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When servicing transmission lines on serial bit stream basis there are certain timing consider-
ations which must be taken into account. In the table below is shown the service rate for the six
most common transmission speeds.
Bits per second
45
50
56.25
75
110
150
Service Rate
22.2 milliseconds
20.0 milliseconds
17.
7
milliseconds
13.3 milliseconds
9.09 milliseconds
6.
67 milliseconds
When scanning the bit buffers, the program initiates scanning at a rate of slightly faster than the
service rate.
For a 45 bit/ second transmission line having a service rate of 22.2 milliseconds,
the line would be scanned approximately every 21.0 milliseconds to insure that any speed varia-
tions in the remote terminal will not result in data lost at the DATANET-30.
BASIC PROGRAM CYCLE
A real time program response time to certain events must be very small. The communications
programs can be divided into the following sequence of events.
1.
Receive bits
2.
Assemble bits into characters
3.
Assemble characters into words
4. ·
Assemble words into blocks
5.
Assemble blocks into messages
6.
Assign message routing
The program to do this is divided into two basic cycles.
1.
Scan cycle (hardware scan) - when each buffer is sampled within a bit time and the bit
present is moved from the buffer into an assembly area.
2.
Processing cycle - when all the rest of the processing to be done by the program must
be accomplished.
Since a basic premise of the DATANET-30 is to service each line in time to receive (or transmit)
each bit within rigid time limitations, both the scan cycle and the process cycle must be com-
pleted within a certain amount of time.
interrupt
scan
process
SI
scan
process
I
full cycle
The time will vary with the line service rate required by the remote terminals. One full cycle
must therefore be completed at a rate slightly faster than the fastest service rate. In order to do
[ID/A\lr/A\~~lr
c=l
~®------------
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