Intek RheoVac CMS - Manual (Page 28)

September 2013 24 © Intek, Inc. 2013
Revision D
SECTION 4 - COMMUNICATION METHODS
The CMS system supports Modbus TCP (Modbus over Ethernet), RS232/422 serial Modbus and
OPC communication protocols. Analog 4-20mA outputs can be supplied as an option. Note: Intek
recommends using the network connection for all data transmissions and CMS communications.
4.1 ANALOG OUTPUT
All 4-20 mA output signals are linearly scaled such that 4 mA represents 0% of the rated full
scale value (except temperature, which is 0°C) and 20 mA represents 100% of the rated full scale value
(temperature is 100°C). See SECTION 8 - CUSTOM INFORMATION for custom outputs.
4.2 MODBUS
Modbus is a communication protocol that can be used to read process variables from the CMS
system. This section demonstrates the basics of Modbus communication with the CMS.
Registers - Modbus stores variables in memory locations referred to as registers. Modbus is
capable of storing variables as coils, discrete inputs, input registers, and holding registers. The CMS
conserves memory space by not supporting coils or discrete inputs. This creates more space for holding
and input registers whose type definition fits more closely with the CMS process variables. This
modification does not affect the formation of a Modbus request packet, which is demonstrated in the
packet section. However, if an attempt to poll the removed register banks is made a timeout error will
occur.
Table 2: CMS input registers
Address Type Description
20 – 3999 PT
Read 16 bit registers (“input registers”)
4000 – 7999 FM Read 16 bit registers (“input registers”)
8000 – 11999 TC
Read 16 bit registers (“input registers”)
12000 – 15999 RV
Read 16 bit registers (“input registers”)
16000 – 19999 CC
Read 16 bit registers (“input registers”)
20000 – 23999 MA Read 16 bit registers (“input registers”)
CMS process variables are stored as IEEE 751 floating point numbers unless specified otherwise.
Therefore, multiple registers must be used to account for the size of the floating point variables. Modbus
protocol specifies that multiple registers containing floating point values are transmitted with the most
significant byte of the register first.
Table 3: Multiple register example
Address Variable Description
24 - 25 Temperature Temperature measured by the first referenced PT Probe