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The Character/Word Unit (CWU) provides buffering for one character (5, 6, 7 or 8 bits) of infor-
mation between the DATANET-30 and the communications lines; or provides buffering for one
word (20 bits) of information between two DATANET-30 data communications processors. As
a character buffer, it is used on lines with transmission rates from 300 to 2400 bits per second
with stop-start asynchronous format (e.g., normal Teletype).
THE CHARACTER BUFFER CHANNEL (CBC 930). The character buffer channel provides the
interface between the DATANET-30 and a high speed remote terminal device over a half-duplex
transmission line. Two character buffer channels are needed for full duplex operation.
A character buffer channel buffers a character length bit stream where the length is determined
by the code of the remote terminal. Each CBC has a code level plug and a timing connector plug.
The code level plug is specially wired for the character length of each CBC. The standard
character lengths are 5, 6, 7 and 8 level code characters. System considerations require that a
CBC be used on lines operating at 300 or more bits per second. The timing connector plug for a
CBC would be wired for transmission speeds of 300, 600, 800, 1050, 1200, or 2400 bits per
second. The transmission rate for each CBC may be independently selected.
Receive Sequence of the Character Buffer. The following is a brief description of the operation of
an 8-bit character buffer channel in the receive mode. A word buffer channel will operate in
exactly the same way except for the number of data bits. Assume that the CBC has been put in
the receive mode by the program, that the receive flag is reset, and that the sending unit is trans-
mitting marks, (i.e., it is idle). Now the sending unit starts to transmit an 8-bit word. The word
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 there is shifted into the shift register.
When the shift register is full, the 8-data 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 CBC 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.
Since the CBC will transfer a character from the shift register 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 another one is transferred into the data register.
When the program takes the character from the data register, the data register and the receive
flag are automatically reset.
Transmit Sequence of the CBC. The following is a brief description of the operation of an 8-bit
CBC in the transmit mode. 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 character has been shifted onto the line, the CBC will transfer the character
in the data register to the shift register. At this time, the transmit flag will automatically be set.
The 8 bits transferred into the shift register will automatically be preceded by a start bit and
followed by 2 stop bits for transmission on the line, i.e., a total of 11 bits. When the shift register
is again empty, the CBC
will
transfer the character in the data register to the shift register and
repeat the process if the transmit
flag is reset. However, i(the transmit fhtg is still set,
iridicating that the
p:ro~r:;tll!
. has not put a new chctracter intc>'
th·e--data--regisfer~
the CB-c-·wm
-~U:.fi11tfetf.pu-CS-fo!J'"1Jils "{m-ar_~s) oI1. the . . line until the transmit flag is reset. When the prc»grain
transferira· -character int<:> Ure-data, register, the transmit flag will be automatically reset and the
above process will be repeated. For maximum line utilization, the program must test the transmit
flag and supply a new character before the current one has been completely shifted onto the line.
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