This collection of wires that I keep referring
to between the tank and the monitoring location can be called a bus
or a network. The distinction between these two terms is more
semantic than technical, and the two may be used interchangeably for all
practical purposes. In my experience, the term "bus" is usually used in
reference to a set of wires connecting digital components within the
enclosure of a computer device, and "network" is for something that is
physically more widespread. In recent years, however, the word "bus" has
gained popularity in describing networks that specialize in interconnecting
discrete instrumentation sensors over long distances ("Fieldbus" and "Profibus"
are two examples). In either case, we are making reference to the means by
which two or more digital devices are connected together so that data can be
communicated between them.
Names like "Fieldbus" or "Profibus" encompass not only the physical
wiring of the bus or network, but also the specified voltage levels for
communication, their timing sequences (especially for serial data
transmission), connector pinout specifications, and all other distinguishing
technical features of the network. In other words, when we speak of a
certain type of bus or network by name, we're actually speaking of a
communications standard, roughly analogous to the rules and
vocabulary of a written language. For example, before two or more people can
become pen-pals, they must be able to write to one another in a common
format. To merely have a mail system that is able to deliver their letters
to each other is not enough. If they agree to write to each other in French,
they agree to hold to the conventions of character set, vocabulary,
spelling, and grammar that is specified by the standard of the French
language. Likewise, if we connect two Profibus devices together, they will
be able to communicate with each other only because the Profibus standard
has specified such important details as voltage levels, timing sequences,
etc. Simply having a set of wires strung between multiple devices is not
enough to construct a working system (especially if the devices were built
by different manufacturers!).
To illustrate in detail, let's design our own bus standard. Taking the
crude water tank measurement system with five switches to detect varying
levels of water, and using (at least) five wires to conduct the signals to
their destination, we can lay the foundation for the mighty BogusBus:
The physical wiring for the BogusBus consists of seven wires between the
transmitter device (switches) and the receiver device (lamps). The
transmitter consists of all components and wiring connections to the left of
the leftmost connectors (the "-->>--" symbols). Each connector symbol
represents a complementary male and female element. The bus wiring consists
of the seven wires between the connector pairs. Finally, the receiver and
all of its constituent wiring lies to the right of the rightmost connectors.
Five of the network wires (labeled 1 through 5) carry the data while two of
those wires (labeled +V and -V) provide connections for DC power supplies.
There is a standard for the 7-pin connector plugs, as well. The pin layout
is asymmetrical to prevent "backward" connection.
In order for manufacturers to receive the awe-inspiring "BogusBus-compliant"
certification on their products, they would have to comply with the
specifications set by the designers of BogusBus (most likely another
company, which designed the bus for a specific task and ended up marketing
it for a wide variety of purposes). For instance, all devices must be able
to use the 24 Volt DC supply power of BogusBus: the switch contacts in the
transmitter must be rated for switching that DC voltage, the lamps must
definitely be rated for being powered by that voltage, and the connectors
must be able to handle it all. Wiring, of course, must be in compliance with
that same standard: lamps 1 through 5, for example, must be wired to the
appropriate pins so that when LS4 of Manufacturer XYZ's transmitter closes,
lamp 4 of Manufacturer ABC's receiver lights up, and so on. Since both
transmitter and receiver contain DC power supplies rated at an output of 24
Volts, all transmitter/receiver combinations (from all certified
manufacturers) must have power supplies that can be safely wired in
parallel. Consider what could happen if Manufacturer XYZ made a transmitter
with the negative (-) side of their 24VDC power supply attached to earth
ground and Manufacturer ABC made a receiver with the positive (+) side of
their 24VDC power supply attached to earth ground. If both earth grounds are
relatively "solid" (that is, a low resistance between them, such as might be
the case if the two grounds were made on the metal structure of an
industrial building), the two power supplies would short-circuit each other!
BogusBus, of course, is a completely hypothetical and very impractical
example of a digital network. It has incredibly poor data resolution,
requires substantial wiring to connect devices, and communicates in only a
single direction (from transmitter to receiver). It does, however, suffice
as a tutorial example of what a network is and some of the considerations
associated with network selection and operation.
There are many types of buses and networks that you might come across in
your profession. Each one has its own applications, advantages, and
disadvantages. It is worthwhile to associate yourself with some of the
"alphabet soup" that is used to label the various designs:
Short-distance busses
PC/AT Bus used in early IBM-compatible computers to connect
peripheral devices such as disk drive and sound cards to the motherboard of
the computer.
PCI Another bus used in personal computers, but not limited to
IBM-compatibles. Much faster than PC/AT. Typical data transfer rate of 100
Mbytes/second (32 bit) and 200 Mbytes/second (64 bit).
PCMCIA A bus designed to connect peripherals to laptop and
notebook sized personal computers. Has a very small physical "footprint,"
but is considerably slower than other popular PC buses.
VME A high-performance bus (co-designed by Motorola, and based on
Motorola's earlier Versa-Bus standard) for constructing versatile industrial
and military computers, where multiple memory, peripheral, and even
microprocessor cards could be plugged in to a passive "rack" or "card cage"
to facilitate custom system designs. Typical data transfer rate of 50
Mbytes/second (64 bits wide).
VXI Actually an expansion of the VME bus, VXI (VME eXtension for
Instrumentation) includes the standard VME bus along with connectors for
analog signals between cards in the rack.
