Transcript Unit 14 PowerPoint Slides

EET 2259 Unit 14
Instrument Control

Read Bishop, Appendix A.

Homework #14 due next week.
Floyd, Digital Fundamentals, 10th ed
Instrument Control
You can use LabVIEW to control
instruments (multimeters, oscilloscopes,
etc.) if your computer and the instrument
have ports that let them talk to each other.
Two ports commonly used for instrument
control are:




RS-232 port (the “serial port” found on many
computers)
GPIB port (General Purpose Interface Bus)
Floyd, Digital Fundamentals, 10th ed
Many Bus Standards
•There are dozens of bus standards in common
use. From Wikipedia’s article on the USB bus:
Terminology: Serial vs. Parallel
•Some bus standards apply to serial
communication (1 data bit transferred at a
time).
•Others apply to parallel communication
(several data bits—usually 8—transferred at a
time).
Terminology: Bits per Second and
Baud Rate
•These are two common measures of speed in
communications. Many writers loosely treat
these as being synonyms, but this is not strictly
correct.
•Bits per second (bps) is the easier to
understand. Often expressed as kbps or Mbps.
•In the simplest cases, baud rate equals bps.
In more sophisticated schemes, the two are
related but not equal. Traditional baud rates
are 300, 600, 1200, 2400, 4800, 9600, 19200.
RS-232 Standard
•First version created in early 1960’s.
•Obsolete in some respects, but still very widely
used. Until recently, most PCs had one or two
RS-232 ports, but they’re becoming less
common as USB becomes more popular.
•Has been applied in ways that its original
creators never imagined, sometimes leading to
problems.
•Original spec defined 25 lines, but often only 9
or fewer are used.
Terminology: DCE vs DTE
•In any RS-232 application, each device is
designated as either Data Terminal Equipment
(DTE) or Data Communications Equipment
(DCE).
•Simple case: When you connect a personal
computer to a modem, the computer is the DTE
and the modem is the DCE.
Connectors
•Original RS-232 standard called for a DB-25
connector. Since many later applications didn’t
use most of the pins, it became common to use
DE-9 connectors (often mistakenly referred to
as DB-9).
RS-232 Signals
•The nine most important signals:
Description
Abbrev.
Direction
DTE - DCE
DB-25
Pin #
DE-9
Pin #
Transmitted data
TxD

2
3
Received data
RxD

3
2
Request to send
RTS

4
7
Clear to send
CTS

5
8
Signal Ground
7
5
Protective Ground
1
Data set Ready
DSR

6
6
Data carrier detect
DCD

8
1
Data terminal ready
DTR

20
4
RS-232 Handshaking
•The four handshaking signals defined in RS232 are:
•Request to Send (RTS)
•Clear to Send (CTS)
•Data Terminal Ready (DTR)
•Data Set Ready (DSR)
RS-232 Voltage Levels
•Roughly speaking, TTL voltage levels are:
• 0 V for a binary 0
• +5 V for a binary 1.
•This scheme is “unipolar” because it doesn’t
use negative voltages.
•For transmission over a cable, it’s undesirable
to have either logic level close to 0 V.
•So RS-232 uses a “bipolar” scheme, with:
• +3 V to +25 V for a binary 0 (“space”)
• -3 V to -25 V for a binary 1 (“mark”)
Limitations of RS-232
•While RS-232 is still widely used, it has many
technical limitations. The most obvious are:
•Maximum speed: 20 kbaud
•Maximum cable length: 50 feet
•“Point-to-point” communication: just two
users communicating
Some Parallel Bus Standards
•Original PC Printer Port (“Centronix Port”)
•IEEE 488 (“GPIB”)
•SCSI (Small Computer System Interface)
IEEE 488 Standard (“GPIB”)
•GPIB = “General Purpose Interface Bus”
•Formerly known as HPIB (“Hewlett-Packard
Interface Bus”)
•First version created in late 1960’s.
•Primarily used to connect test equipment
(power supplies, function generators,
multimeters, oscilloscopes, etc.) and let them
be controlled by a computer.
GPIB Controller, Talker, & Listener
•In any GPIB application there must be one
(and only one) controller.
•There can also be up to 14 other devices,
which are categorized as talkers, listeners, or
talker/listeners.
•Each device must be assigned a unique
address within the network.
GPIB Connectors
•The GPIB standard calls for a 24-pin
connector:
•8 data lines
•8 ground lines
•8 control lines (= 3 handshake lines and 5
bus-management lines)
GPIB Control Lines
•The 3 handshake lines:
Description
Abbrev.
Originates from
Data valid
DAV
Talker
Not ready for data
NRFD
Listener
Not data accepted
NDAC
Listener
•The 5 bus management lines:
Description
Abbrev.
Originates from
Interface clear
IFC
Controller
Attention
ATN
Controller
Remote enable
REN
Controller
Service request
SRQ
Any device
End or identify
EOI
Talker
Ports on our Fluke 45 DMMs
IEEE 488 (“GPIB”) option
not installed (Parallel)
RS-232
(Serial)
Ports on our Tektronix TDS2014s
RS-232
(Serial)
Printer
(Parallel)
IEEE 488 (“GPIB”)
(Parallel)
Instrument Control without
LabVIEW
•Without LabVIEW, it’s still possible to establish
communications between a computer and an
instrument with an appropriate port.
•Free software such as Windows Hyperterminal
lets you communicate with any device
connected to the computer’s RS-232 port.
Using Hyperterminal with a Fluke
45 Multimeter
1. Connect serial cable.
2. Meter setup:
2nd Rate, 9600, n, on.
3. Connect meter’s leads to func gen set for
half-max amplitude & 5 kHz.
4. Hyperterminal setup:
COM1, 9600, 8, N, 1, XON/XOFF.
5. Commands:
*IDN?
VAC
FREQ2
VAL?
Using Hyperterminal with a
Tektronix TDS2014 Oscilloscope
1. Connect serial cable.
2. Scope setup: Utility> Options> RS232> 19200, Hard,
LF/CR, None.
3. Connect probe to compensation terminal.
4. Hyperterminal setup: COM1, 19200, 8, N, 1,
HARDWARE.
5. Use Notepad to create text file:
*IDN?
Autoset execute
measurement:immed:source ch1
measurement:immed:type pk2
measurement:immed:value?
6. Save file & send it from Hypterminal using Transfer
> Send Text File.
Instrument Control Using LabVIEW
• LabVIEW gives you a few different ways to
control instruments:
1. Using the same low-level commands that
we’ve been using in Hyperterminal.
2. Using an Instrument Driver, which
typically is written by an employee of the
company that manufactured the
instrument.
3. Using the Instrument I/O Assistant.
• Option #2 is usually the best and easiest
way to go, assuming that an Instrument
Driver is available.
Option #1: Using Low-Level
Commands
• Within LabVIEW’s Instrument I/O palette
are several sub-palettes, including the
Serial sub-palette. This contains functions
that let you send the same commands we’ve
been entering in Hypterterminal.
Option #2: Using an Instrument
Driver
• NI’s website contains Instrument Drivers for
thousands of instruments manufactured by
dozens of companies:
http://www.ni.com/downloads/instrument-drivers/
• A typical instrument driver consists of one or
more llb (LabVIEW library) files containing
VIs to designed to control a specific device.
• Usually one of these VIs is named “Getting
Started,” and you should start with that one.
Example: Instrument Driver for
Fluke 45
Option #3: Using the Instrument
I/O Assistant
• If you can’t find an Instrument Driver for your
instrument, LabVIEW’s Instrument I/O
Assistant provides a wizard-type dialog box
to help you configure and control your
device.