RS-422 and RS-485
November 2002 Vol.4 No.11
Inside this issue:
• RS-422: An Alternative to
RS-232 Serial
• New Lava RS-422 Boards
• DOS on Steroids: USB for DOS
• Distributor Profile
Lava I/O News
RS-422: An Alternative
to RS-232
Users of serial devices who need high data
rates, long-distance serial cabling, or multiple
devices on one port might consider RS-422 as
an alternative to RS-232. RS-232, the most
common serial interface, has limits on speed,
cable distance, and device support that can be
problems when designing Point-of-Sale (POS)
systems, factory automation installations,
building automation systems, and setups
facing high levels of electrical noise.
Demanding installations of these sorts are
often better served by RS-422 serial
connections.
Signalling Method
RS-422 differs from RS-232 primarily in the
electrical method it uses to carry its signals.
RS-422's transmit and receive signals use a
method of signalling called "differential
signalling" or "balanced transmission." With
differential signalling, a given signal uses a
pair of wires dedicated to that purpose. The
signal is determined by the difference in
voltage between these two wires. By contrast,
RS-232 signals are determined with reference
to a common ground signal carried on its own
wire, in a method termed "unbalanced
transmission." The diagrams below show this
difference for a single signal.
Balanced Transmission
Transmitter
Receiver
Line A
Line B
Balanced signalling of the sort used by RS-422
is also used in LAN protocols such as Ethernet
and Token Ring, as well as in other connection
methods that use the familiar DB connector
format. RS-485, RS-449 (DB-37 connector),
RS-530 (DB-25 connector), and x.21 (DB-15
connector) are examples.
The advantage of differential signalling
comes mainly from its strong ability to
maintain signal integrity. Because a signal is
determined by the difference in voltage
between a pair of wires, it will be unaffected
by electrical changes–such as electrical noise
or end-to-end differences in ground
signal–that influence both wires in the same
degree. This signal integrity is especially
strong when each wire pair has its strands
twisted together: hence the term "twisted pair
wiring." When wires are in twisted pairs,
electrical noise influencing a cable will tend to
affect both wires of a pair equally.
Because of this, the voltage differential
between Line A and Line B for RS-422 can be
as little as ± 0.2 V and still be considered a valid
signal.That is, a voltage less than -0.2 volts has
a truth value of 0, and a voltage greater than
+0.2 volts has a truth value of 1 for the
purpose of representing a signal. For RS-232,
the voltage differentials between the signal
line and the ground line must be ± 5.0 V.It can
be seen that balanced transmission can
operate within much finer tolerances than
RS-232.
Unbalanced Transmission
Transmitter
Receiver
Line A
Ground
Cable Runs
Consequently, RS-422 is capable of
substantially longer cable runs than RS-232.
How much longer is a function of the
signalling rate: the faster the signalling rate,
the shorter the maximum cable length. At a 10
Mbps signalling rate, RS-422 cables max out at
about 15 feet. On the other hand, the
permissible cable length increases
exponentially as speeds drop; at a signalling
Balanced Transmission: RS-422 Cable Lengths
10,000
SS-PCI RS422
P.N. MOKO L93.X
1,000
MADE
IN
PN MOKO S-1
0255351
CANADA
LAVA COMPUTERS
RS422 SS-PCI
100
10
10K
MADE
IN
CANADA
100K
1M
Data Rate (Bits per second)
10M
DS-PCI RS422
P.N. MOKO L97.0
LAVA
COMPUTERS
PN MOKO S-1
MADE IN JAPAN
0255351
RS-422 Topology
RS-422 employs a master/slave device
structure. This structure allows one master
device to communicate with up to 10 slave
devices; slaves must communicate to other
slaves through the master device. Again, this
feature contrasts with RS-232, which is simply
device-to-device connection.
RS-422 Drawbacks
On the down side, RS-422 cabling is
necessarily more expensive than that needed
for RS-232, mainly because RS-422 requires
more wires to carry a given number of signals,
and because this cabling should be twisted
pair. The RS-422 specification stipulates that
cabling be 24 AWG twisted pair with a shunt
capacitance of 16 pF per foot and 100 ohm
characteristic impedance. Specialized RS-422
cabling to meet these requirements can be
expensive, but fortunately typical CAT 5
networking cable (UTP or STP) meets or
exceeds RS-422 requirements, and generally
at a lower cost.
A second drawback is that RS-422 is simply
not as common as RS-232.However,an RS-232
device can be easily attached to RS-422 with
readily available and inexpensive interface
adapters.
Finally, avoiding signal reflections that can
occur because of propagation delays may
become important when setting up an RS422 connection with long cable runs or
particularly high data rates. To eliminate
signal reflections, RS-422 lines should be
terminated with a resistor that has the effect
of making the cable, electrically speaking,
infinitely "long" from the point of view of the
transmitter/receiver.In the case of RS-422, the
resistance value is the same as the
characteristic impedance of the cable–100
ohms.
RS422 DS-PCI
LAVA COMPUTERS
QUAD-PCI RS422
MADE
IN
PN MOKO S-1
MADE IN JAPAN
0255351
RS422 Quattro-PCI
All in all, RS-422 fills a need for features in serial
connectivity that cannot be found in RS-232
serial. For those who need these features, RS422 is an effective and low cost technology.
