Cameron NUFLO MC-III, User Manual

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C-1
MC-III™ Panel Mount Flow Analyzer Appendix C
Appendix C—Communications Protocol
Firmware Version 2.00
Register Table Version 1
Introduction
The communications protocol for the MC-III is in accordance with Modicon, Inc. RTU Mode Modbus
®
as
described in Modicon Modbus Protocol Reference Guide , PI-MBUS-300 Rev. J, June 1996. All registers are
implemented as 4X or holding registers. Reading of registers is implemented via function code 03H (Read
Holding Registers). Writing to registers is implemented via function code 10H (Preset Multiple Registers).
The instrument provides Enron Modbus
®
compliant downloads for hourly, daily and event records. For details
on Enron Modbus
®
, refer to Specifications and Requirements for an Electronic Flow Measurement Remote
Terminal Unit for Enron Corp. , Dec. 5, 1994.
Supported Commands
The Modbus
®
functions supported by the MC-III are as follows:
Function Code
(Hex)
Description
03 Read Holding Registers
05 Preset Boolean
(for Enron event record acknowledgement)
10 Preset Multiple Registers
For the read holding and preset multiple registers, the instrument supports the full 250 bytes of data in a
message. This corresponds to 125 registers in 16-bit holding register size and 62 registers in 32-bit holding
register size.
Data Types
Various data types are implemented in the MC-III. The following table lists the formats and the numbers of
bytes and registers associated with each type.
Data Type Byte Count Register Count
Floating Point (FP) 4 2
Floating Point (FP32) 4 1
Unsigned Word (U16) 2 1
Packed ASCII (PA) 2 1
The Floating Point (FP) type follows the IEEE-754 format and consists of 32 bits contained in two registers.
It is utilized for parameters and values that are not integers. For example, Instantaneous Flow Rate is a
parameter that is a floating-point data type. It can be interpreted by reading two registers, starting with register
17014.