S-100 Sometimes called the Altair bus, this bus standard was the
product of a conference in 1976, intended to serve as an interface to the
Intel 8080 microprocessor chip. Similar in philosophy to the VME, where
multiple function cards could be plugged in to a passive "rack,"
facilitating the construction of custom systems.
MC6800 The Motorola equivalent of the Intel-centric S-100 bus,
designed to interface peripheral devices to the popular Motorola 6800
microprocessor chip.
STD Stands for Simple-To-Design, and is yet another passive
"rack" similar to the PC/AT bus, and lends itself well toward designs based
on IBM-compatible hardware. Designed by Pro-Log, it is 8 bits wide
(parallel), accommodating relatively small (4.5 inch by 6.5 inch) circuit
cards.
Multibus I and II Another bus intended for the flexible design of
custom computer systems, designed by Intel. 16 bits wide (parallel).
CompactPCI An industrial adaptation of the personal computer PCI
standard, designed as a higher-performance alternative to the older VME bus.
At a bus clock speed of 66 MHz, data transfer rates are 200 Mbytes/ second
(32 bit) or 400 Mbytes/sec (64 bit).
Microchannel Yet another bus, this one designed by IBM for their
ill-fated PS/2 series of computers, intended for the interfacing of PC
motherboards to peripheral devices.
IDE A bus used primarily for connecting personal computer hard
disk drives with the appropriate peripheral cards. Widely used in today's
personal computers for hard drive and CD-ROM drive interfacing.
SCSI An alternative (technically superior to IDE) bus used for
personal computer disk drives. SCSI stands for Small Computer System
Interface. Used in some IBM-compatible PC's, as well as Macintosh
(Apple), and many mini and mainframe business computers. Used to interface
hard drives, CD-ROM drives, floppy disk drives, printers, scanners, modems,
and a host of other peripheral devices. Speeds up to 1.5 Mbytes per second
for the original standard.
GPIB (IEEE 488) General Purpose Interface Bus, also known
as HPIB or IEEE 488, which was intended for the interfacing of electronic
test equipment such as oscilloscopes and multimeters to personal computers.
8 bit wide address/data "path" with 8 additional lines for communications
control.
Centronics parallel Widely used on personal computers for
interfacing printer and plotter devices. Sometimes used to interface with
other peripheral devices, such as external ZIP (100 Mbyte floppy) disk
drives and tape drives.
USB Universal Serial Bus, which is intended to interconnect
many external peripheral devices (such as keyboards, modems, mice, etc.) to
personal computers. Long used on Macintosh PC's, it is now being installed
as new equipment on IBM-compatible machines.
FireWire (IEEE 1394) A high-speed serial network capable of
operating at 100, 200, or 400 Mbps with versatile features such as "hot
swapping" (adding or removing devices with the power on) and flexible
topology. Designed for high-performance personal computer interfacing.
Bluetooth A radio-based communications network designed for office
linking of computer devices. Provisions for data security designed into this
network standard.
Extended-distance networks
20 mA current loop Not to be confused with the common
instrumentation 4-20 mA analog standard, this is a digital communications
network based on interrupting a 20 mA (or sometimes 60 mA) current loop to
represent binary data. Although the low impedance gives good noise immunity,
it is susceptible to wiring faults (such as breaks) which would fail the
entire network.
RS-232C The most common serial network used in computer systems,
often used to link peripheral devices such as printers and mice to a
personal computer. Limited in speed and distance (typically 45 feet and 20
kbps, although higher speeds can be run with shorter distances). I've been
able to run RS-232 reliably at speeds in excess of 100 kbps, but this was
using a cable only 6 feet long! RS-232C is often referred to simply as
RS-232 (no "C").
RS-422A/RS-485 Two serial networks designed to overcome some of
the distance and versatility limitations of RS-232C. Used widely in industry
to link serial devices together in electrically "noisy" plant environments.
Much greater distance and speed limitations than RS-232C, typically over
half a mile and at speeds approaching 10 Mbps.
Ethernet (IEEE 802.3) A high-speed network which links computers
and some types of peripheral devices together. "Normal" Ethernet runs at a
speed of 10 million bits/second, and "Fast" Ethernet runs at 100 million
bits/second. The slower (10 Mbps) Ethernet has been implemented in a variety
of means on copper wire (thick coax = "10BASE5", thin coax = "10BASE2",
twisted-pair = "10BASE-T"), radio, and on optical fiber ("10BASE-F"). The
Fast Ethernet has also been implemented on a few different means
(twisted-pair, 2 pair = 100BASE-TX; twisted-pair, 4 pair = 100BASE-T4;
optical fiber = 100BASE-FX).
Token ring Another high-speed network linking computer devices
together, using a philosophy of communication that is much different from
Ethernet, allowing for more precise response times from individual network
devices, and greater immunity to network wiring damage.
FDDI A very high-speed network exclusively implemented on
fiber-optic cabling.
Modbus/Modbus Plus Originally implemented by the Modicon
corporation, a large maker of Programmable Logic Controllers (PLCs) for
linking remote I/O (Input/Output) racks with a PLC processor. Still quite
popular.
Profibus Originally implemented by the Siemens corporation,
another large maker of PLC equipment.
Foundation Fieldbus A high-performance bus expressly designed to
allow multiple process instruments (transmitters, controllers, valve
positioners) to communicate with host computers and with each other. May
ultimately displace the 4-20 mA analog signal as the standard means of
interconnecting process control instrumentation in the future. |