RS-232, RS485 and RS422 - 09.05.02

I am often asked about RS-232, RS-422 and RS-485 communications. Without delving into the EIA Standards, I hope to outline a few key points.

RS-232, RS-422 and RS-485 are all considered SERIAL communications, although they differ in transmission distance and wiring configuration. The RS-232 protocol is more widely used and most computers come with a RS-232 port. Unfortunately for industrial applications, RS-232 does have some limitations. When specifying you next system, be cognizant of the following:

Limiting Distance: Typically RS-232 is limited to 50 feet or less while RS-422 and 485 can span 500 feet.

Multiple Drops: One major difference between RS-232 and RS-422/485 is that, RS-232 does not allow for multiple control devices on a single port. RS-422 and RS-485 support multi-drop connection where up to 32 devices can be connected per data line. This is advantageous when multiple drives or controllers are located within the same building.

RS-422 and RS-485 have better noise immunity than RS-232: RS-232 is a ground reference signal. Therefore, if the controller is located in an electrically noisy environment (like most are), the signal may be impacted causing data interruption and dropped transitions. Both RS-422 and RS-485 are balanced signals. They use a pair of signal lines to transfer one signal, usually in the 2 to 6 volt range. The advantage of having a balanced pair is that the system is more immune to grounding and 60 Hz. noise problems.

RS-232 to RS-422/485 Converters: One disadvantage of RS-422 and RS-485 is that you will need to purchase a converter. PC's don't come with a standard RS-422 or RS-485 port so a converter is used to adapt the signal to RS-232. Protocol converters are not expensive but make sure to purchase an industrial model if the unit will be installed in the field.

In general, I have found that most oilfield applications use RS-232, RS-485 or the combination of both. RS-485 is usually easier to implement and troubleshoot as long as the system does not require multiple protocol converters. Whenever possible, it is best avoid converting back an fourth between protocols.

 

Common Electrical and Controls Acronyms - 05.25.02

A - Amp (ampere)
A/D - Analog-to-digital
AC - Alternating Current
ADC - Analog-to-digital Converter
ANSI - American National Standards Institute
ASCII - American Standard Code for Information Interchange
bps - Bits Per Second
CPU - Central Processing Unit
D/A - Digital-to-analog
DAC - Digital-to-analog Converter
dB - Decibel
DC - Direct Current
DCE - Data Communications Equipment
DCS - Distributed Control System
DDE - Dynamic Data Exchange
DSP - Digital Signal Processing
DTE - Data Terminal Equipment
EEPROM - Electrically Erasable Programmable ROM
EMI - Electromagnetic Interference
EPROM - Erasable Programmable ROM
GUI - Graphical User Interface
HMI - Human-machine Interface
Hz - Hertz
i - Current
I/O - Input/Output
IEEE - Institute of Electrical and Electronic Engineers
LED - Light Emitting Diode
LSB - Least Significant Bit
MUX - Multiplexer
OLE - Object Linking and Embedding
PC - Personal Computer
PCI - Peripheral Component Interconnect
PCMCIA - Personal Computer Memory Card International Association
PID - Proportional-Integral-Derivative
PLC - Programmable Logic Controller
R - Resistance
RFI - Radio Frequency Interference
ROM - Read-only Memory
SCADA - Supervisory Control and Data Acquisition
SNR - Signal-to-noise Ratio
SPC - Statistical Process Control
SQL - Structured Query Language
STP - Shielded Twisted Pair
TCP/IP - Transmission Control Protocol/Internet Protocol
TTL - Transistor-to-transistor Logic
V - Volt