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Hardware Load has a special format.
The generation of paper tape in the hardware load format
is described in the section on programming the paper tape reader.
The Elapsed Time Clock (Q-counter)
The DATANET-30 is a real time data communications processor. Real time programs have
a periodic nature of operation.
The elapsed time clock (the Q-counter) provides an efficient
technique for achieving this.
The Q-counter is loaded by the program, and is counted down one each word time. This serves
as a word/time counter.
Q can be loaded with any number between -32 and +16,351.
If
loaded
with 16,351, this is equal to approximately 112 milliseconds.
When Q counts down to zero, a program interrupt is initiated, thus permitting the periodic
execution of programs at any period up to 112 milliseconds. The Q-counter may be used as
a relatively accurate real time clock by counting the number of program interrupts when they
occur.
For example, if a delay of 900 milliseconds is desired and the communication lines
are scanned every 12.5 milliseconds, then a count of 72 interrupts equals 900 milliseconds.
The Q-counter and Hardware Load
The Q-counter also serves as a reliability check on the system. When Q counts down to -32,
the DATANET-30 assumes a circuit failure and automatically initiates loading a restart pro-
gram by initiating hardware load.
Successful operation of the programs depends on preventing
Q from counting to -32 and reading in a restart program. This is achieved in the Program
Interrupt Routine by loading the Q-counter before it counts down to -32. Also, in the operate
mode, protection against a
11
dead loop" which includes an instruction to load the Q-counter,
has been achieved by counting the number of times the counter has been loaded since the last
program interrupt. Hardware load will be initiated upon execution of the second Load Q instruction.
This assures that the Program Interrupt Routine is executed periodically. The Program Inter-
rupt Routine may be written to check the program and initiate a hardware load if a fault is found.
This hardware-software feature provides a very adequate check on the proper operation of the
program.
In the event that certain programs do not require a periodic interrupt, this feature
may be inhibited by the Q-counter switch on the operating panel.
Upon the completion of loading the restart program, control is returned to the program and
the necessary details involved in the restart process are completed.
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I-23
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