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TRANSMIT OPERATION
Assume that the program has put the CBC in the transmit mode, the CBC is in the process
of sending a word out on the line, and a word is waiting in the data register. When the current
word has been shifted into the line, the CBC will transfer the word in the data register to the
shift register.
At this time, the transmit flag will automatically be set. The 5 bits transferred
into the shift register will automatically be preceded by a start bit and followed by 2 stop bits
when transmitted onto the line for a total of 8 bits. When the shift register is again empty,
the CBC will transfer the word in the data register to the shift register and repeat the process
if the transmit flag is reset. However, if the transmit flag is still set, indicating that the program
has not put a new word into the data register, the CBC will continue to put stop bits (marks)
on the line until the transmit flag is reset. When the program transfers a new word into the
data register, the transmit flag will be automatically reset and the above process will be re-
peated.
For maximum line utilization, the program must test the transmit flag and supply a
new word before the current word has been completely shifted onto the line.
1
DEF 4
-----~--------
2
Transmit Clock
3
Transfer
Data
n
n
Register to
Transmit-----~11
... _ _ _ _ _ _ _ _ _ _ _ __.I
J&.-__ _
Buffer shift register
Start
Data
Stop
+
r----------"'--~--, ~
4
Transmit Buffer
0
0
1 0 1 0
1
1
5
Transmit Line
6
Transmit Flag
Figure 19. CBC Transmit Timing Diagram
The timing diagram (Figure 19) illustrates graphically what happens when a 5-bit character
is transmitted onto a communications line by a character buffer channel.
[IDfillf&~~lf ~ ~@------------
V-24
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