RS-232 tutorial
- an overview or tutorial about the RS232 communication standard that is
widely used for the transmission of serial data
The RS-232 communications standard has been in use for very
many years and is one of the most widely used standards for serial data
communications. There are several different specifications that are used to
define RS-232: RS-232C, RS-232D; V24; V28 and V10, but they are all very similar
and can be used to interface to each other. Additionally, RS232 communication
can be used in either an asynchronous mode, which is the most common, or a
synchronous mode.
Originally the RS232 communication specification was devised
in 1962 when the need to be able to transmit data along a variety of types of
line started to grow. The telephone companies saw the need to introduce some
standards. As a result the Electrical Industries Association in the U.S.A.
created a standard for serial data transfer or communication known as RS232C. It
defines the electrical characteristics for transmission of data between a Data
Terminal Equipment (DTE) and the Data Communications Equipment (DCE). Normally
the data communications equipment is the modem (modulator/demodulator) which
that encodes the data into a form that can be transferred along the telephone
line. A Data Terminal Equipment could be a computer.
Since it was first introduced there have been a number of
updates to the RS-232 standard. These have included EIA-232 (Electronic
Industries Alliance) and later EIA/TIA-232 (Telecommunications Industry
Association).
The original RS-232 specification was developed in the USA
and sponsored by organisations there. On a more international scale the
International Telecommunications Union (ITU) developed a standard known as ITU
v.24. However this is often just written V24. This standard is totally
compatible with RS232, and its aim was to enable manufacturers to conform to
global standards and thereby allow products that would work in all countries
around the world.
The success of the RS232 standard has meant that it is now
used for many more types of equipment. As a result many lines defined in the
specification are rarely used. This means that care has to be taken when
connecting any new equipment or defining which lines are to be used in a new
design.
Interface Basics
The interface is intended to operate over distances of up to 15 metres. This is
because any modem is likely to be near the terminal. Data rates are also
limited. The maximum for RS-232C is 19.2 k baud or bits per second although
slower rates are often used. In theory it is possible to use any baud rate, but
there area number of standard transmission speeds used.
Common Data Transmission Rates
50
75
110
150
300
600
1200
2400
4800
9600
19200
38400
76800
Note: speeds up to 19200 bits per second are
normally used. Above this noise that is picked up, especially over long cable
runs can introduce data errors.
Connections
The RS-232C specification does not include a description of the connector to be
used. However, the most common type found is the 25 pin D-type connector.
RS232 signal levels
The voltage levels are one of the main items in the specification. For RS232
data signals a voltage of between -3V and -25V represents a logic 1. The logic 0
is represented by a voltage of between +3V and +25V. Control signals are in the
"ON" state if their voltage is between +3V and +25V and "OFF" if they are
negative, i.e. between -3V and -25V.
The data is sent serially on RS232, each bit is sent one
after the next because there is only one data line in each direction. This mode
of data transmission also requires that the receiver knows when the actual data
bits are arriving so that it can synchronise itself to the incoming data. To
achieve this a logic 0 is sent as a start bit for the synchronisation. This is
followed by the data itself and there are normally seven or eight bits. The
receiver obviously has to know how many data bits to expect, and there are often
small dual in line switches either on the back of the equipment or inside it to
set this information.
Data on RS232 is normally sent using ASCII (American Standard
Code for Information Interchange). However other codes including the Murray Code
or EBCDIC (Extended Binary Coded Decimal Interchange Code) can be used equally
well.
After the data itself a parity bit is sent. Again this
requires setting because it is optional and it can be even or odd parity. This
is used to check the correctness of the received data and it can indicate
whether the data has an odd or even number of logic ones. Unlike many systems
these days there is no facility for error correction.
Finally a stop bit is sent. This is normally one bit long and
is used to signify the end of a particular byte. Sometimes two stop bits are
required and again this is an option that can often be set on the equipment.
RS232 data transmission is normally asynchronous. However
transmit and receive speeds must obviously be the same. A certain degree of
tolerance is allowed. Once the start bit is sent the receiver will sample the
centre of each bit to see the level. Within each data word the synchronisation
must not differ by more than half a bit length otherwise the incorrect data will
be seen. Fortunately this is very easy to achieve with today's accurate bit or
baud rate generators.
Lines and their usage
Their are four types of line defined in the RS232 specification. They are Data,
Control, Timing and Ground. Not all of them are required all the time. It is
possible to set up a very simple communication using very few lines. When
looking at the lines and their functions it is necessary to remember that they
are defined for a connection between a modem (the data set or communications
equipment) and a terminal or computer (data terminal equipment) in mind. All the
lines have directions, and when used in this way a one to one cable operates
correctly.
The most obvious lines are the data lines. There are two of
these, one for data travelling in each direction. Transmit data is carried on
pin 2 and the receive data is carried on line three.
The most basic of the control circuits is Data Carrier
Detected (DCD). This shows when the modem has detected a carrier on the
telephone line and a connection appears to have been made. It produces a high,
which is maintained until the connection is lost.
Data Terminal Ready (DTR) and Data Set Ready (DSR) are the
main control circuits. They convey the main information between the terminal and
modem. When the terminal is ready to start handling data it flags this on the
DTR line. If the modem is also ready then it returns its signal on the DSR line.
These circuits are mainly used for telephone circuits. After a connection has
been made the modem will be connected to the line by using DTR. This connection
will remain until the terminal is switched off line when the DTR line is
dropped. The modem will detect this and release the telephone line.
Sometimes pin 20 is not assigned to DTR. Instead another
signal named, Connect Data Set To Line (CDSTL) is used. This is virtually the
same as DTR, but differs in that DTR merely enables the modem to be switched
onto the telephone line. CDSTL commands the modem to switch, despite anything
else it may be doing.
A further two circuits, Request To Send (RTS) and Clear To
Send (CTS) are also used. This pair of circuits are used together. The terminal
equipment will flag that it has data to send. The modem will then return the CTS
signal to give the all clear after a short delay.
This signalling is used particularly when switched carriers
are used. It means that the carrier is only present on the line when there is
data to send. It finds its uses when one central modem is servicing several
others at remote locations.
Secondary lines
There are two types of lines that are specified in the RS-232 specification.
There are the primary channels that are normally used, and operate at the normal
or higher data rates. However, there is also provision for a secondary channel
for providing control information. If it is used it will usually send data at a
much slower rate than the primary channel.
As the secondary lines are rarely used or even implemented on
equipment, manufacturers often use these connector pins for other purposes. In
view of this it is worth checking that the lines are not being used for other
purposes before considering using them. When the secondary system is in use, the
handshaking signals operate in the same way as for the primary circuit.
Grounding
The ground connections are also important. There are two. First the protective
ground ensures that both equipments are at the same earth potential. This is
very useful when there is a possibility that either equipment is not earthed.
The signal ground is used as the return for the digital signals travelling along
the data link. It is important that large currents that are not part of the
signalling do not flow along this line otherwise data errors may occur.
Summary
The RS-232 specification is still widely used. Although faster specifications
exist, it is likely to remain in use for many years to come. One of the reasons
for this is the fact that it is found on most of today's personal computers.
Although the parallel "LPT" ports are used almost universally for printers, it
still used for many other purposes, including connecting the computer to a
modem.
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