PDP11 PeripheralsHbk 1972 - Trailing-Edge

3. Receive Procedure.
The programmmer initializes the receiver by setting the Receiver Enable bit. The
Bit Assembly Table (BAT) and the Circular Buffer (CB) are used for receiving. Nei·
ther table need be set up by software. BAT need not be referenced by software; as
it is merely the place in which incoming characters are assembled. Once a charac·
ter is completely assembled it will be placed in the Circular Buffer and a character
done interrupt will occur, if the receive interrupt is enabled. The program must
then go to the Receiver Service Routine and service any entries in the Circular
Buffer. Bits 0-7 of the CB entries contain the received data character right justified.
The line number is contained in Bits 9-12. Bit 13 will contain a one if the received
character contained an odd number of one's and this can be used for parity
checking. Bit 14 will be set if the fjrst stop bit on a character is a space;
software then must determine by inspecting the character whether it is a valid
break or a dropped bit. Bit 15 is the Flag Bit for the word. If Bit 15 is set then the
core word contains a received character. The program must clear this bit after
ser'/icing the character in the Circular Buffer. Therefore, the Receive Service Routine
sequentially processes characters until it discovers a 0 in Bit 15 of a word in
the Circular Buffer. This indicates that all characters have been processed.
4. Transmit Procedure.
The Current Address Table (CAT) and the Byte Count Table (BCT) are used for
transmitting. Software must create an output buffer containing each message
somewhere in core. The starting address of this buffer is loaded into/the word of
the CAT which corresponds to the line on which the message is to be transmitted.
The two's complement of the total number of characters to be transmitted is
loaded into the corresponding word of the BCT as two's complement. Characters
in" the buffer are packed one per byte, right justified, in sequential bytes. Once the
BCT and CAT entries are loaded, the appropriate bit of the Buffer Active Register
(BAR) is set and transmission will proceed until all characters have been transmitted.
At the end of the transmission an interrupt is generated and software
checks the BAR to see which lines have completed message transmission.
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