RS-422

a
Tutorial

RS422 BALANCED
DIFFERENTIAL DRIVERS

Line drivers and receivers are commonly used to
exchange data between two or more points (nodes) on a network. Reliable data
communications can be difficult in the presence of induced noise, ground level
differences, impedance mismatches, failure to effectively bias for idle line
conditions, and other hazards associated with installation of a network.

Standards have been developed to insure compatibility between units
provided by different manufacturers, and to allow for reasonable success in
transferring data over specified distances and/or data rates. The Electronics
Industry Association (EIA) has produced standards for RS485, RS422, RS232, and
RS423 that deal with data communications. Suggestions are often made to deal
with practical problems that might be encountered in a typical network. EIA
standards where previously marked with the prefix "RS" to indicate recommended
standard; however, the standards are now generally indicated as "EIA" standards
to identify the standards organization. While the standards bring uniformity to
data communications, many areas are not specifically covered and remain as "gray
areas" for the used to discover (usually during installation) on his
own

The balanced voltage digital interface circuit
will normally be utilized on data, timing, or control circuits where the data
signaling rate is up to 10 Mbit/s. Balanced voltage digital interface devices
meeting the electrical characteristics of  need not operate over the entire
data signaling rate range specified. They may be designed to operate over
narrower ranges to satisfy more economically specific applications, particularly
at the lower modulation rates.

When communicating at high data rates, or over long
distances in real world environments, single-ended methods are often inadequate.
Differential data transmission (balanced differential signal) offers superior
performance in most applications. Differential signals can help nullify the
effects of ground shifts and induced noise signals that can appear as common
mode voltages on a network.

RS422 (differential) was designed for
greater distances and higher Baud rates than RS232. In its simplest form, a pair
of converters from RS232 to RS422 (and back again) can be used to form an "RS232
extension cord." Data rates of up to 100K bits / second and distances up to 4000
Ft. can be accommodated with RS422. RS422 is also specified for multi-drop
(party-line) applications where only one driver is connected to, and transmits
on, a "bus" of up to 10 receivers.

While a multi-drop "type" application
has many desirable advantages, RS422 devices cannot be used to construct a truly
multi-point network. A true multi-point network consists of multiple drivers and
receivers connected on a single bus, where any node can transmit or receive
data.

"Quasi" multi-drop networks (4-wire) are often constructed using
RS422 devices. These networks are often used in a half-duplex mode, where a
single master in a system sends a command to one of several "slave" devices on a
network. Typically one device (node) is addressed by the host computer and a
response is received from that device. Systems of this type (4-wire,
half-duplex) are often constructed to avoid "data collision" (bus contention)
problems on a multi-drop network (more about solving this problem on a two-wire
network in a moment).

Compatibility With Other Interfaces

Both RS-422 and RS-485 use a twisted-pair wire (i.e.
2 wires) for each signal. They both use the same differential drive with
identical voltage swings: 0 to +5V. The main difference between RS-422 and
RS-485 is that while RS-422 is strictly for point-to-point communications (and
the driver is always enabled), RS-485 can be used for multidrop systems (and the
driver has a tri-state capability).

As stated in the scope of this Standard, generators and receivers
meeting the requirements of RS-422-A are compatible with those meeting CCITT
Recommendations V. 11 and X.27. The electrical characteristics of the balanced
voltage digital interface are designed to allow use of both balanced and
unbalanced (see EIA Standard RS-423-A) circuits within the same interconnection
cable sheath. For example, the balanced circuits may be used for data and timing
while the unbalanced circuits may be used for low speed control
functions.

Since the basic differential receivers of
RS-423-A and RS422-A are electrically identical, it is possible to interconnect
an equipment using RS423-A receivers and generators on one side of the interface
with an equipment using RS422-A generators and receivers on the other side of
the interface, if the leads of the receivers and generators are properly
configured to accommodate such an arrangement and the cable is not
terminated.

The balanced interface circuit is not intended
for interoperation with other interface electrical characteristics such as
RS-232-C. MIL-STD-188C and MIL-STD-188-100, and CCITT Recommendations V.28 and
V.35. Under certain conditions. interoperation with circuits of some of the
above interfaces may be possible but may require modification in the interface
or within the equipment; therefore, satisfactory operation is not assured. and
additional provisions not specified herein may be required.

