Glossary
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rack number A number assigned to a Rack according to the order that it is connected to the
CPU Rack, with the CPU Rack generally being rack number 0.
Rack PC A PC that is composed of Units mounted to one or more Racks. This configura-
tion is the most flexible, and most large PCs are Rack PCs. A Rack PC is the
opposite of a Package-type PC, which has all of the basic I/O, storage, and con-
trol functions built into a single package.
RAM Random access memory; a data storage media. RAM will not retain data when
power is disconnected.
RAS An acronym for reliability, assurance, safety.
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 parameter (table) A table of settings that specifies which words in the data links for a System are to
be refreshed for a particular PC. See common link parameter table.
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.
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.
response code A code sent with the response to a data transmission that specifies how the
transmitted data was processed.
response format A format specifying the data required in a response to a data transmission.
Restart Bit A bit used to restart a Unit mounted to a PC.
restart continuation A process that 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.
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 that has resulted in an error
message from the receiving device.
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.
rising edge The point where a signal actually changes from an OFF to an ON status.
ROM Read only memory; a type of digital storage that cannot be written to. A ROM
chip is manufactured with its program or data already stored in it and can never
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