Channel Scanning Sequence - Automation Direct DL405 User Manual

10–12
F4-08THM 8-Channel Thermocouple 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 F4–08THM 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.
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 thermocouple 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