LAVA
RS-422
MADE IN
CANADA
LAVA COMPUTERS
P.N. MOKO L26.2
RS422 SS-550
2
CANADA
P.N. MOKO L98.0
SERIAL 2
rate of 115.2 kbps, RS-422 cables can be up to
4000 feet long. The graph on the next page
shows the relationship between data rate and
cable length for RS-422. At any given data rate
RS-422 supports much longer cabling than
RS-232.
SERIAL 1
Cable Length (Feet)
4,000
Profile
New Lava RS-422 Boards
Lava has added three new RS-422 boards to its lineup: the RS422 SS-PCI, the RS422 DSPCI, and the RS422 Quattro-PCI.The boards add one,two, or four RS-422 ports to PCI-bus
equipped PCs.
The connectors on these boards have nine-pin RS-422 pinouts, and all boards are fully
Plug and Play compatible. Each of these boards uses just one IRQ. This makes the twoand four-port boards particularly efficient in their use of system resources when
available IRQs are scarce.
The RS422 SS-PCI, RS422 DS-PCI, and RS422 Quattro-PCI all use 16550 UARTs on their
ports, making them capable of data throughput rates up to 115.2 kbps.
These new boards join Lava's RS422-550 ISA-bus two-port card, and are ideally suited to
industrial and build customers who are looking for robust and long-distance serial
connectivity. Like all Lava boards, the RS422 SS-PCI, RS422 DS-PCI, and RS422 QuattroPCI are covered by the Lava LifetimeWarranty.
DOS on Steriods
Sometimes it seems that adopting
new technologies pushes us to evernewer versions of operating systems
and hardware. USB 2.0 is a case in point:
it's got great speed and convenience, but operating
system support for USB 2.0 is not always available. PC users with
Windows 3.x, 95,98, Me,or Nt 4.0 have had to accept that Microsoft does not plan to add
native USB 2.0 support to these operating systems, ever.
Now a solution has come along for USB storage devices such as the Kazan, Lava's USB 2.0
external IDE hard drive enclosure. It arrives for an unexpected, but logical platform:good
old DOS. Cypress Semiconductors, a maker of USB-to-IDE bridge chips (including the
bridging chip used in Lava's Kazan), has written DOS drivers for USB 1.1 and USB 2.0
storage devices.
These drivers support USB hardware that meets the Open Host Controller Interface
(OHCI) specification, the Universal Host Controller Interface (UHCI) specification, and the
Enhanced Host Controller Interface (EHCI) specification; that is,USB 1.1 and USB 2.0. On
the storage device side, the drivers work with many devices, such as USB ZIP drives, USB
hard disks, USB Super Floppy/HiFD drives, USB CD-ROMs, and USB MO drives.
Specifically, the driver handles devices that use the SL11R-IDE, FX/FX2,and ISD chipsets
for their USB-to-storage interface (Lava's Kazan uses the ISD chipset). On the host
controller side, the drivers have been tested with various popular Intel, VIA, and NEC
EHCI controllers; as well as with various USB 1.1 chipsets and configurations. The Lava
USB 1.1 and USB 2.0 Host Adapters work perfectly, for example. Finally, four different
versions of DOS have been tested to date:DOS 6.22, Windows 98SE DOS, Windows 98 Me
DOS, and Windows XP DOS.
The driver is a small file that can be loaded by CONFIG.SYS on booting, or run as a
separate executable. USB operates as simply in DOS as it does in other operating
systems: when attached to a USB port, a USB mass storage device will automatically
mount and be assigned the next available drive letter.If a DOS system has a single drive
designated as drive C: and a USB drive is inserted,then the USB driver will assign drive D:
to the new drive. The drive will be accessible as a regular drive in a DOS system. If the
drive is removed and then reinserted, the driver will reassign the D: drive letter and the
device will become accessible once again.
Best of all, this software is compact, simple to use, and free. Download a copy from
www.lavalink.com and watch the Kazan USB 2.0 external hard drive enclosure fly. The
advantages of USB 2.0 storage are suddenly accessible from DOS.
3
Trimex Marketing Inc. (TMI) is one of
Canada's oldest companies in their
field–they started business in 1982.
Since then, TMI has been providing
wholesale computer accessories,
peripherals, and supplies to a global
market of computer resellers.
TMI is a distributor for over 3,000
computer product resellers in Canada
alone. Their main focus is in several key
areas, including computer product
retailers, value added resellers, and
system integrators. They pride ourselves
on having the most comprehensive
product mix in the industry.
TMI also places the highest priority on
high order fill rates, efficient, friendly
and helpful service, and aggressive
pricing. Their goal is to offer their
customers and clients the best overall
value and quality, which in turn allows
them to compete effectively in today's
exciting and fast changing market.
Trimex carries many national and
international brands, and they also carry
exclusive brands, including their own
PerfectMicro and PerfectLink series of
accessories, cables, peripherals and and
supplies. They also actively support,
promote, and sell products made by
local and North American
manufacturers, including Lava boards.
Trimex Marketing Inc.
Trimex Marketing Inc.
120-4611 Viking Way
Richmond, BC
Canada V6V 2K9
TEL: 1.800.663.8883
FAX: 1.888.869.3882
www.trimex.com
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