Tektronix 2714, Руководство программиста

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Appendix B: GPIB System Concepts
B–6
2714 & 2715 Programmer Manual
Parallel Poll Enable (PPE) messages are derived from the characters in the first
column under Lower Case letters in Table B–4 (decimal coded characters 96
through 111). The standard recommends the use of decimal code 112 (lower case
letter p) for the Parallel Poll Disable (PPD) command. All parallel poll-config-
ured instruments respond with status information at the same time when the EOI
line is asserted and ATN is true.
Device-Dependent Messages
The IEEE standard does not specify the coding of device-dependent messages
that control the internal operating functions of a device. After addressing a talker
and the required number of listeners with interface control messages, the
controller unasserts the ATN line (false or high) on the bus. When ATN becomes
false (high), any commonly understood 8-bit binary code may be used to
represent a device-dependent message.
The standard recommends that the alphanumeric codes associated with the
numbers, symbols, and uppercase characters (decimal 32 to decimal 94) in the
ASCII Code Chart (Table B–4) be used to compose device-dependent messages.
One example of a device-dependent message could be the following ASCII
character string that controls the signal generator from Figure B–2:
MODE V;VOLTS 2.5E-3;FREQ 1.0E3
The ASCII character string from this example, sent when the ATN line is
unasserted, tells the signal generator to set its front panel controls to the voltage
mode (
MODE V;VOLTS
) and produce a 2.5 mV signal (
2.5EĆ3;
) at a frequency of
1000 Hz (
FREQ 1.0E3
).
When 8-bit binary codes other than the ISO 7-bit are used for device-dependent
messages, the most significant bit should be on data line DI08 (for bit 8).
To summarize the difference between interface control messages and device-de-
pendent messages on the data bus, remember that any message sent or received
when the ATN line is asserted (low) is an interface control message. Any
message (data bytes) sent or received when the ATN line is unasserted (high) is a
device-dependent message.