GE IC697ALG230 Base Converter Module Expansion Bus

Expansion Bus

The bottom six terminals (35 through 40) on the ter

minal board on the Base Converter and Expander

modules make up an expansion bus for connecting

input Expander modules to the Base Converter mod

ule.  An analog multiplexer on the Expander module

acts as a switching circuit to connect analog inputs,

one at a time, to the A/D (Analog to Digital) converter

on the Base Converter module.

Input Sampling Techniques

The objective of the input sampling technique for the

analog subsystem is to provide 8 input channels on

the base module that have a fast (approximately 3 ms)

update rate and additional expander channels that are

updated less frequently, but have a lower cost per

channel.

Operation is such that the base module initially up

dates all eight channels plus one expander channel.

On each successive scan all eight channels of the base

converter are again updated - plus the next expander

channel in sequence.  After 16 analog input scans all 16

channels of the first expander have been sampled; on

the next scan, all eight base converter channels plus

the first channel of the next Expander module are

scanned.

This sampling technique continues until all available

expander channels (16 x number of Expander mod

ules) have been scanned, at which time the sequence

starts over.  The number of analog scans required to

include sampling of all expander channels is equal to

the total number of Expander modules x 16 (16 chan

nels per Expander module) in the system.

With no Expander modules present, each base con

verter channel is updated once every 2.4 milliseconds.

With one or more Expander modules present, this

update time increases to 2.8 milliseconds.

Each expander channel is updated every 2.8 x N ms

(where N = total number of Expander channels pres

ent).  Note that the scan sequence is free running and

it cannot be synchronized with any external event.

Also note that all inputs of an Expander module will be

scanned even if they are not being used.