HP 7470A Interfacing And Programming Manual page 114

)
^ maximum number of bytes sent by an output statement to allow
J room for the overshoot.
) 5. Once the Xoff trigger character has been sent, when the amount of
\ stored data drops to the Xon threshold level, the plotter sends the
Xon trigger character to signal the computer to resume sending
J data. The Xon threshold level is automatically set at 128 bytes. If the
Xoff threshold level is greater than 128, the Xon threshold is reset to
send the Xon character when one more byte than required by the
} Xoff threshold is available in the plotter's buffer.
6. Data is again stored in the buffer until all the data are transferred or
until the Xoff threshold level is exceeded again.
)
)
)
The following conditions can be specified for the Xon-Xoff handshake
J mode to match the requirements of the computer system, by using the
appropriate command:
• Xoff threshold level (ESC . I command)
• Xon trigger character (ESC . I command)
• Xoff trigger character (ESC . N command)
• Intercharacter delay (ESC . N command)
The enquiry character ( E S C . I command) must either be defaulted or
) specified as zero.
inquire/Acknowledge Handshake
With the enquire/acknowledge handshake, the computer's operating
system or application program initiates the data exchange process by
querying the plotter about the availability of buffer space. The format
of the exchange is dependent upon the requirements of the computer.
The following conditions can be specified for the enquire/acknowledge
handshake mode by using the appropriate command:
• Turnaround delay (ESC . M command)
• Output trigger character (ESC . M command)
• Echo terminate character (ESC . M command)
• Output initiator character (ESC . M command)
• Output terminator (ESC . M command)
• Intercharacter delay (ESC . N command)
• Immediate response string (ESC . N command)
• Data block size (ESC . I or ESC . H command)
-20 RS-232-C/CCiTT V.24 INTERFACING
• Enquiry character (ESC . I or ESC . H command)
• Acknowledgment string (ESC . I or ESC . H command)
In its simplest form, the data exchange looks like this:
C O M P U T E R
DO Y O U HAVE B U F F E R SPACE FOR A D A T A B L O C K ?
YES. T H E R E IS R O O M I N M Y B U F F E R
DATA
•EIMQ"
" A C K "
P L O T T E R
ENQ/ACK Handshake Protocol Example 1
In a more complex form, the communication might look like the
following example, where the two commands i^w - M250;17;10; 13:
and ES3 • H100; 5; 6: have been sent to specify the variables as:
turnaround delay = 250 ms
output trigger character = ASCII character DC1 (decimal equiva­
lent 17)
echo terminate character = ASCII character LF (decimal equiva­
lent 10)
output terminator = ASCII character CR (decimal equivalent 13)
data block size = 100 bytes
enquiry character = ASCII character ENQ (decimal equivalent 5)
acknowledgment string = ASCII character ACK (decimal equiva­
lent 6)
RS-232-C CCITT/V.24 INTERFACING 10-21