Configuration of Data and Power loops

The Family of Remote Function Modules (RFM) enables the construction of simple data loops with minimised cabling.

RFM-ISR is a combined Power and Communications barrier Module that is mounted in the Safe area and protects all Hazardous area equipment in accordance with ATEX regulations.

A typical installation will have several RFM-ISR modules in the safe area, each protecting equipment connected to it and feeding the communication data to the central Monitor via a 2-wire RS485 Data bus.

The data cabling from each RFM-ISR is configured as a closed loop for redundancy, meaning that a cable failure at any single point does not cause loss of communication.

The RFM-ISR also includes input / output buffers for communications. These effectively isolate each of the Sub Level Loops from the Top Level Loop, meaning an issue with one loop does not affect all instruments. They also provide a local booster for reliable communication in “noisy” environments.

Since the RFM-ISR’s primary function is to limit to safe levels the amount of power into the hazardous area (output is 300mA @ 12V), the number of instruments that may be connected per RFM-ISR depends upon what they are and what power requirements they have.

The iCT transmitter has a very low power requirement of less then 3mA when run in digital mode only. In this mode up to 100 iCT’s can be connected to a single RFM-ISR. Radar or other 4-20mA Transmitters require more power for operation (100mA +) and so only 2 transmitters (and up to 8 iCT’s in digital mode) can be connected per RFM-ISR.

The Radar transmitters are connected to the data and power cabling via an RFM2-A module. This unit provides a local increase of the loop voltage level of 12V to supply sufficient voltage for powering the Radar transmitter. RFM2-A then converts the radar’s level signal to the Modbus communication protocol used on the network.

The RFM1-1 and RFM1-4 are simple termination modules which provide a means of connecting the iCT into the data / power network.

The RFM1-1 accepts a single iCT and for where a number of iCT’s are in the same location the RFM1-4 will accept up to 4 iCT’s. 

The RFM1-1, RFM1-4, and RFM2-A are available in an IP67 Galvanized steel enclosure suitable for mounting in harsh environments.

Where equipment is mounted in the safe area it may be connected directly to the Top Level Loop, it is not necessary to use an RFM-ISR barrier.

The overall Top Level Loop may have any number of RFM-ISR, RFM1-1, or RFM1-4, units connected to it. The only limitation is that the total number of connected instruments may not exceed 247, and each data cable loop should not exceed 1500 metres in length.

For applications needing more than 247 measurement points a second network can be constructed using a different serial port on the Master.