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1.
When a start pulse is received the clock in the receive unit is started and the line is
sampled in the center of each bit period of the character.
2.
The receive flag is set when the line is sampled and the bit is sent to the receive data
buffer.
3.
The data buffer temporarily stores the bit which has just come in from the line.
4.
The program tests to see if the flag is set.
If
it is, the program will transfer the bit
to a register.
Transferring the bit will automatically reset the receive flag and data
buffer by issuing a DEFl instruction.
5.
After the complete character is received the program initiates a DEF5 instruction
which resets the clock. The clockwillnot be set again until another start bit is received.
TRANSMIT OPERATION
Assume that the program is not transmitting and that the transmit flag is set. This means that
the BBC is ready to take a new bit from the program. The program sends a bit to the transmit
data buffer. This automatically resets the transmit flag. At regular intervals, the BBC transfers
the bit in the transmit data buffer to the transmission line. When this happens, the transmit
data buffer shifts a bit onto the line, whether or not a new bit has been supplied. The program
must test the transmit flag and provide a new bit before this transfer occurs. This process will
repeat for each bit in the bit stream. At the end of the bit stream, the last bit will remain in
the transmit data buffer and will be transferred to the line regularly. Therefore, the last bit
in a bit stream will be a 1, so that the line remains in the mark condition when no information
is being transmitted.
Note that with a BBC the length of the bit stream is completely under
program control.
The next diagram illustrates how the character R would be transmitted to a communications
line.
The character R would be represented in memory as 11101010, where the right-hand 0
is the start bit and the two left-hand 1' s are the stop bits. The 5 bits in between the start bit
and stop bits represent the 5-level teletype code for the letter R.
1.
The transmit clock occurs every bit period as specified by the data timing unit.
2.
The transmit flag is set each time the transmit clock occurs and is reset when the
data is transferred to the transmit buffer.
3.
When the program finds the transmit flag set, it transfers the next data bit to the BBC,
which automatically resets the transmit flag.
4.
This shows how the transmit buffer would look over a period of one character time.
5.
This shows the signal as it appears on the line.
@&u&~~uc:J ~@------------
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