Exchange/RJE Manual
Appendix C BSC Line Control
104698 Tandem Computers Incorporated C–1
This appendix briefly reviews the BSC control characters and their use in the line-
control protocol.
BSC Line-Control
Characters
The following paragraphs list the BSC line-control characters, briefly describe the
function of each, and show the corresponding codes. Each character is described in
terms of the station that is receiving the character.
The SYN Character In both the control state and the message state, the SYN (synchronous idle) character
establishes and maintains character synchronization over the data communications
line.
The ENQ Character In the control state, the ENQ (enquire) character is a request for permission to transmit
(in other words, a bid for the line). In the message state, the ENQ character has two
meanings depending on where it appears. If the ENQ character is sent between
blocks, it is a request for the receiving station to respond or repeat the last response. If
the ENQ character terminates a block, it tells the receiving station to discard the block
and respond with a NAK. The ENQ character always causes a line turnaround (the
station receiving the ENQ transmits next).
Between blocks or during a bid for the line, the ENQ character is transmitted as
shown:
ASCII line code EBCDIC line code
pad pad
SYN SYN
SYN SYN
ENQ ENQ
pad pad
At the end of block, the ENQ character is transmitted as shown:
ASCII line code EBCDIC line code
text text
ENQ ENQ
pad pad
The ACK 0 Character In the control state, the ACK 0 (affirmative acknowledgment 0) character is the
positive response to a request for permission to transmit (a bid for the line). The
station receiving the ACK 0 can begin transmission of the first block of data. In the
message state, ACK 0 is the positive response to receipt of an even-numbered block. It
means that the last block was received and validated (the BCC calculated at the
receiving station agreed with the BCC transmitted with the block). The station
receiving the ACK 0 can transmit the next block of data. The ACK 0 character always
causes a line turnaround (the station receiving the ACK 0 transmits next).