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38 Series 90-70 Hot Standby CPU Redundancy User’s Guide – December 1993
GFK-0827
Note that the memory for the Shared I/O data that is stored at configuration must be
subtracted from the amount of memory on the configured expansion memory module.
The calculation for the size of the memory for Shared I/O data is (bytes of Input data
transfer plus bytes of Output data transfer plus 8 Kbytes for synchronization
information).
For example if the selected expansion memory is 512 Kbytes, and the Shared I/O data is
10 Kbytes of Input data transfer and 20 Kbytes of Output data transfer, then 10K + 20K
+ 8K Bytes = 38 KBytes is reserved for data transfers. This is subtracted from the 512
KBytes of available user memory (512K – 38K = 474K Bytes, which means that 474
KBytes is available for the user’s control program.
Table 6. Transfer Data Size
Reference Type Reference Size
Calculation for Number of Bytes
If point faults are DISABLED use the following calculations.
%I Bit
(%I references x 4 bits/reference) ÷ 8 bits/byte
%AI Word (%AI references x 2 bytes/reference)
%Q Bit
(%Q references x 4 bits/reference) ÷ 8 bits/byte
%M Bit
(%M references x 4 bits/reference) ÷ 8 bits/byte
%AQ Word (%AQ references x 2 bytes/reference)
%R Word (%R references x 2 bytes/reference)
If point faults are ENABLED use the following calculations.
%I Bit
(%I references x 5 bits/reference) ÷ 8 bits/byte
%AI Word (%AI references x 3 bytes/reference)
%Q Bit
(%Q references x 5 bits/reference) ÷ 8 bits/byte
%M Bit
(%M references x 4 bits/reference) ÷ 8 bits/byte
%AQ Word (%AQ references x 3 bytes/reference)
%R Word (%R references x 2 bytes/reference)
Interrupts
Interrupts cannot be ENABLED for Input Modules or VME modules when the
configured CPU is a CPU 780. The current configuration will be checked to ensure that
there are no modules that have interrupts ENABLED. If modules do exist in the current
configuration that have interrupts ENABLED and you configure a CPU 780, a prompt
will appear asking whether or not all module interrupts should be DISABLED
automatically. You cannot configure a Redundant CPU (CPU 780) until all module
interrupts are DISABLED.