RS485, RS232, RS422, RS423, Quick Reference Guide Page 1 of 3
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
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RS485, RS232, RS422, RS423, Quick Reference Guide Page 2 of 3
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
http://www.rs485.com/rs485spec.html 11/05/2010
RS485, RS232, RS422, RS423, Quick Reference Guide Page 3 of 3
SPECIFICATIONS RS232 RS423 RS422 RS485
SINGLE SINGLE
Mode of Operation DIFFERENTIAL DIFFERENTIAL
-ENDED -ENDED
Total Number of Drivers and 1
1
Receivers on One Line (One driver DRIVER 1 DRIVER 32 DRIVER
DRIVER
active at a time for RS485 10 10 RECVR 32 RECVR
1 RECVR
networks) RECVR
Maximum Cable Length 50 FT. 4000 FT. 4000 FT. 4000 FT.
Maximum Data Rate (40ft. - 4000ft.
20kb/s 100kb/s 10Mb/s-100Kb/s 10Mb/s-100Kb/s
for RS422/RS485)
Maximum Driver Output Voltage +/-25V +/-6V -0.25V to +6V -7V to +12V
Driver Output Signal +/-5V to
Loaded +/-3.6V +/-2.0V +/-1.5V
Level (Loaded Min.) +/-15V
Driver Output Signal
Unloaded +/-25V +/-6V +/-6V +/-6V
Level (Unloaded Max)
Driver Load Impedance (Ohms) 3k to 7k >=450 100 54
Max. Driver Current in
Power On N/A N/A N/A +/-100uA
High Z State
Max. Driver Current in +/-6mA
Power Off +/-100uA +/-100uA +/-100uA
High Z State @ +/-2v
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),
3k to 7k 4k min. 4k min. >=12k
(1 Standard Load for RS485)
Please call us at: 513-874-4796
Contact Information:
R.E. Smith, 10330 Chester Road, Cincinnati, Ohio 45215
513-874-4796 Phone, 513-874-1236 Fax., rs485.com
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