Channel Scanning Sequence - Automation Direct DL405 User Manual

9–10
F4–08RTD 8-Channel RTD Input
Channel Scanning
Sequence
Before you begin writing the control program, it is important to take a few minutes to
understand how the module processes and represents the analog signals.
The F4–08RTD module supplies one channel of data per each CPU scan. Since
there are eight channels, it can take up to eight scans to get data for all channels.
Once all channels have been scanned the process starts over with channel 1. There
are ways around this. Later we'll show you how to write a program that will get all
eight channels in one scan.
Unused channels are not processed, so if you select only two channels, then each
channel will be updated every other scan.
Scan
Read inputs
Execute Application Program
Read the data
Store data
Write to outputs
Even though the channel updates to the CPU are synchronous with the CPU scan,
the module asynchronously monitors the RTD transmitter signal and converts the
signal to a 16-bit binary representation. This enables the module to continuously
provide accurate measurements without slowing down the discrete control logic in
the RLL program.
Scan N
Channel 1
Scan N+1
Channel 2
Scan N+2
Channel 3
Scan N+3
Channel 4
Scan N+4
Channel 5
Scan N+5
Channel 6
Scan N+6
Channel 7
Scan N+7
Channel 8