Tektronix 2714, Руководство программиста

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Appendix B: GPIB System Concepts
B–10
2714 & 2715 Programmer Manual
GPIB Signal Line Definitions
Figure B–2 shows how the sixteen active signal lines on the GPIB are function-
ally divided into three component buses: an 8-line data bus, a 3-line data byte
transfer control (handshake) bus, and a 5-line general interface management bus.
The data bus contains eight bidirectional signal lines, DI01 through DI08.
Information in the form of data bytes is transferred over this bus. A handshake
timing sequence between the enabled talker and the enabled listeners on the
three-line data transfer control bus transfers one data byte (eight bits) at a time.
These data bytes are sent and received in a byte-serial, bit-parallel fashion.
Since the handshake sequence is an asynchronous operation (no clock signal on
the bus), the data transfer rate is only as fast as the slowest instrument involved
in a data byte transfer.
A talker cannot place data bytes on the bus faster than the slowest listener can
accept them.
Figure B–3 illustrates the flow of data bytes on the bus when a typical controller
sends ASCII data to an assigned listener. The first data byte (decimal 44) enables
an instrument at address 12 as a primary listener. The second data byte (decimal
108) is optional; for example, enabling a plug-in device at secondary address 12
as the final destination of the data to follow. The data is the two ASCII charac-
ters A and B (decimal 65 and decimal 66). Note that the ATN line is asserted for
the first two data bytes and unasserted for the device-dependent character to
indicate the last data byte in the message.