Output Override and I/O Update Instructions
Chapter 7
74
Figure 7.2
Scan Sequence
I/O Scan
Performs I/O
Updating
(Typically
0.5ms/128 I/O)
Start of
Program
Instruction
End of
Program
Instruction
Program Scan,
Instructions
(typically 6ms/1K)
Upon power up, the processor begins the scan sequence with the program
scan and then the I/O scan. During the I/O scan, data from the input
modules is transferred to the input image table. Data from the output image
table is transferred to the output modules.
After completing the I/O scan, the processor begins the program scan for
the second time. Here, all user program instructions are scanned and
executed in the order in which they were entered except where jumps and
subroutines are used. The scan sequence when jumps and subroutines are
employed is described in those sections.
The I/O scan and program scan are synchronously performed in a local
system, one after the other. The time required to complete both scans is
typically 6 msec/1K instructions plus 0.5 msec/128 I/O Rack. Refer to
Chapter 5 for remote systems.
It is clear that 40-50 msec may pass before I/O data is updated with a 16K
memory. The purpose of the immediate I/O instructions is to interrupt the
program scan to update a word of critical input data or output device
response in advance of the normal update sequence.
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