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Paper tape may be read under program control in two modes, continuous mode at 300 cps or step
mode up to 50 cps. Five to eight-level tape may be read in the unit, but normally only eight-level
tape will be used. The paper tape reader is always buffer selector address zero.
Hardware Scan
Hardware scan is a special purpose communication oriented instruction.
It
is designed to greatly
reduce the amount of real time needed to service low speed communication lines attached to bit
buffer channels.
This is accomplished with hardware which does the same things as a group of
instructions which check for flags, receive bits for a character, and transmit bits for a character.
Hardware scan requires 3 word times to service a full duplex line.
After the channel number of the first line to service has been placed in the C counter, the program
initiates the hardware scan instruction.
When the hardware scan process is finished, the
DATANET-30 executes the instruction immediately following.
The program must examine, at a
character rate, the hardware scan's transmit character flags and receive character flags, take
new characters as necessary, and reset the appropriate character flags.
The hardware scan instruction is a unique feature of the DATANET-30 hardware and adds signi-
ficant value and higher performance with minimum equipment.
ELAPSED TIME CLOCK
The elapsed time clock (Q counter) can be loaded or read out under program control. The Q
counter is decremented by one each word time, and thus serves as a word-time counter.
The Q counter has three primary functions:
1.
To interrupt the program when a given amount of time has passed and transfer control to
another part of the program (the part which services the input/ output on a regular basis).
This is accomplished by loading Q with a number; the interrupt will occur when
Q
counts
down to zero.
The number is chosen to give the desired amount of elapsed real time.
2.
To serve as a "dead man" switch in conjunction with hardware load.
If
Q
is not reloaded
by the program interrupt instructions,
Q
will continue to count down for a given number
of additional word times and then a hardware load will automatically be initiated. This is
a method of detecting fault conditions in the operating program and automatically
initiating a restart.
3.
To measure the amount of "real time" used since the last time Q was changed. Q can be
used in this way to provide an inexpensive real time clock, with the appropriate program-
ming.
LINE SERVICE RATE
The service rate can be defined as the cycle time for the operation of the receive or transmit
flag of the bit buffer.
IDJ/A\lr/A\~~lf
c:J
~@------------
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