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Secondary Receiver Ready (SRRlSCF) line must also be
high. The terminal will revert back to the receive state if
the condition is not met within 2.6 seconds. There is no
timing delay when switching from transmit state to
receive state; an indefinite wait for Receiver Ready (RRlCF)
may, however, occur.
Line-turnarounds in half duplex operation are governed by
the following protocols:
1.
Reverse Channel. This protocol requires the secondary
control lines RRlCF and SRRlSCF and is generally driv-
. driven by a host computer. A drop on the Secondary
Receiver Ready (SRRlSCF) line will cause the terminal
to enter the receive state. A drop on the Receiver Ready
(RRlCF) line will cause the terminal to attempt to enter
the transmit state.
2. Mainchannel (strict). This protocol uses start of data
(SOD) and end of data (EOD) framing characters. The
SOD is optional and the EOD is mandatory. Any data
received after an EOD is discarded. This protocol, which
is common in Europe, is used in applications where a
drop on the Receiver Ready (RRlCF) line results in an
extraneous received character. Only primary control
lines are used.
a. An EOD transmitted by the terminal causes the ter-
minal to enter receive state.
b. An EOD received from the host computer causes the
terminal to attempt to enter transmit state.
c. An EOD is automatically affixed to the end of each
block transmitted by the terminal.
d. When an SOD character is required, any data
received since the prior EOD and prior to an SOD is
discarded. When an SOD character is required, the
terminal automatically affixes one to the beginning
of each block transmitted by the terminal.
e. The SOD and EOD characters may be defined dif-
ferently for transmit and receive state.
3. Mainchannel (non-strict). This protocol is the same as
described above except that transitions on control lines
may also be used for causing transmit/receive state
transitions.
a. An EOD transmitted by the terminal or a drop (high
to low) on the Secondary Receiver Ready (SRRlSCF)
line, whichever occurs first, will cause the terminal
to enter the receive state.
b. An EOD received from the host computer and/or a
drop on the Receiver Ready (RRlCF) line will cause
the terminal to attempt to enter transmit state. An
EOD received alone will cause the terminal to
remain in receive state untill}RlCF drops, at which
time the terminal then attempts to enter transmit
state.
Pacing Mechanisms
In a full duplex environment, the HP 2624A can par-
ticipate in either ofthe following forms of transmit pacing:
1.
Hardware handshake. A host computer or external
printer can temporarily restrain the terminal from
transmitting by:
Data Communications
a. Lowering the Clear to Send (CB) line; or
b. 'furning off Secondary Receiver Ready (SRRlSCF).
Normally a low state is interpreted as "off", but you
can use the InvertSRR field in the terminal's data
comm configuration menu to invert the sense of the
SRRlSCF line so that a high state is interpreted as
"off"; or
c. Both of the above simultaneously.
Note that this type of transmit pacing can only be used
in a hardwired configuration.
2. XON-XOFF handshake. The host computer or external
printer uses the ASCII control codes XOH «DC1» and
XOFF «DC3» to start and stop the terminal from
transmitting. Note that a single XOH code cancels any
number of preceding XOFF codes.
In a full duplex environment, the HP 2624A can also par-
ticipate in the following forms ofreceive pacing:
1.
Terminal Ready handshake. The terminal can tem-
porarily restrain the host computer from transmitting
by lowering the Data Terminal Ready (TRlCD) line. It
does this when its receive "working" buffer is full. When
enough data has been processed so that the receive
buffer is only half full, the terminal restarts transmis-
sion from the host by raising the TRlCD line.
Note that this type of receive pacing can only be used in
a hardwired configuration.
2. XOH-XOFF handshake. The terminal uses the ASCII
control codes XON «DC1» andXOFF «DC3» to start and
stop the host computer from transmitting. Note that a
single XON code cancels any number ofXOFF codes.
In either a full or half duplex environment, the terminal
can also participate in an ENG-ACK handshake which is a
Hewlett-Packard handshaking mechanism. With this form
of handshaking, the host computer transmits a block of
data and the then sends an ASCII <ENG) control code. The
terminal responds to the <ENG) by sending back an
ASCII <ACK) control code when it has processed all of the
data preceding the <ENG). The general interpretation of
these two control codes is as follows:
ENG: "Have you processed the data up to this point?"
ACK: "Yes, I have."
The above pacing mechanisms are responded to by the
terminal in the following order of precedence:
1.
Hardware handshake pacing
(highest priority)
2. XON-XOFF transmit pacing
3. XON-XOFF receive pacing
4. ENG-ACK pacing
(lowest priority)
WARNING: If both XOH-XOFF transmit pacing and XON-
XOFF receive pacing are enabled, it is the
responsibility of the host computer program
to prevent (or to detect and correct) any cases
of "deadlock" that may arise.
If
the terminal
sends an XOFF and then receives an XOFF, it
will not send an XON until first receiving one
from the host computer.
7-15
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