Rockwell Automation 1771-SPI,D17716.5.122 SPI PROTOCOL INTERFACE - Manual (Page 11)

Overview of an SPI Communication Network
Chapter 1
1-5
You establish the order (polling queue) in which the SPI module
communicates with devices on the network. You do this by listing device
ID codes in the module configuration command (MCC) block. When
entering this list, you may also choose to have:
each device polled once
one or more devices polled more frequently in the queue
the SPI module return system status (SYS) if desired
The SPI module communicates with its devices under two circumstances:
automatic polling
in response to a CDB command
Automatic Polling
Once the SPI module has successfully stored a custom configuration block
(CCB) for each device on its network (and for each device listed in queued
order in the MCC), the SPI module begins automatic polling to return
device status to the PLC processor as follows:
1. receives (polls) status from the first device
2. formats the status into a CDS and returns it to the processor by BTR
3. if it detects an error,
enters an error code in CDS word 3
clears status words of their values
4. goes to the next device
5. returns SYS (only if you listed SYS in the polling sequence)
6. when it receives a CDB command during automatic polling,
executes the command only after completing the current polling
sequence
In Response to CDB Commands
When the SPI module receives CDB commands addressed to one or more
devices on the SPI network, it stores them until it completes the current
automatic polling sequence. Then it:
1. sends the CDB to the first target device
2. if it detects no errors, returns the device’s CDS to the processor
3. if it detects an error or that it cannot find the device,
enters an error code in CDS word 3
clears CDS status words of their values
4. sends the next CDB to the next target device and repeats steps 2 and 3
5. returns to automatic polling after servicing the last target device
How the SPI Module
Communicates With Its
Network Devices