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4.8.5
BUS_ Functions
Four program functions allow the PACSystems CPU to communicate with modules in the system.
Bus Read (BUS_RD)
■
Bus Write (BUS_WRT)
■
Bus Read/Modify/Write (BUS_RMW)
■
Bus Test and Set (BUS_TS)
■
These functions use the same parameters to specify which module on the bus will exchange data
with the CPU.
Note: Additional information related to addressing modules is required to use the BUS_ functions.
For open VME modules in an RX7i system, refer to the PACSystems RX7i User's Guide to
Integration of VME Modules, GFK-2235. For other modules, refer to the product
documentation provided by the manufacturer.
Rack, Slot, Subslot, Region, and Offset Parameters
The rack and slot parameters refer to a module in the hardware configuration. The region parameter
refers to a memory region configured for that module. The sub-slot is ordinarily set to 0. The offset is
a 0-based number that the function adds to the module's base address (which is part of the memory
region configuration) to compute the address to be read or written.
BUS Read
The BUS_RD function reads data from the bus.
This function should be executed before the data is needed in the
program. If the amount of data to be read is greater than 32767
BYTES, WORDS, or DWORDS, use multiple instructions to read the
data.
When BUS_RD receives power flow, it accesses the module at the
specified rack (R), slot (S), subslot (SS), address region (RGN) and
offset (OFF). BUS_RD copies the specified number (Length) of data
units (DWORDS, WORDs or BYTEs) from the module to the CPU,
beginning at output reference (Q).
The function passes power to the right when its operation is
successful. The status of the operation is reported in the status
location (ST).
Note: For each BUS_RD function type, use the corresponding data
type for the Q operand. For example, BUS_RD_BYTE requires
Q to be a BYTE variable.
Note: An interrupt block can preempt the execution of a BUS_RD
function. On the bus, only 256 bytes are read coherently (i.e.,
read without being preempted by an interrupt).
GFK-2950C
Chapter 4. Ladder Diagram (LD) Programming
February 2018
Mnemonics:
BUS_RD_DINT
BUS_RD_DWORD
BUS_RD_WORD
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