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   An Overview of the MOD-MUX System...


MOD-MUX is an innovative modular I/O system which provides a simple low cost solution for distributed I/O requirements. The MOD-MUX system consists of stand-alone Digital and Analog Input and Output modules which are connected together on a RS485 two wire multi-drop network using the MODBUS protocol. Two baud rates are selectable, 9600 and 19200 baud. Typical response times are under 50ms.

All MOD-MUX modules plug into industry standard DIN rail mount 11 pin relay bases. All modules have isolation between the field and logic.

There are a number of configurations in which the MOD-MUX modules may be used in a system. Some are listed as follows:

Simple Point to Point I/O

This is the basic configuration were a maximum of 8 Digital or Analog Inputs will be transmitted to 8 Digital or Analog Outputs at a remote location on the network. The primary advantage of this configuration is cost saving where 9 wires are replaced by a single twisted pair. It may also be used on existing installations to avoid having to install additional cabling for expansion purposes.


Complex Point to Point I/O

This configuration allows for a larger number of I/O to be connected to the network at random points on the network. A maximum of 32 Digital Input Modules and 16 Analog Modules may be placed on the network, and the inputs are transmitted to a corresponding number of Digital or Analog Output Modules. In this configuration the Modules are set to operate in the MODBUS Slave mode and a Master Module called an I/O MAPPER is used to transmit the Inputs from the Input Modules to the Outputs on the Output Modules.

I/O Expansion

There are a number of devices such as PLC’s (Programmable Logic Controllers) which have a MODBUS Communications facility available. When configured as a MODBUS Master, and attached to the RS485 network, MOD-MUX Modules may be used as remote I/O reducing cabling costs and increasing the I/O capability of the PLC.

Data Acquisition

Another use of the MOD-MUX Modules is for Data Acquisition where a PC (Personal Computer) is connected to the Network. Many SCADA software packages support the MODBUS Master Protocol and can hence retrieve data from Input Modules or send data to Output Modules. The serial port of the PC is connected to an RS232/RS485 Converter which in turn is connected to the Network.


Procon has developed some unique modules which allow up to four Modbus masters to be connected to the same Modbus network. These Modbus masters take turns on the network to avoid corruption of messages. The technique used is token passing. The Modbus slave modules (I/O Modules) ignore the tokens and are standard Modbus Slave devices. The advantage of the multi-master system is to enable more than one PC to be connected to the network, or for a PLC to be connected for control purposes while a PC is connected for monitoring purposes.




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