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The maximum number of lines which can operate simultaneously is dependent on the bit rate in
use in the system, the volume of traffic and other factors. The slower the transmission rate, the
higher is the number of remote stations that can be handled simultaneously.
More than one code level may be used within a bit buffer unit module. The program recognizes
different code levels in use in the same system.
Receive Operation. The following is a brief description of the operation of the receive section of
a bit buffer channel.
Assume that the terminal device is sending marks, i.e., it is idle. Now the
terminal device starts to send a bit stream (e.g., a character). 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 and a 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, trans-
ferring the data on the line to the receive data buffer, and setting the receive flag continues until
the clock is stopped by the program.
Since the bit buffer channel 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. 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 bit buffer channel 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 bit buffer 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.
TO
DATANET-30
REGISTERS
CONTROL
..,.
___ _
DATA
CONTROL
FLAG
RECEIVE
DATA BUFFER
RECEIVE LINE
---i
f"FIRsT
L__j
BIT
START
Figure 12. Receive Mode
Transmit Operation. The following is a brief description of the operation of the transmit section.
Assume that the program is not sending anything and that the transmit flag is set. This means
that the bit buffer is ready to take a new bit. The program sends a bit to the transmit data buffer.
This automatically resets the transmit flag. At regular intervals, the bit buffer transfers the bit
in the transmit data buffer to the transmission line. When this happens, the transmit flag is
automatically set by the bit buffer unit.
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