HP 7470A Interfacing And Programming Manual page 158

0 Enquiry Character —In some systems the computer sends an enquiry
character to ask the plotter if it has room for a block of data, thereby
initiating the handshake process. If Xon—Xoff handshake mode is to
be established, a NULL character (decimal equivalent 0) must be
specified as the enquiry character. If enquire/acknowledge is to be
established, an ENQ character (decimal equivalent 5) or any other
ASCII character besides the NULL is used.
0 Immediate Response String — Certain system environments require
an immediate response from the plotter acknowledging the enquiry
from the computer. Systems of this type include a computer that
transmits data to the plotter after a certain time interval but before
receiving a go—aheadsignal from the plotter. If the plotter's buffer is
full and the computer sends more data, the buffer will overflow. The
immediate response string prevents this inadvertent tfansmission of
data before the plotter is ready. It is transmitted by the plotter
immediately after receipt of an enquiry character and tells the com­
puter, "Wait, I am here and checking my buffer space." Computers
frequently require a DC3 character (decimal equivalent 19) for the
immediate response.
0 Acknowledgment String — The acknowledgment string specifies the
character or characters that the plotter will send to the computer
when the plotter's input buffer has room for another block of data.
Computers frequently require that the ACK character (decimal equiv­
alent 6) be used for the acknowledgment string.
0 Data Block Size — This is the maximum size of each data block the
computer will transmit to the plotter.
0 Data Terminal Ready (CD) Line Control — This variable sets the
configuration of the plotter's Data Terminal Ready control line (pin 20)
to enable or disable the hardwire handshake mode. Pin 20 is held on
(+12 V) if hardwire handshake is disabled.
0 Xoff Threshold Level — In the Xon—Xoff handshake mode this defines
how many empty bytes remain in the buffer when the plotter sends
the Xoff trigger character to the computer, telling it to stop sending
data.
0 Xoff Trigger Character — This specifies the character string the
plotter will use to signal the computer to temporarily stop sending
data while the plotter processes what it has already received. The
DC3 character (decimal equivalent 19) is generally used for the Xoff
trigger.
10-16 RS-232-C/CCITT V.24INTERFACING