Interrupts and Aborts
Forward Abort
A Block of text can be terminated prematurely by using an ENQ character which signals the receiver to "disregard this block" NAK is always the reply in this situation.RVI Reverse Interrupt
The RVI control sequence is a positive response used in place of the ACK0
or ACK1 positive acknowlegement. RVI is transmed by a receiving station to request
termination of the current transmission because of a high priority message which
it must transmit to the sending station.
Flow Control
WACK (Wait-before-transmit-positive-acknowledgement) allows a receiving station
to indicate a "temporary not ready to receive"
------------
S S | S |
Y Y | O | ---> message
N N | H |
+---+--------
---
S S | W |
<--- Y Y | A |
N N | C |
+---|
K
---
S S | E |
Y Y | N | ---->
N N | Q |
+---|
. continues to send WACK in response
. to ENQ until ready to continue
.
---
S S | E |
Y Y | N | ---->
N N | Q |
+---|
-----
S S | A |
<---- Y Y | C |
N N | K |
+-----|
0/1
------------------
S S | S |
Y Y | O | next message -->
N N | H |
+---+--------------
TTD - Temporary Text Delay
The TTD control sequence is sent by a sending station in message transfer
state when it wishs to retain the line but is not ready to transmit.
The normal response to TTD is a NAK
TTD = STX ENQ
------------------>
NAK
<------------------
Transparent Data (optional - can be used in EBCDIC or ascii)
If information which is
not EBCDIC or ascii
text is to be sent in a message then a bit pattern equivalant to a control code
may occur, to avoid this transparent mode must be used. In transparent mode
all data-link control sequences must start with DLE as follows,
DLE STX = STX
DLE ETB = ETB
DLE ETX = ETX
DLE SYN = SYN inside message only
message starts with SYN SYN as usual
DLE ENQ = ENQ
DLE DLE used to permit transmission of DLE as data when a bit pattern
equivalent to DLE appears within the transparent data.
One DLE is disregarded the other is treated as data.
DLE ITB = ITB
EOT and NAK must be followed by a trailing 'FF' PAD character
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