MODBUS - INTRODUCTION

Because it is so commonly used, because it is so limited, because some vendors went to a lot of trouble and because some vendors hired bad programmers, Modbus, as simple as it seems, can offer lots of complications. Modbus was invented to transfer data as well as to program/configure PLC’s. For the purposes of this article, we are only interested in the data transfer functions.

There are 4 types of data

Holding Registers

An area of 16 bit words. Intended as read / write. Originally used as programmer scratch pad area and for analog outputs in old Modicon PLC’s. Also known as 4xxxx registers (xxxx is the place holder for the specific holding register’s point number).

Input Registers

Think Analog inputs. 16 bit words.
Also known as 3xxxx registers (xxxx is the place holder for the specific input register’s point number).

Inputs

Think Binary inputs.
Also known as Inputs.
Also known as 1xxxx inputs (xxxx is the place holder for the specific input’s point number).

Coils
Think Binary outputs. Named coils after the coil in a relay which is activated to energize a circuit. The original PLC’s were relay replacement machines.
Also known as Outputs.
Also known as 0xxxx inputs (xxxx is the place holder for the specific input’s point number).

5 Digit VS 6 Digit Addressing