Windmill Software Ltd
Data Acquisition Intelligence

multiplexing
November 2000

-----------------------------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.

We only send this newsletter to people who have subscribed -- 
should you wish to cancel your free subscription please visit 
http://www.windmill.co.uk/newsletter.html

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 [email protected] or 
visit http://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
(http://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
http://www.windmill.co.uk/freewm.html
Issue 15, RS-232 Communications
http://www.windmill.co.uk/rs-232.html
Issue 24, Universal Serial Bus
http://www.windmill.co.uk/monitor24.html
_________________________________________________________________

* Copyright Windmill Software Ltd
* Reprinting permitted with this notice included
* For more articles see http://www.windmill.co.uk

We are happy for you to copy and distribute this 
newsletter, and use extracts from it on your own web site 
or publication, providing the above notice is 
included and a link back to our website is in place.

An archive of previous issues is at 
http://www.windmill.co.uk/newsletter.html
and an index of articles at 
http://www.windmill.co.uk/newsletter.html

Windmill Software Ltd, PO Box 58, North District Office,
Manchester, M8 8QR, UK
Telephone: +44 (0)161 833 2782
Facsimile: +44 (0)161 833 2190
E-mail: [email protected]

http://www.windmill.co.uk/

https://www.windmillsoft.com/



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