QUICK REFERENCE
FOR
RS485, RS422, RS232
AND RS423

INTRODUCTION

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.

The connection between two or more elements (drivers and
receivers) should be considered a transmission line if the rise and/or
fall time is less than half the time for the signal to travel from the
transmitter to the receiver.

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 user to discover (usually
during installation) on his own.

SINGLE-ENDED DATA TRANSMISSION

Electronic data communications between elements will generally
fall into two broad categories: single-ended and differential. RS232
(single-ended) was introduced in 1962, and despite rumors for its early
demise, has remained widely used through the industry. The specification
allows for data transmission from one transmitter to one receiver at
relatively slow data rates (up to 20K bits/second) and short distances (up
to 50Ft. @ the maximum data rate).

Independent channels are
established for two-way (full-duplex) communications. The RS232 signals
are represented by voltage levels with respect to a system common (power /
logic ground). The "idle" state (MARK) has the signal level negative with
respect to common, and the "active" state (SPACE) has the signal level
positive with respect to common. RS232 has numerous handshaking lines
(primarily used with modems), and also specifies a communications
protocol. In general if you are not connected to a modem the handshaking
lines can present a lot of problems if not disabled in software or
accounted for in the hardware (loop-back or pulled-up). RTS (Request to
send) does have some utility in certain applications. RS423 is another
single ended specification with enhanced operation over RS232; however, it
has not been widely used in the industry.

DIFFERENTIAL DATA TRANSMISSION

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).

RS485 meets the requirements for a truly multi-point
communications network, and the standard specifies up to 32 drivers and 32
receivers on a single (2-wire) bus. With the introduction of "automatic"
repeaters and high-impedance drivers / receivers this "limitation" can be
extended to hundreds (or even thousands) of nodes on a network. RS485
extends the common mode range for both drivers and receivers in the
"tri-state" mode and with power off. Also, RS485 drivers are able to
withstand "data collisions" (bus contention) problems and bus fault
conditions.

To solve the "data collision" problem often present in
multi-drop networks hardware units (converters, repeaters, micro-processor
controls) can be constructed to remain in a receive mode until they are
ready to transmit data. Single master systems (many other communications
schemes are available) offer a straight forward and simple means of
avoiding "data collisions" in a typical 2-wire, half-duplex, multi-drop
system. The master initiates a communications request to a "slave node" by
addressing that unit. The hardware detects the start-bit of the
transmission and automatically enables (on the fly) the RS485 transmitter.
Once a character is sent the hardware reverts back into a receive mode in
about 1-2 microseconds (at least with R.E. Smith converters, repeaters,
and remote I/O boards).

Any number of characters can be sent, and
the transmitter will automatically re-trigger with each new character (or
in many cases a "bit-oriented" timing scheme is used in conjunction with
network biasing for fully automatic operation, including any Baud rate
and/or any communications specification, eg. 9600,N,8,1). Once a "slave"
unit is addressed it is able to respond immediately because of the fast
transmitter turn-off time of the automatic device. It is NOT necessary to
introduce long delays in a network to avoid "data collisions." Because
delays are NOT required, networks can be constructed, that will utilize
the data communications bandwidth with up to 100% through put.

Below are the specifications for RS232, RS423, RS422, and RS485.
Please give us a call at 513-874-4796 if
further information is required. We have solutions to most problems that
are encountered in this area. Any comments and/or corrections would be
appreciated.

Thanks, Ron Smith

SPECIFICATIONS

RS232

RS423

RS422

RS485

Mode of Operation

SINGLE
-ENDED

SINGLE
-ENDED

DIFFERENTIAL

DIFFERENTIAL

Total Number of Drivers and Receivers on One Line (One
driver active at a time for RS485 networks)

1 DRIVER
1 RECVR

1 DRIVER
10 RECVR

1 DRIVER
10 RECVR

32 DRIVER
32 RECVR

Maximum Cable Length

50 FT.

4000 FT.

4000 FT.

4000 FT.

Maximum Data Rate (40ft. - 4000ft. for
RS422/RS485)

20kb/s

100kb/s

10Mb/s-100Kb/s

10Mb/s-100Kb/s

Maximum Driver Output Voltage

+/-25V

+/-6V

-0.25V to +6V

-7V to +12V

Driver Output Signal Level (Loaded Min.)

Loaded

+/-5V to +/-15V

+/-3.6V

+/-2.0V

+/-1.5V

Driver Output Signal Level (Unloaded Max)

Unloaded

+/-25V

+/-6V

+/-6V

+/-6V

Driver Load Impedance (Ohms)

3k to 7k

>=450

100

54

Max. Driver Current in High Z State

Power On

N/A

N/A

N/A

+/-100uA

Max. Driver Current in High Z State

Power Off

+/-6mA @ +/-2v

+/-100uA

+/-100uA

+/-100uA

Slew Rate (Max.)

30V/uS

Adjustable

N/A

N/A

Receiver Input Voltage Range

+/-15V

+/-12V

-10V to +10V

-7V to +12V

Receiver Input Sensitivity

+/-3V

+/-200mV

+/-200mV

+/-200mV

Receiver Input Resistance (Ohms), (1 Standard
Load for RS485)

3k to 7k

4k min.

4k min.

>=12k

Please call us at: 513-874-4796

Contact
Information:
R.E. Smith,  4311 R. E. Smith Dr., West Chester, Ohio
45069
513-874-4796 Phone, 513-874-1236 Fax., rs485.com

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