SPECIFICATIONS

RS423

RS422

Mode of
Operation

SINGLE - ENDED

DIFFERENTIAL

Total Number of Drivers and
Receivers on One Line

1 DRIVER
10 RECVR

1 DRIVER
10 RECVR

Maximum Cable
Length

4000 FT.

4000 FT.

Maximum Data
Rate

100kb/s

10Mb/s

Maximum Driver Output
Voltage

+/-6V

-0.25V to +6V

Driver Output Signal Level (Loaded
Min.)

Loaded

+/-3.6V

+/-2.0V

Driver Output Signal Level (Unloaded
Max)

Unloaded

+/-6V

+/-6V

Driver Load Impedance
(Ohms)

>450

100

Max. Driver Current in High Z
State

Power On

N/A

N/A

Max. Driver Current in High Z
State

Power Off

+/-100uA

+/-100uA

Slew Rate (Max.)

Adjustable

N/A

Receiver Input Voltage
Range

+/-12V

-10V to +10V

Receiver Input
Sensitivity

+/-200mV

+/-200mV

Receiver Input Resistance
(Ohms)

4k min.

4k
min.

Making sense of cable
specifications

Selecting data cable for an RS-422 or RS-485
system isn't difficult, but often gets lost in the shuffle of larger system
issues. Care should be taken. however, because intermittent problems caused by
marginal cable can be very difficult to troubleshoot.

Beyond the obvious
traits such as number of conductors and wire gauge, cable specifications include
a handful of less intuitive terms.

Characteristic Impedance (Ohms): A
value based on the inherent conductance, resistance, capacitance and inductance
of a cable that represents the impedance of an infinitely long cable. When the
cable is out to any length and terminated with this Characteristic Impedance,
measurements of the cable will be identical to values obtained from the infinite
length cable. That is to say that the termination of the cable with this
impedance gives the cable the appearance of being infinite length, allowing no
reflections of the transmitted signal. If termination is required in a system,
the termination impedance value should match the Characteristic Impedance of the
cable.

Shunt Capacitance (pF/ft): The amount of equivalent capacitive
load of the cable, typically listed in a per foot basis One of the factors
limiting total cable length is the capacitive load. Systems with long lengths
benefits from using low capacitance cable.

Propagation velocity (% of c):
The speed at which an electrical signal travels in the cable. The value given
typically must be multiplied by the speed of light (c) to obtain units of meters
per second. For example, a cable that lists a propagation velocity of 78% gives
a velocity of 0.78 X 300 X 106 - 234 X 106 meters per
second.

Plenum cable

Plenum rated cable is fire
resistant and less toxic when burning than non-plenum rated cable. Check
building and fire codes for requirements. Plenum cable is generally more
expensive due to the sheathing material used.

The RS-422 specification
recommends 24AWG twisted pair cable with a shunt capacitance of 16 pF per foot
and 100 ohm characteristic impedance. While the RS-485 specification does not
specify cabling, these recommendations should be used for RS485 systems as
well.

It can be difficult to quantity whether shielding is required in a
particular system or not, until problems arise. We recommend erring on the safe
side and using shielded cable. Shielded cable is only slightly more expensive
than unshielded.

There are many cables available meeting
the recommendations of RS-422 and RS-485, made specifically for that
application. Another choice is the same cable commonly used in the misted pair
Ethernet cabling. This cable, commonly referred to as Category 5 cable, is
defined by the ElA/TIA/ANSI 568 specification The extremely high volume of
Category 5 cable used makes it widely available and very inexpensive, often less
than half the price of specialty RS422/485 cabling. The cable has a maximum
capacitance of 17 pF/ft (14.5 pF typical) and characteristic impedance of 100
ohms.

Category 5 cable is available as shielded twisted pair (STP) as
well as unshielded twisted pair (UTP) and generally exceeds the recommendations
for RS-422 making it an excellent choice for RS-422 and RS-485
systems.

RS232 & Cable
Diagrams info
(how it works and spec. & pin outs)
X.21  info (how it
works and spec. & pin outs)
RS423 info (spec. & pin
outs)
RS485 info (how it works and spec. & pin outs)
RS449
info (how it works and spec. & pin
outs)
RS530 info
spec.
V.35 info (how it
works and spec.& pin outs)
IEEE-488 info (pin
layout)
USCO Codes
RJ-48C and RJ48S jack pin
out
RJ 11C thru RJ48 Jacks - a Glossary

jump to
Interface Converters pages
jump to more technical
information pages

ARC
ELECTRONICS
1-800-926-0226

jump to ... Home
Page

[email-masked]