Handshaking - HP 7470A Interfacing And Programming Manual

NOTE: A buffer overflow condition may also cause an HP-GL'error to
occur. In this case, an HP-GL IN or OE command or a front-panel reset
must be executed in order to clear the ERROR light. See Chapter 7 for an
explanation of the output error instruction, OE. I
Handshaking
The 7470 uses a 255-byte input buffer to synchronize the processing of
data with the rate at which it is received. The presence of an input
buffer requires that the computer and the plotter transfer information
to one another in such a way that data will not be lost or misinterpreted.
This is the purpose of handshaking.
The 7470is capable of using any one of four handshaking methods to
prevent buffer overflow and the resulting loss of data. The computer
system's capabilities and requirements dictate which handshake method
is appropriate.
0 Hardwire Handshake — uses a physical wire, pin 20 of the RS-232-C
cable, to control handshaking. It can be used if the computer system
can or does monitor pin 20 (DTR).
0 Xon-XoffHandshake —is managed by the peripheral device. It can
be used if the computer system follows an Xon-Xoffprotocol (control
characters are transmitted from the peripheral to the computer).
0 Enquire/Acknowledge Handshake — is managed by the computer
system and interface. This handshake is often used in Hewlett­
Packard systems and is so named because the ASCII characters
ENQ and ACK may be used to control the handshake.
0 Software Checking Handshake — is managed by the applications
programmer. It can be used on almost any computer system, but it
must be used if the system cannot implement any of the other three
handshaking methods.
Once the handshake method is selected, the 7470 can be program­
matically instructed to match the computer system requirements, imple­
ment the chosen handshake method, and function properly within the
system-dependent communication environment. This is done by specify­
ing certain variables in device control commands which are issued to
the 7470at the beginning of each computer session or graphics program.
The variables, which may be specified by using the decimal value of
the character desired to establish one of the four handshake methods
available to the 7470, are:
0 Output Trigger Character — The output trigger character, when
used, is the last character output by the computer when making a
request of a graphics peripheral. Defining this character in a com­
mand tells the plotter, "Don't respond to my request until you receive
10-14 RS-232—C/CCITTV.24 INTERFACING