Installation guide

Communications Overview

890 USE 101 00 October 2006

RIO Network Communications

Overview Modicon’s RIO network is a high speed (1.544 Mbit/s) local area network (LAN) that

uses commercially available coaxial cable and CATV media technology. RIO

supports:

z Discrete and register data to input and output module communications

z ASCII message transmissions to and from certain RIO drop adapters

Data Transfer

Consistency

PLCs service their drop adapters at the beginning and end of logic segments when

Ladder programming is used, or, for IEC, before and after all sections have been

executed. Most data transfers between head and remote drop take less than 1 ms.

A CRC 16 message frame check assures that RIO messages will arrive reliably and

completely error-checked at the proper destination node.

How Messages

are Transmitted

A message initiated by the RIO head processor travels along the network’s cable

system and is received by all RIO adapters. The RIO adapter with the address

specified in the message can then transmit a response message back to the RIO

head within a specific time period. If the drop adapter does not respond, the same

message is sent again. The process of resending the message after no response is

called a retry.

If the adapter does not respond to several retries, the drop is declared dead. On

each successive scan of the PLC, the RIO head attempts to re-establish

communications with the adapter—only one attempt per scan will be made to

communicate with a dead drop until the adapter is successfully brought back up.

Predictable

Speeds for Time-

critical

Applications

As a high speed LAN, RIO must support applications that are very time-critical. In

this respect, RIO has several advantages over other proprietary PLC

communication methods. These advantages include:

z HDLC protocol implementation makes the RIO data transfer speed very

predictable

z The PLC services each node using a consistent communications method—the I/

O drops are always updated in a determinate time period that can be calculated

based on the number of segments or sections in the user logic program

z Only one node transmits at a given time, so message collisions do not occur—

each node is able to transmit on the network in a determinate time period

z RIO has high data integrity due to the frame check sequence and error checking

at the physical protocol layer

This document provided by Barr-Thorp Electric Co., Inc. 800-473-9123 www.barr-thorp.com