eeworldonline.com | designworldonline.com 6 • 2019 DESIGN WORLD — EE NETWORK 41Getting started with USBTMC communicationUSBTMC
connections via USB. Although USB provides the hardware, it is
the TMC (Test and Measurement Class) driver that standardizes the
process of initializing and programming test instruments remotely.
The USBTMC driver provides simple plug-and-play operation
and GPIB-like communication at a relatively low cost. Remote
communication is made possible by a standard USB cable where data
transfers in a binary and/or ASCII format. The TMC driver on the PC
and the instrument TMC firmware provide two-way communication
for reading and writing instrument commands.
Unlike USB raw connections, the TMC driver does not
require any complicated setup or configuration procedures. The
only requirement for plug-and-play operation with a USBTMC-
supported device is installation of a VISA (Virtual Instrument Software
Architecture) I/O library which contains all the necessary drivers for
remote communication. Input and output functions are handled with
the VISA write command strings such as “*IDN?” and the VISA read
response string from the instrument.WHY USB/USBTMC?
The application determines whether or not a USBTMC scheme makes
sense. For simple benchtop or laboratory applications, USBTMC is
often the preferred interface for remote communication. In addition
to USB and LAN interfaces, GPIB also remains widely used, especially
in larger ATE systems.
Although GPIB has been the staple interface for over 40 years,
its cost has remained rather high. Compared to GPIB, the ubiquitous
use of USB by system and computer manufacturers make it commonly
available and easily accessible. Most modern PCs
have at a minimum of four to six standard USB
interfaces. In contrast, GPIB takes place via controller
cards added to a PC and can cost an additional
$200 or more. Alternatively, USB-to-GPIB controller
adapters range from $500 to $1,000. The GPIB
cables alone (depending on length), cost anywhere
from $80 to over $300.
All in all, GPIB can be an expensive proposition
for simple benchtop applications. Most instrument
manufacturers, including B&K Precision, now provideUSB connections and support for their equipment, making USBTMC a
more attractive option.
Nevertheless, both interfaces have their own advantages and
disadvantages. Specifically, consider the approximate cost for a
five-meter cable – a mere $6 for USB compared to $170 for GPIB.
USBTMC is built upon the USB 2.0 standard which supports a
theoretical bandwidth of up to 480 Mbps (60 MB/sec). However, with
isolation it is de-rated to 12 MB/sec. This speed is still much higher
than GPIB at 1 MB/sec.
USBTMC-compliant instruments are also backwards-compatible
with GPIB protocols that comply with IEEE 488.1 and IEEE 488.2
standards. These standards are important because they define a set
of instrument-specific commands known as Standard Commands for
Programmable Instrumentation (SCPI). In a nutshell, USBTMC requires
little effort to setup thanks to its true plug-and-play operation,
especially useful in cases where instruments are often interchanged.
Rated for a longer cable length, GPIB does still have some
advantages over USB. The higher noise immunity and ruggedness of
GPIB connectors appeals to many engineers working around large
ATE systems and numerous arrays of test racks. And sometimes
engineers are more comfortable with GPIB and cannot justify
spending the time or money to replace existing GPIB systems.
Many manufacturers, including B&K Precision, label some USB-
compliant instruments ‘USBTMC’ directly. Other instruments labeled
‘USB’ or ‘Device’ might still be USBTMC compliant, so it’s best to
check the instrument specifications or documentation to
verify compatibility.USBTMC (USB 2.0) GPIBIntegration Plug-and-play auto-detection Requires setup procedureTypical latency 125 μsec 300 μsecBandwidth
(transfer speed) 60 MB/sec 1 MB/secMax cable length 5 m 20 mApproximate cost
for 5m cable $6 $170USBTMC (USB 2.0) GPIBA comparison of GPIB and USBTMC shows why
the latter standard is becoming more widely used.A view of the B&K 8600 DC Load Rear
Panel shows the USBTMC marking.B&K — Test and Measurement HB 06-19.indd 41 6/7/19 1:56 PM