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CPU Processing Modes Section 6-8
Parallel Processing Modes
There are two different Parallel Processing Modes: Parallel Processing with
Synchronous Memory Access and Parallel Processing with Asynchronous
Memory Access.
■ Parallel Processing with Asynchronous Memory Access
In this mode, I/O memory access for peripheral servicing is not synchronized
with I/O memory access for program execution. In other words, all peripheral
servicing is executed in parallel with program execution, including memory
access. This mode will provide the fastest execution (compared to the other
modes) for both program execution and event processing when there is a
heavy peripheral servicing load.
■ Parallel Processing with Synchronous Memory Access
In this mode, I/O memory access for peripheral servicing is not executed in
parallel with program execution, but rather is executed following program exe-
cution, just like it is in the normal execution mode, i.e., following the I/O refresh
period. All other peripheral servicing is executed in parallel with program exe-
cution.
This mode will provide faster execution that the normal execution mode for
both program execution and event processing. The program execution cycle
time will be longer than that for Parallel Processing with Asynchronous Mem-
ory Access by the time required to refresh I/O for peripheral servicing.
The cycle times and peripheral servicing responses for Normal, Parallel Pro-
cessing with Asynchronous Memory Access, and Parallel Processing with
Synchronous Memory Access are listed in the following table. (These values
are for a program consisting of basic instructions with a cycle time of 10 ms
and with one Ethernet Unit. These values are provided for reference only and
will vary with the system.)
Note 1. Peripheral servicing includes event servicing (e.g., communications for
FINS commands) for Special I/O Units, CPU Bus Units, and Inner Boards
(CS Series only), as well as communications port servicing for the periph-
eral and RS-232C ports (but not including data links and other special I/O
refreshing for CPU Bus Units).
2. The CS1G/H-CPU@@-V1, CS1-H, and CJ-series CPU Units also support
a Peripheral Servicing Priority Mode that will perform peripheral servicing
at a fixed cycle during program execution. It will provide faster peripheral
servicing than the normal processing mode, but program execution will be
slower. Event response, however, will not be as fast as the Parallel Pro-
cessing Modes. Parallel Processing with Asynchronous Memory Access
should thus be used whenever response to events is to be given priority in
processing.
3. Peripheral servicing cycle time over errors can occur in the CPU Unit as
described in a) and b), below, when parallel processing is used. If this error
occurs, the display on the Programming Device will indicate that the cycle
time is too long, A40515 (Peripheral Servicing Cycle Time Over) will turn
ON, and operation will stop (fatal error)
Item Normal Mode Parallel Processing with
Asynchronous Memory
Access
Parallel Processing
with Synchronous
Memory Access
Cycle time Arbitrarily set to 1 0.9 0.9
Peripheral
servicing
Arbitrarily set to 1 0.4 1.0
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