May
2005, Issue 178
Network
GPIB Controller
The
GPIB protocol enables you to connect various pieces
of test equipment. This team built a low-cost eZ80F91-based
network GPIB controller for controlling equipment
over Ethernet.
by
Ron Battles, Patrick Jackson, & Scott Shumate
Test
equipment such as oscilloscopes, power supplies, and
signal generators is often used in manufacturing and
compliance applications that require you to control
and automate its behavior. Back in the 1960s, Hewlett-Packard
recognized this requirement and invented a generic bus
protocol that allows you to connect and control test
equipment. Today the protocol is called the general-purpose
interface bus (GPIB), although it’s also known as IEEE
488.1.
GPIB
is a multipoint, 8-bit parallel bus that uses a three-wire
handshake to acknowledge each data byte. The bus is
organized in a master-slave arrangement in which at
least one controller is responsible for directing bus
communication and a number of talkers that send data
and listeners that receive data. Every device on the
bus can serve any combination of these three roles.
The bus allows for a total of 15 devices with up to
2 m of separation between them and a maximum bus length
of 20 m. The maximum nominal transfer rate of the bus
is 1 MBps, although there are nonstandard enhancements
that can allow it to operate faster.
GPIB,
which is supported on a large percentage of test equipment,
has proven to be an extremely popular protocol. But
the rise of the Internet has made network-enabled electronics
all the rage. This reality has also affected the test
equipment industry.
A
lot of test equipment now includes built-in Ethernet
interfaces with embedded web servers and other network
control interfaces in addition to (or completely replacing)
the GPIB interface. The main advantages of network-enabled
control are that the cabling costs are much lower and
there aren’t inherent distance limitations.
In
addition, embedding a web server or client application
allows you to control the equipment with any PC that
has a web browser. You don’t have to install the client
software first.
A
few devices enable you to control equipment with only
a GPIB interface over an Ethernet connection. These
devices suffer from two major drawbacks. First, they’re
unnecessarily expensive, which can put them out of reach
for many designers and small companies. Second, they
don’t embed the client application for controlling the
GPIB equipment, so you must install software on each
PC. We designed a network GPIB controller that solves
both of these problems. Our low-cost solution makes
it easy to embed client applications.
