Hardware Reference
In-Depth Information
This enhanced specification more precisely defines the ways in which con-
trollers and instruments communicate with one another. A further improvement
in the IEEE-488 standard is the
Standard Commands for Programmable
Instruments
(SCPI) specification which provides the IEEE-488.2 specifica-
tion
with
a
comprehensive
command
set suitable for all instruments
(see
Figure 8.1).
The most recent version of the IEEE-488 standard (IEEE.488.1-2003) intro-
duces the high-speed protocol, HS488, for data transfers. This specification
supports transfers data rates of up to 8 MB/s although actual rates will depend
upon host architecture and system configuration. Furthermore, because HS488
is a superset of the IEEE-488.1 standard, it is possible to mix non-HS488 GPIB
devices with devices that are high-speed compatible without having to modify
software applications.
Figure 8.1
Relationship
between the SCPI, IEEE-488.2,
and IEEE-488.1 standards
IEEE-488 devices
The IEEE-488 standard provides for the following categories of device: listen-
ers, talkers, talkers and listeners, and controllers. We shall briefly examine the
role of each type of device.
Listeners
Listeners can receive data and control signals from other devices connected to
the bus but are not capable of generating data. An obvious example of a listener
is a signal generator.
Talkers
Talkers are only capable of placing data on the bus and cannot receive data.
Typical examples of talkers are magnetic tape, magnetic stripe, and bar code
readers. Note that, whilst only one talker can be active (i.e. presenting data to the
bus) at a given time, it is possible for a number of listeners to be simultaneously
active (i.e. receiving and/or processing the data).
Talkers and listeners
The function of a talker and listener can be combined in a single instrument.
Such instruments can both send data to and receive from the bus. A digital
multimeter is a typical example of a talker and listener. Data is sent to it in
order to change ranges and returned to the bus in the form of digitized readings
of voltage, current, and resistance.
Controllers
Controllers
are
used
to
supervise
the
flow
of
data
on
the
bus
and
pro-
vide
processing
facilities.
The
controller
within
an
IEEE-488
system
is
