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EIA-RS Serial Communications GE Infrastructure
Sensing
EIA-RS Serial Communications
All Models
Users Guide GE Infrastructure
Sensing
EIA-RS Serial Communications
All Models
Users Guide
916-054B
October 2004 October 2004
ii
Table of Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Serial Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Simplex & Duplex Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Asynchronous & Synchronous Transmission. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
DCE and DTE Communication Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Terminal Communications Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Setting the Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
EIA-RS-232 Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
EIA -RS-232 Pin Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Constructing a Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
EIA-RS-422/485 Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
EIA-RS-422/485 Pin Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Converting the EIA-RS-232 to EIA-RS-485 Serial Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Wiring the EIA-RS-485 Interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Comparison of EIA-RS Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
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2004, GE Infrastructure Sensing. EIA-RS Serial Communications
1
October 2004
Introduction
Serial ports are used to establish two-way communications between a
PC and a variety of external devices. This type of port is included on
most computers and is popular because it is relatively easy to
program and wire. However, in order for this system to work, it is
necessary to clearly define the electrical specifications of the interface
so that there can be consistency in the hardware and software
implementation of the connection. This has resulted in several serial
interface definitions, known as RS (recommended standard)
specifications, which are also known as EIA (Electronic Industries
Association) standards.
Note: Standards will be referred to using both prefixes throughout
this document (e.g., EIA-RS-232).
This appendix discusses the following: defines serial communication discusses the various types of communication devices lists various communications software packages lists pin assignments and describes how to construct a cable for use
with EIA-RS-232 standard describes and lists pin assignments for EIA-RS-422/485 standards discusses converting EIA-RS-232 to EIA-RS-485 provides a comparison of EIA-RS-232, 422 and 485 standards
Serial Communication
Serial transmission, also known as bit-serial transmission, conveys a
message one bit at a time through a channel. A channel is a pathway
over which information is sent. A channel can be a physical wire,
radio wave, laser or other source. Serial communication transfers
individual data bits sequentially (serially) via a channel. The data bits
are then reassembled at the destination to compose the message. Only
serial communication is possible in systems with just a single
channel.
Byte-serial communication transfers eight bits at a time through eight
parallel channels. The data transfer rate is eight times greater than in
bit-serial communication; however, eight channels are needed which
means it is also likely to be eight times more expensive. When high
data rates are desired, this type of transmission is most economic and
feasible when data is being transmitted over a short distance.
For successful communication, both devices must be programmed for
the same transmission parameters, such as baud rate, data bits, stop
bits, and parity. Serial devices consist of a number of characteristics
which are discussed in the following sections. October 2004
2
EIA-RS Serial Communications
Simplex & Duplex
Channels
Serial devices consists of one or more channels and all channels have
a direction of data flow. Channels can be one of the three types listed
below. Refer to Figure 1 for an illustration of each channel type: Simplex: A simplex channel refers to a system in which data can
flow in only one direction. That is, one device is always the
transmitter and the other device is always the receiver. A simple
example is a radio station transmitting its signal to your table radio.
The radio cannot send information to the station. Half-duplex:: A half-duplex channel is a single physical channel in
which data can flow in two directions, but never at the same time.
A telephone call is a good example of half-duplex communication.
When one person speaks, the other person listens, if both people
speak at the same time, the information gets garbled. Full-duplex: A full-duplex channel has the ability to exchange data
in two directions simultaneously. This type of system consists of
two simplex channels: one forward and one reverse channel (e.g.,
an external modem connected to the serial port of a personal
computer).
Figure 1: Channel Types
Transmitter
Receiver
Simplex
Transmitter
Half Duplex
Receiver
Receiver
Transmitter
Transmitter
Receiver
Receiver
Transmitter
Full Duplex EIA-RS Serial Communications
3
October 2004
Asynchronous &
Synchronous
Transmission
Serial data is not sent at a uniform rate, but in bursts of regularly
spaced binary data bits followed by a pause, then another data burst.
A series of bursts is sent until the message is fully transmitted. The
length of pauses between bursts may be uniform or variable;
therefore, in order for data to be transmitted properly, this timing
information must be shared between the communication devices.
There are two basic transmission techniques that are used to ensure
correct communication: Synchronous: When the timing between bursts and data bits is
known, this is called synchronous transmission since the timing is
synchronized between the transmitter and receiver. In synchronous
transmission, separate channels are used to transmit data and
timing information. The transmitter uses one channel to send clock
pulses to the receiver. Once the clock pulse is received, the
receiver reads the other channel for data. The receiver does not
read the data channel again until the next clock pulse arrives. Asynchronous: When no clocking signal is present, the data may
be sent with any arbitrary time spacing. This is called
asynchronous transmission. In this type of transmission one
channel is used. Highly accurate oscillators in the transmitter and
receiver are used. The receiver generates an internal clock signal
that is equal to the transmitters. Data is sent in bursts of 10 or 11
bits, eight of which is the actual message. When the channel is idle,
the signal voltage is equivalent to a continuous logic 1. A data
burst always begins with a logic 0 (start bit) to signal the receiver
that a transmission is starting. The start bit triggers an internal
timer in the receiver that generates the clock pulses. The eight bit
message follows the start bit at the agreed upon baud rate. The
burst is completed with a parity bit and a stop bit. See Figure 2 on
page 4 for an illustration of a completed data burst. October 2004
4
EIA-RS Serial Communications
Asynchronous &
Synchronous
Transmission (cont.)
Figure 2: One Completed Data Burst
0
1
2
3
4
5
6
7
P
8 Bit Message
Parity Bit Check
Check Accuracy
Stop Bit
Start Bit
Typical Format
1 Start Bit
8 Data Bits
1 Parity Bit
1 Stop Bit