Tektronix 2714, Programmer'S Manual

Page: 290 / 308

Appendix B: GPIB System Concepts
B–14
2714 & 2715 Programmer Manual
A talker can use the EOI signal line to indicate the end of a data transfer
sequence. The talker asserts EOI as the last byte of data is transmitted. In this
case, the EOI line is essentially a ninth data bit and must observe the same
settling time as the data on the data bus.
When an instrument controller is listening, it assumes that a data byte sent with
EOI asserted is the last data byte in the complete message. When the instrument
controller is talking, it may assert the EOI signal line as the last data byte is
transferred. The EOI line is also asserted when the ATN line is true if the
controller conducts a parallel polling sequence on the bus. The EOI line is not
used for a serial polling sequence.
Interface Functions and Messages
The ten major interface functions listed in Table B–1 provide a variety of
capabilities and options for an instrumentation system. These functions may be
implemented in, or for, any particular instrument with instrument hardware or
with a programming routine (software).
Only those functions necessary for an instrument’s purpose must be implemented
by the instrument’s designer. An instrument will seldom have all ten interface
functions. For example, an instrument generally does not need to implement the
Parallel Poll (PP) function if the instrument can respond to a serial polling
sequence from the controller in charge of the GPIB system.
The interface functions and their relationship to the interface control messages in
Table B–4 are discussed below. All interface control messages discussed are sent
and received over the GPIB when the ATN line is asserted (low).
The RL function provides an instrument with the capability to select between
two sources of input information. This function indicates to the instrument that
its internal device-dependent functions are to respond to information input from
the front panel (Local), or to corresponding programming information from the
GPIB (Remote). Only the system controller is permitted to assert the REN
(Remote Enable) line, whether or not it is the controller in charge at the time.
When the system controller asserts the REN line, an instrument on the GPIB
goes to a remote mode when it is addressed as a listener with its listen address.
An instrument remains in a remote mode until the REN line is released (high), or
an optional front-panel switch on the instrument is activated to request the local
mode, or a GTL (Go To Local) command is received while the instrument is
enabled as a listener.
The controller can also disable the instrument’s front-panel “return to local”
switch(es) by sending a LLO (Local Lockout) command. The LLO command
must be preceded or followed by a listen address (MLA) to cause the instrument
EOI (End Or Identify)
RL (Remote-Local
Function)