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RECEIVE OPERATION
Assume that a remote terminal device is sending out a continuous stream of marks, (the line
is in the idle condition}. Then the operator at the remote terminal begins transmitting information.
When the start bit (a space} is received, a clock is started. The clock is used to time the future
sampling of the line.
The start bit is transferred into the receive data buffer by the bit buffer
channel (BBC}, and the receive flag is set. When the clock reaches the proper time, the line
is sampled again, the bit on the line is transferred to the receive data buffer, and the receive
flag is set.
This process of sampling the line at regular intervals, transferring the data on
the line to the receive data buffer, and setting the receive flag continues until the clock of the
BBC is stopped by the program.
Since the BBC will transfer the information from the line into
the receive data buffer every bit time, the program must test the receive flag and take away
the bit in the receive data buffer before the line is sampled again .by the BBC.
Whenever the bit is taken, the receive flag and the receive data buffer are automatically reset.
At some point, the program decides that the appropriate number of bits have been received and
sends a signal to the BBC which stops the clock. The receive flag will remain reset until another
start bit is received.
As a protection against noise on the transmission line causing the clock
to start running, the BBC circuitry requires the space condition to exist on the line for at least
one-half of a bit-time to start the clock. Thus, noise of less duration than one-half of a bit-time
will have no effect.
A BBC can be used with a half-duplex line by ignoring the receive section when sending and by
ignoring the transmit section when receiving.
If
a subset is used, control of the carrier is
accomplished by activating the appropriate external function driver (with a DEF instruction}.
The following timing diagram shows how the character Y would be received by a bit buffer as
a 5-level teletype character.
Receive Line
1
Clock
2
Flag
3
Data Buffer
4
Data Transfer
5
Reset Clock
DEF 5
Start
Pulse
1
n
0
1
n
0
I
I
Stop
I
1
f Pulsel
n
n
[ID£1J&~~!Yc:J ~@------------
V-9
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