Host Responsibilities - Allen-Bradley 1784-KTX Reference Manual

Chapter 1
Understanding I/O Scanner Concepts

Host Responsibilities

Allen-Bradley Parts
1-8
The KTx scanner has built-in collision detection; an arbitrator governs
traffic in the dual-port RAM. When the KTx processor and the host
processor try to access the same dual-port RAM location simultaneously,
the processor that made the first request receives access.
The host has two primary responsibilities; it:
changes and monitors the flow of discrete data
issues scanner commands
Data Flow and Data Paths
Here is the path followed by a discrete input bit:
An external device causes an input terminal of a discrete input module
to turn "on."
When next asked by the adapter, the input module reports the new input
information. The adapter updates its internal input image table by
setting the bit corresponding to the particular input point.
When next scanned by the scanner, the adapter reports the new input
information. The scanner updates the input image table in the dual-port
RAM by setting the bit corresponding to the particular input point.
Host may read the dual port (discrete data) at any time.
The path of an output bit is essentially the reverse of the input path:
Your program sets a bit in its output image table. Your program knows
that this bit maps to an output terminal on a particular output module.
When the scanner next scans the adapter controlling the particular
output module, it tells the adapter to update its output image table with
the new information.
The adapter tells the discrete output module to update its outputs with
the new information.
The discrete output module turns on the output. Any external device
attached to the output module then becomes active.
For timing information, see Timing of Discrete I/O in chapter 7,
"Understanding Discrete I/O."