How The Scanner Scans Remote I/O - Allen-Bradley 1747-SN User Manual

1–4 Overview
How the Scanner Scans
Remote I/O
Publication 1747 6.6 - July 1996
The scanner communicates with each logical device in a sequential
fashion. First, the scanner initiates communication with a device by
sending output data to the device. The device then responds by
sending its input data back to the scanner, as illustrated below. You
refer to this exchange as a discrete I/O transfer. After the scanner
completes its discrete I/O transfer with the last configured network
device, it begins another discrete I/O transfer with the first device.
It is important to understand that the scanner transfers RIO data on a
logical device basis not on an adapter basis. A logical device is a
full logical rack or portion of a logical rack assigned to an adapter.
RIO Scanner Scan
The scanner updates its
input image file each time
it scans a logical device.
Scanner
Output
Input
Device 3
Image File
Input
Device 3
Output
Device 2
Scanner Output
Image File
Input
Device 1
Output
Device 1
Input
Device 2