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Data Communications
In a full duplex environment, this terminal can also participate in the following forms of
~,
receive pacing:
~
1. Terminal Ready Pacing. The terminal can temporarily 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
~~working"
buffer is only one quarter full, the terminal restarts transmission from the host
by raising the TRIeD line.
This type of receive pacing can only be used in a hardwired configuration.
2. Receiver Ready Pacing. When the Receiver Ready line is lowered, the terminal does not
in~erpret
received characters as data; it discards them.
3. XON-XOFF pacing. The terminal uses the ASCII control codes XON «DCl >)' and XOFF
«DC2» to start and stop the host computer from transmitting. Note that a single XON
code cancels any number of XOFF codes.
4. ENQ-ACK handshake. This is a Hewlett-Packard handshaking mechanism. With this form
of handshaking, the host computer transmits a block of data and then sends an ASCII
<ENQ> control code. The terminal responds to the <ENQ> by sending back an ASCII
<ACK> control code when it has processed all of the data preceding the the <ENQ>. The
general interpretation of these two control codes is as follows:
ENQ:
"Have you processed the data up to this point?"
ACK: ' ,
Ye s, I have."
If the host computer is an HP 1000 or HP 3000, it does not send any data following the
<ENQ> until it has received the <ACK>, or until a timeout period (several seconds) has
elapsed.
The above pacing mechanisms are responded to by the terminal in the following order of
precedence:
1. Hardware handshaking pacing (highest priority)
2. XON/XOFF
receive pacing
3. XON/XOFF
transmit pacing
4. ENQ/ACK pacing (lowest priority)
NOTE: If both
XON/XOFF
transmit pacing and
XON/XOFF
receive pacing are enabled, the
receive pacing has priority, so that if the host computer sends XOFF, followed by data,
the terminal can still respond with an XOFF before its buffer overflows. This al-
gorithm should also be used by the host computer, as the terminal may send XOFF and
follow it with transmit data. If both parties function in this way, then deadlock is
prevented, and both parties should prevent buffer overrun at all times.
7-14
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