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Mnemonic
Operand
Word Times
'NES
I
1
NES 1
NES 2
NES
3
N~S
4
NES 5
NES 6
NES 7
NES 8-10
Receive flag set (data register contains a new character).
Transmit flag set (data register is ready for a new character).
Call in progress.
Request answer.
Data mode.
Carrier on.
Clear to send.
Not used.
LDT - Do not use.
SCN - Do not use."
Register 'Transfer
5,6,7, or 8 bits
8
RECEIVE OPERATION
5
R,
(TRA from R to _ _ )
The 5, 6, 7 or 8 bits as specified by the size
of the character buffer are transferred from
R.
The receive data buffer and flag are
reset (DEFl).
1
- - - - - - - - R (8,7,6, or 5 ".' 1)
Assume that the character buffer channel (CBC) has been put in the receive mode by the pro-
gram, that the receive flag is reset, and that the sending unit is transmitting a continuous stream
of marks.
(The line is in the idle condition.) The sending unit starts transmitting a character.
The character is preceded by a start bit (a space) and followed by a stop bit (a mark). When
the start bit is received, a clock is started. The clock is used to time the future sampling
of the line.
The start bit is shifted into the shift register. At regular intervals, the line is
sampled and the bit which is present at sampling time is shifted into the shift register. When
the shift register is full, the character bits are automatically transferred into the data register,
the receive flag is set, and the clock is stopped. The clock will start again and the above process
will repeat when the next start bit is received on the transmission line. As a protection against
noise on the transmission line causing the clock to start running, the character buffer circuitry
requires that the space condition 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. Since
the character buffer will transfer a word into the data register whether or not the data register
and receive flag are reset, the program must test .the receive flag and take the character before
[ID&\1f
&\~~1f
c:J
~@------------
V-22
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