May
2005, Issue 178
Network
GPIB Controller
by
Ron Battles, Patrick Jackson, & Scott Shumate
CONTROLLER
HARDWARE
We
used an eZ80F91 microcontroller to integrate a number
of critical functions into a single device. The GPIB
driver is the heart of the controller (see Figure 1).
It implements an IEEE 488.1-compliant GPIB bus controller
in the software by way of the eZ80F91 microcontroller’s
external GPIO lines. The eZ80F91 microcontroller also
incorporates an embedded Ethernet MAC, which helped
reduce both the cost and complexity.
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(Click
here to enlarge)
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Figure
1—The brains behind the network GPIB controller
is an eZ80F91 microcontroller running at 50 MHz.
A MAX6309 programmable reset device monitors the
supply voltage and provides a stable reset. Run-time
memory for the software is provided by 512 KB of
SRAM. |
The
server software in the controller was built on top of
ZTP, the free TCP/IP protocol stack provided by Zilog.
We selected the XML remote procedure call (XML-RPC)
protocol to be the main means of control between the
remote client applications and the controller. XML-RPC
is a simple protocol based on XML. It’s transported
via HTTP. The XML-RPC’s main advantages are that it’s
lightweight and well supported under a number of programming
environments.
To
demonstrate the ability to embed a client application
in the controller and use it to control GPIB equipment,
we also implemented a GUI application in Java for the
Tektronix TDS-210 oscilloscope. The application has
the ability to graphically display waveforms as well
as set and control operation, all via the oscilloscope’s
GPIB connection. The Java application uses the XML-RPC
protocol to control the operation of the GPIB bus via
the controller.
The
primary hardware design criterion for the network GPIB
controller was that it must be inexpensive. To accomplish
this goal, we focused on a design that integrated as
many functions as possible in a minimum number of devices.
The
eZ80F91 microcontroller is the heart of the system because
of its high level of integration. It includes the primary
flash memory storage for the software, an integrated
10/100 Ethernet MAC, address and data buses for external
run-time memory, and ample GPIO pins.
Although
dedicated GPIB controllers such as the industry-standard
Texas Instruments TMS9914A and NEC Electronics uPD7210
are easy to use, they’re expensive and require additional
real estate on the board. The eZ80F91 microcontroller
is the perfect solution because it can implement the
GPIB controller function in software. The GPIO pins
on the eZ80F91 microcontroller make it easy to interface
to the GPIB bus signals. Cutting out the dedicated GPIB
controller not only reduces cost, but it gives you a
measure of investment protection because any updates
to the GPIB protocol can be made to the system in the
field via a software update.
The
GPIB bus signals require line drivers external to the
eZ80F91 microcontroller. We chose two commonly used
Texas Instruments parts for this task: SN75160 and SN75161
transceivers. The former, which drives the GPIB data
bus, is interfaced to the eZ80F91 microcontroller via
GPIO port A. The latter, which drives the GPIB control
and handshaking signals, is interfaced to the eZ80F91
microcontroller via GPIO port B. Both devices have control
signals that are interfaced via four pins on GPIO port
C.
The
integrated 10/100 Ethernet MAC on the eZ80F91 microcontroller
reduces the cost and parts count. An MII bus connection
interfaces the MAC to an external PHY, in this case
the Advanced Micro Devices Am79C874 PHY, which we chose
because it’s commonly available and Zilog already produces
a driver for the ZTP protocol stack. An RJ-45 jack with
integrated magnetics and LEDs rounds out the Ethernet
interface (see Figure 2).
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(Click
here to enlarge)
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Figure
2—The Am79C874 is an Ethernet 10/100 PHY device
that connects directly to the eZ80F91’s media-independent
interface (MII) on one side and an RJ-45 jack (with
integrated magnetics and LEDs) on the other. The
SN75160 and SN75161 provide some state decoding
logic as well as line drivers that are electrically
compliant with GPIB. The 5- and 3.3-V power supplies
are standard fare. |
The
eZ80F91 microcontroller includes 256 KB of integrated
flash memory that provides you with more than sufficient
room to store the software image. The software boots
out of this flash memory and copies itself to external
SRAM for execution. This is done to provide maximum
run-time performance and to give the network stack and
server plenty of heap and stack space in which to run
(see Figure 3).
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(Click
here to enlarge)
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Figure
3—We satisfied our hardware design goal of having
a highly integrated, low-cost system. The eZ80F91
microcontroller performs all of the important functions
in the system. The other devices serve only to interface
it to the outside world. |
To
accelerate the development schedule and to minimize
initial costs, we prototyped the design with the eZ80F91
development kit because it contains the same processor
and 10/100 Ethernet PHY as this design. We constructed
an external circuit on a small prototype board for the
GPIB line drivers. Because the development kit exposes
the eZ80F91 GPIO pins on a set of 0.1" headers,
it was easy to interface the circuit to the development
kit via a standard ribbon cable. To connect the GPIB
bus to the line drivers, we sacrificed a GPIB cable
by cutting the connector off on one end to expose the
wires inside.
