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PF2100

Modbus Card Instruction Manual v1.1

Jan 5, 2012

Page 16

Appendix A – Modbus Background Info

Modbus is an industrial control and monitoring protocol originally designed in the 1970s to connect a PLC (“Programmable Logic Controller”) or some other type

of Supervisory Computer with one or more RTUs (“Remote Terminal Units”). Modbus is a Master-Slave protocol where the An RTU typically encoding some

physical property such as temperature, pressure, pH, or flow rate into a digitally represented number and then sending it in a framed packet on the bus.

There are many different variants of Modbus such as Modbus RTU, Modbus ASCII, Modbus TCP, Modbus+, etc. The PF2100 Modbus Card only supports Modbus

RTU and the rest of this Appendix is dedicated to describing this variant.

There are four required components in a typical Modbus RTU Implementation:

1. Master – This device is the only device that is allowed to “speak” first on the bus. It will send read or write commands to one or more slave devices.

There can be only one Master on a Modbus RTU bus.

2. Slave – This device only “speaks” on the bus if first spoken to by a Master Device. There can be up to 247 slave devices on a single bus.

3. Wire – Three wires are required to be run between the master and all slaves: a pair of differential signal wires and a common ground wire. The bus

should be arranged as single bus configuration with short “drops” to individual devices located along the bus. Star or other wiring topologies are not

supported. Cat-5e twisted pair cable will result in the best performance especially for long cable lengths and high bit rates.

4. Terminating Resistors – The device at each end of the bus must be terminated using a terminating resistor. Many Modbus devices have these built in

which can be enabled or disabled using a jumper or other user selectable setting.

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