HP 7470A Interfacing And Programming Manual page 179

output terminator = carriage return (decimal equivalent 13),
intercharacter delay = 5,
no immediate response string,
block size = 80,
enquiry character = DC2 (decimal equivalent 18),and
acknowledgment string = 1 (decimal equivalent 49).
The subroutine in line 100 contains the handshake. It causes the
following chronological action. The enquiry character, DC2, is sent
asking if the plotter has room for an 80—byte b lock. The plotter does not
send an immediate response because that has been specified as null by
its omission in the ESC . N command. The plotter holds its response T
until after it receives the output trigger character, ?. The question mark
is sent by the computer when it interprets the BASIC statement
INPUT to prompt for the input, Z. Z is the variable into which the
acknowledgment string, 1, is read. If the acknowledgment string had
been specified to contain nonnumeric characters, a string variable such
as Z$ would have been used instead of Z.
The plotter waits approximately five milliseconds, the intercharacter
delay, before sending the 1 and between the 1 and the output terminator,
carriage return. Note the carriage return parameter could have been
omitted, but carriage return still would have been sent as the output
terminator because that is the default value for output terminator. If
ESC . I had been used instead of ESC . H, the output terminator would
not have been sent after the acknowledgment string (but it would
follow responses to HP-GL output commands). The carriage return
character is a logical choice, because it is expected by the computer to
delineate the end of data read by the INPUT statement.
The computer is now free to send the string OUT$, which contains HP­
GL commands, to the plotter. Note the enquiry character must be sent
each time data is sent to the plotter.
Another handshake which would work using ESC . I is
40 PRINTCHR$(2?);".IBO;?;33;13:"
50 PRINTCHR$(2?);".M500:";CHR$(2?);".N5:"
100 PRINT CHR$(?):INPUT Z$: PRINT UUT$: RETURN
RS-232-C/CCITT V.24INTERFACING 10-37