Exchange/RJE Manual

The WACK Character
BSC Line Control
104698 Tandem Computers Incorporated C–3
The NAK Character The NAK (negative acknowledgment) character is a negative response. In the control
state, a NAK character in response to a bid for the line means that permission to
transmit is not granted. In the message state, a NAK in response to a block means the
last block was received with an error and should be transmitted again. The NAK
character is also used in several other line-control operations. The NAK character
always causes a line turnaround (the station receiving the NAK transmits next).
The NAK character is transmitted as shown:
ASCII line code EBCDIC line code
pad pad
SYN SYN
SYN SYN
NAK NAK
pad pad
The TTD Character The TTD (temporary text delay) character is not used in the control state. In the
message state, the TTD character indicates that the sending station is unable to
transmit the next (or initial) block immediately. The station receiving the TTD
responds with a NAK and waits for the transmission to begin. The sending station can
repeat the TTD character until it is able to send the next block. The TTD character
always causes a line turnaround (the station receiving the TTD transmits next).
The TTD character is transmitted as shown:
ASCII line code EBCDIC line code
pad pad
SYN SYN
SYN SYN
STX STX
ENQ ENQ
pad pad
The WACK Character In both the control state and the message state, the WACK (wait-before-transmit
positive acknowledgment) character is a positive acknowledgment that also indicates
that the station sending the WACK is temporarily unable to receive. The station
receiving the WACK responds with an ENQ, which means that the station is willing to
wait before transmitting, or with an EOT or DLE EOT, which terminates the
transmission. Assuming that the station sending the WACK received an ENQ in
response, that station can repeat the WACK, causing the other station to wait longer,
or that station can respond with an ACK 0 or ACK 1 as required, which indicates the
station is now ready to receive. The WACK character always causes a line turnaround
(the station receiving the WACK transmits next).