Intek RheoVac 950A - Manual (Page 42)

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I:\OFFICE\WPMANUAL\RV950A Appendix C revB.doc Feb. 4, 2010
Serial Modbus for RheoVac Model 950
C.2.1 INTRODUCTION
Modbus protocol is a messaging structure developed by Modicon in 1979. It is used to establish master-
slave/client-server communication between intelligent devices. Modbus is used in multiple master-slave
applications to monitor and program devices, to communicate between intelligent devices and sensors and
instruments, to monitor field devices using PCs and HMIs.
The RheoVac Model 950 supports the Modbus protocol over RS-232, acting as a slave device. There are
many existing commercial products that function as a Modbus master, allowing for data collection from
the RheoVac Model 950 using serial Modbus. Detailed information on the Modbus protocol can be found
at
www.modbus.org .
C.2.2 SUPPORTED MODBUS FUNCTIONS
The RheoVac Model 950 supports the following Modbus function codes:
x 03 (0x03) Read Holding Registers
x 06 (0x06) Write Single Register
x 08 (0x08) Diagnostics
All of the RheoVac Model 950 data available via serial Modbus exists as Modbus registers (16-bit
values). Modbus coils (1-bit values) are not used in any way. Most of the registers are read-only, with
the exception of the Modbus server global settings discussed in Section C.2.3. Functions 03 and 06 are
used to read and write registers, respectively. Function 08 can be used to test connectivity, as it will
produce a reply from the RheoVac Model 950 identical to the sent packet.
C.2.3 MODBUS SERVER GLOBAL SETTINGS
A number of global settings are used to configure the RheoVac Model 950 serial Modbus server. Many
of these settings can be adjusted by the end user to facilitate integration of a RheoVac Model 950 with an
existing Modbus network. Table C2.1 lists the RheoVac Model 950 serial Modbus global settings and
their default values.
The settings can be modified in two ways: either by modifying a configuration file within the RheoVac
Model 950 or by sending a properly formatted Modbus Function 06 message to write the address listed in
Table C2.1. Refer to Section C.2.6 for more information on modifying the configuration file within the
RheoVac Model 950.
Ten different process variables (PVs) are produced by the RheoVac Model 950 serial Modbus server.
These values take one of the two formats: multiplied integer or single float. The “Conversion Mode”
(Register 0009) setting listed in Table C2.1 determines the data format used for the PVs. In multiplied
integer mode, the RheoVac 950 data values are multiplied by a power of ten specified in a set of
additional registers. The resulting values are then transmitted as 16-bit unsigned integers. In single float
mode, all of the data values are converted into a 4-byte hexadecimal string corresponding to their single-
precision float representation.