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3. SPEclFlcATloNs
/MELSE
(2) Requests for communication transactions with the PC CPU may
also come from other sources, these are listed below and are
processed in the same way as AD51 transaction requests. Only
one transaction may be processed per PC CPU scan so that a
delay of
1
t o 5 scans is possible before the AD51 transaction is
processed i f several of these requests overlap. The following list
gives the transaction requests in priority order.
-
1. Programmable controller CPU OS program
2. Peripheral equipment (e.g. A6GPP)
3. Optical or coaxial data link unit incorporated in CPU
unit
A1 (E)CPUP21/R21
Hence, i f continuous processing requests are received from the
A6GPP and AJ71C24, communication between the AD51 and
PC CPU is only made once every three scans.
/
(3) When a system subroutine is called which accesses the PC CPU
there is a delay while the PC CPU prepares the appropriate data.
During this delay time, the AD51 switches tasks t o optimize
scan time. In the example shown below, three tasks are executing
subroutine which access the PC CPU. Task one provides the first
processing request t o the PC CPU which prepares the requested
data. During this delay the AD51 switches t o task 2 which is
unable t o pass, its processing request t o the PC which is still deal-
ing with the one from task 1. The AD51 therefore switches to
task 3 for which the same situation exists. Only after task 1s
request has been fully processed can task 2s request be dealt
with. Similarly task 3 must wait until task 2 been processed.
For details of other task switching, refer to the GPC-BASIC
-
Handbooks.
END
END
EN0
END
END
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