Rockwell Automation 1771-SPI,D17716.5.122 SPI PROTOCOL INTERFACE - Manual (Page 23)

Chapter
3
3-1
Defining Command and Status Blocks
You communicate with devices on the SPI network by transferring data
blocks between a PLC processor and an SPI module. In this chapter, we
present the structure of data blocks and show relationships between them.
In this chapter we cover:
command and status blocks
module communication command block (MCC)
system status block (SYS)
custom configuration block (CCB)
custom data block (CDB)
custom configuration status (CCS)
custom data status (CDS)
The processor uses command and status blocks to communicate with the
SPI module. The processor writes command blocks to the module with
block-transfer write (BTW) instructions, and reads status blocks from the
module with block-transfer read (BTR) instructions. The status block can
partially or totally reflect the data transferred in the command block.
SPI protocol uses three types of command and status blocks:
module-specific configuration block sent to the SPI module to configure
it, and an optional system status block returned by the SPI module
device-specific configuration blocks sent to the SPI module to configure
target devices on the SPI network, and configuration status blocks
returned by the device via the SPI module
device-specific data blocks sent to the SPI module destined for target
devices on the SPI network, and device status blocks returned by the
device via the SPI module
The SPI specification defines standard device types (types of devices) for
use on the SPI network by ID code. Command and status blocks for each
type of device have unique acronyms such as MCC for Module
Configuration Command (block) or CDB for Custom Data Block.
Chapter Objectives
Command and Status
Blocks