With the term "serial port" we will usually mean
the hardware RS-232 and its signal levels, connections etc.
because many of the modern devices still connect to serial
port even after the development of many advanced technologies
in serial communication systems. There may be many reasons
like ease of debugging, cost effective, etc. Serial port
is also termed as COM port.
RS-232 a standard is related to serial data communication
between host systems, commonly known as Data Terminal Equipment,
or DTE and a peripheral system termed, