GE GFK-0804B manual Appendix RTU Protocol, Introduction

Appendix

C

RTU Protocol

This appendix describes the Remote Terminal Unit (RTU) serial communications protocol, which can be used to provide communications between the Micro PLC or other remote device and a host computer.

HMessage Types

HTransmission Sequence

HMessage Fields

HCharacter Format

HMessage Termination

HTimeout Usage

HCyclic Redundancy Check (CRC)

HRTU Message Length

HRTU Table Addresses

HMessage Descriptions

HCommunication Errors

Refer to appendix E for a sample RTU master program.

Introduction

One of the software Setup parameters for the Micro PLC is the selection of either Micro PLC protocol or RTU protocol. RTU protocol must be selected to use the features described here.

RTU protocol is a query±response protocol used for communication between devices, such as the Micro PLC and a host computer. The host computer operates as the master device; the Micro PLC is always a slave.

The data transferred consists of 8±bit binary characters with a parity bit. No control characters are added to the data block; however, an error check (Cyclic Redundancy Check) included as the final field of each query and response to ensure accurate trans- mission of data.

GFK-0804B C-1