OL4.SEP–wy
GPIB throughput takes off
New high-speed protocols help raise the interface's data-transfer rate
from 1 Mbyte/s to as high as 8 Mbytes/s
GPIB manufacturers are taking dead aim at speeding up the interface's
transfer rate to meet the requirements of today's high-data-throughput
applications. Improvements in the IEEE-488 protocol as well as in
triggering methods have raised data throughput rates significantly.
The most-often-voiced complaints concerning the extremely popular
IEEE-488 interface bus, or as it is perhaps better known, the
general-purpose interface bus (GPIB), usually relate to its speed. Or lack
of it! The GPIB's 1-Mbyte/s transfer rate just isn't fast enough for many
of today's applications. Consequently, GPIB suppliers have been hard at
work trying to bring the interface more in line with the latest
applications. For example, two of the major GPIB suppliers, Capital
Engineering Corp. (Burlington, MA) and National Instruments (Austin, TX)
have recently proposed changes to the IEEE-488 standard to speed up data
transfer. And another supplier Keithley Instruments' Data Acquisition Div.
(Taunton, MA) has introduced a GPIB board that provides programmable
trigger control to reduce trigger latency, thereby significantly greatly
increasing test throughput. A streaming-data protocol extension to the
IEEE-488 standard developed by Capital Equipment Corp. (CEC) has been
submitted to the IEEE-488 Standards Committee for consideration. The new
protocol, already implemented in CEC's 488SD interface board (see Fig. 1),
increases GPIB throughput from its longstanding 1 Mbyte/s rate to 5
Mbyte/s. The 488SD protocol introduces a new way of using the IEEE-488
handshake signals to break through the speed barrier of the older
three-wire handshake.It uses streaming-data, or burst-mode, transfers,
sending an entire block of data bytes without repeating the overhead of
the IEEE-488 listener handshake on every byte. The 488-SD handshake
operates as follows (see Fig. 2): At time 1, a source places a data byte
on the bus. The source asserts the DAV signal, indicating that the data is
valid (time 2). The source waits for a fixed time period (time 2 to time
3), then removes DAV. Bus listener devices are required to latch the data
byte within this time period. (Steps 1 through 3 repeat for multiple data
bytes.) When any data byte arrives, one or more listeners can pause the
transfer by asserting the NRFD line low (time 4). When all listeners are
again ready for data (time 5), NRFD goes high (time 6) and the transfer
continues. National Instruments' HS488 high-speed protocol increases the
GPIB theoretical maximum data-transfer rate from 1 Mbyte/s to as high as 8
Mbytes/s, depending on the host computer architecture. The HS488's higher
transfer rates are achieved by removing some of the propagation delays
associated with transferring a data byte. The HS488 high-speed protocol
is incorporated into National Instruments just-introduced TNT4882, the
first single-chip IEEE-488 talker/listener interface (see Electronic
Products, Aug. 1993, p. 67). The TNT4882 chip, in turn, is incorporated
into the company's AT-GPIB/TNT IEEE-488.2 controller board for use in
PC/AT computers with 16-bit plug-in slots. GPIB data-transfer rates up to
4 Mbytes/s are possible for both read and write operations. Like CEC with
its streaming-data protocol, National Instruments intends to propose the
HS488 protocol to the IEEE as an extension of IEEE Standard 488.2 – 1987.
Keithley Instruments' Data Acquisition Div.has taken a different approach
to improving data-transfer rates. Rather than attack data-transfer rates
directly, its Trigger Master interface (see Fig. 3) works to reduce
trigger latency. Trigger latency is the lag that occurs between when an
instrument receives a command to measure and when the measurement actually
occurs (see Fig. 4). As measurement systems evolve to require higher
throughput, instrument synchronization is becoming more critical. Trigger
Master addresses trigger latency and timing uncertainty by generating
precisely timed external trigger inputs, reducing measurement time by as
much as a factor of 8 for multiple-instrument systems. Trigger Master is a
hardware solution that controls the triggering process independently of
the PC, a faster form of communication than diverting all trigger and
measure commands over the IEEE-488 bus.
CAPTIONS:
Fig. 1. CEC's 488SD GPIB interface board incorporates a streaming-data
protocol that increases data-transfer rates to 5 Mbytes/s.
Fig. 2. The 488SD streaming-data interface, like previous GPIB interfaces,
requires a three-wire handshake, but supports much faster throughput.
Fig. 3. Keithley's Trigger Master feature reduces trigger latency in its
IEEE-488 interface board, thereby raising data transfer 50%.
Fig. 4. Trigger latencies have a detrimental effect on measurement time.
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