-----------------------------Monitor------------------------------
The Newsletter for PC-Based Data Acquisition and Control
Issue 28 www.windmill.co.uk November 2000
-------------------------ISSN 1472-0221---------------------------
Welcome to another edition of Monitor. This month we are pleased
to announce the launch of Windmill Remote software, for condition
monitoring over the internet. For those more interested in local
data acquisition, we also discuss the GPIB.
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CONTENTS
========
* Launch of Windmill Remote for Data Acquisition over the
Internet
* Local Monitoring: The GPIB
- Connect up to 14 instruments to 1 PC
- High speed
- Large numbers of measurement and control channels
- Technical details
- Summary
- Further Reading
_________________________________________________________________
Windmill News:
Windmill Software Launch Windmill Remote
Windmill Software is proud to use this issue to announce the
global launch of our new Windmill Remote software. Windmill Remote
lets you monitor conditions at a distance, and view live data over
the internet. It is especially useful for hazardous environments.
Designed to incorporate GPS measurements, it is also ideal for
mobile systems where location needs to be logged.
Windmill Remote lets you monitor as many remote stations as
necessary. With proper permission, Administrators may also control
the monitoring station: setting different parameters to be logged
(temperature, speed, etc), new update intervals, fresh alarm
settings, different telephone numbers to be dialled to alert people
to alarms, and so on.
For more details of Windmill Remote contact sales@biodataltd.com or
visit https://www.windmill.co.uk/wmremote.html
_________________________________________________________________
Local Monitoring - the GPIB?
There are many things to consider when choosing data acquisition
equipment, including how you will connect it to the PC. Many
cabling options exist: USB, RS-232, Ethernet, Modbus, GPIB, etc.
Each method has its advantages and disadvantages, many of which we
summarised in Issue 2 of Monitor
(https://www.windmill.co.uk/freewm.html). This month - why use the
General Purpose Interface Bus?
The original name for the GPIB was HPIB, or Hewlett-Packard
Interface Bus. As the name implies, the bus was designed by
Hewlett-Packard. It was subsequently accepted as the IEEE Standard
488. Originally peripherals like printers and plotters were
commonly connected by the GPIB. Although this is no longer true,
the GPIB continues to be a popular for data acquisition systems -
especially in high speed applications.
*
Connect up to 14 Instruments to one PC
======================================
With the GPIB you can connect up to 14 instruments to one PC. Each
instrument should be less than 2 metres from the next. You need a
GPIB adaptor card in your computer and a GPIB lead. The GPIB
adaptor will come with driver softwar. You might also receive
functions that let you write your own programs to access the GPIB.
Alternatively you could use off-the-shelf applications like
Windmill, Streamer or WaveCap software.
*
High Speed
==========
According to the specification, the GPIB allows speeds of 1 Mbyte
per second. However, in practice, the maximum speeds tend to be
around 250 Kbytes per second.
*
Large Numbers of Measurement and Control Channels
=================================================
Each device on the GPIB must have a unique, primary, address which
is a number from 0 to 30. Secondary addresses are also available
for modular instruments. For example, a frame or case could contain
a number of plug-in boards that perform different functions:
timing, analogue input, digital input and output, etc. Using the
GPIB the computer can address each module individually. As 961
secondary addresses are available, and each module might provide 16
or 32 inputs or outputs, there is potential for vast numbers of
measurement and control channels.
*
Technical Details
=================
At any time there must be one device on the bus that is a
controller. This device issues commands to the other devices, and
in our example is the PC. The other devices may be Talkers -
putting data onto the bus, Listeners - reading from the bus, or
inactive - neither talking nor listening. Only one device may talk
at once, but more than one may listen to the Talker.
The GPIB uses a 16 line parallel connection which has strictly
defined mechanical and electrical properties. The 16 lines are
divided into 8 data lines, 3 handshake lines to synchronise the
transfer and 5 management lines to control use of the bus.
The management lines are
ATN - Attention - Indicates current use of data lines: commands
(true) or data (false)
IFC - Interface Clear - Used by the controller to reset all devices
on the bus to inactive
EOI - End or Identify - Depending whether ATN is in command or data
mode, requests device status or indicates the end of data transfer
SRQ - Service Request - A device which requires attention may use
SRQ to signal the controller
REN - Remote Enable - Used to switch a device between local control
and remote control. Not used on many devices.
The three handshake lines guarantee transmission integrity.
NFRD - Not Ready for Data - indicates when a device is ready for a
message
NDAC - Not Data accepted - Indicates when a device has accepted a
message
DAV - Data Valid - Indicates when signals on the data lines are
stable and valid and can thus be accepted
The GPIB uses TTL levels with negative logic. Lines are true when
they are at the TTL low level: 0.8 V or less. Lines are false when
they are at the TTL high level: 2.0 V or more.
*
Summary of the GPIB
===================
- Can connect 14 devices, from different manufacturers, to one PC
- High speed: the GPIB transmits data bits in parallel rather than
in series
- Individual modules can be addressed, allowing an enormous number
of measurement and control channels
- Reliable and efficient method of communication because each byte
is handshaked
- For systems with instruments close to the computer
- The GPIB specification gives precise talking and forgiving
listening
*
Further Reading
===============
ANSI/IEEE standard 488.1-1987 Standard Digital Interface for
Programmable Instrumentation.
ANSI/IEEE standard 488.2-1992 Codes, Formats, Protocols and
Commands for Programmable Instruments
Microlink Technical Notes: GPIB
http://www.microlink.co.uk/gpib.html
Agilent: Fundamentals of HP-IB
http://www.agilent.com/
Monitor
Issue 2, Summary of Communication Links
https://www.windmill.co.uk/freewm.html
Issue 15, RS-232 Communications
https://www.windmill.co.uk/rs-232.html
Issue 24, Universal Serial Bus
https://www.windmill.co.uk/monitor24.html
_________________________________________________________________
* Copyright Windmill Software Ltd
* Reprinting permitted with this notice included
* For more articles see https://www.windmill.co.uk
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Windmill Software Ltd, PO Box 58, North District Office,
Manchester, M8 8QR, UK
Telephone: +44 (0)161 834 6688
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E-mail: monitor@windmillsoft.com
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