Glossary
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RAS An acronym for reliability, assurance, safety.
read-only area A memory area from which the user can read status but to which data cannot be
written.
refresh The process of updating output status sent to external devices so that it agrees
with the status of output bits held in memory and of updating input bits in memory
so that they agree with the status of inputs from external devices.
refresh period The period during which memory status is actual read and written, e.g., during
the I/O refresh period, input bit status is written to agree with input terminal status
and output terminals are changes to agree with output bit status.
Register Area A memory are that contains both index registers and data registers.
relay-based control The forerunner of PCs. In relay-based control, groups of relays are intercon-
nected to form control circuits. In a PC, these are replaced by programmable cir-
cuits.
remote I/O word An I/O word allocated to a Unit in a Remote I/O System.
reserved bit A bit that is not available for user application.
reserved word A word in memory that is reserved for a special purpose and cannot be accessed
by the user.
reset The process of turning a bit or signal OFF or of changing the present value of a
timer or counter to its set value or to zero.
Restart Bit A bit used to restart a Unit mounted to a PC.
restart continuation A process which allows memory and program execution status to be maintained
so that PC operation can be restarted from the state it was in when operation
was stopped by a power interruption.
result word A word used to hold the results from the execution of an instruction.
retrieve The processes of copying data either from an external device or from a storage
area to an active portion of the system such as a display buffer. Also, an output
device connected to the PC is called a load.
retry The process whereby a device will re-transmit data which has resulted in an er-
ror message from the receiving device.
return The process by which instruction execution shifts from a subroutine back to the
main program (usually the point from which the subroutine was called).
reversible counter A counter that can be both incremented and decremented depending on the
specified conditions.
reversible shift register A shift register that can shift data in either direction depending on the specified
conditions.
right-hand instruction See
terminal instruction
.
rightmost (bit/word) The lowest numbered bits of a group of bits, generally of an entire word, or the
lowest numbered words of a group of words. These bits/words are often called
least-significant bits/words.
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