MODBUS
communications. The MODBUS option card provides an RS485 interface with a host system, while the main circuit
board continues to support RS232 communications for use with a PC running PanaView™ software.
Note:
PanaView™ does not support Modbus.
To properly set up the instrument, use this addendum along with the standard flowmeter
User’s Manual. This
document shows how to install the MODBUS option card and how to program the modified flowmeter to access this
special feature.
When equipped with the optional MODBUS output card, the flow transmitter can send flow data and diagnostic
information to a flow computer (or SCADA) serially, using a Gould-type RTU protocol. In this case, only the MODBUS
function command, 3 (read multiple registers), 6 (write multiple registers) is valid. The format for the data exchange
is as follows:
•
The
send
command (initiated by the host flow computer or controller) comes in the form:
[time delimiter]<Addr><3><First Register MSB>
<First Register LSB><Register Count MSB>
<Register Count LSB><CRC Low><CRC High>[time delimiter]
•
The response (initiated by the host flow computer or controller) comes in the form:
[time delimiter]<Addr><3><Byte count><Data.........>
<CRC Low><CRC High>[time delimiter]
The format for the returned data types is as follows:
•
Integer (16 bit Integer) <MSB><LSB>
1 Register - 16 bit integer
•
Integer (32 bit IntegerI) <MSB><LSB><LSB><LSB>
2 Registers - 32 bit long integer
•
Floating Point (FP) <EXP><MAN><MAN><MAN>
2 Registers - 32 bit IEEE floating point number
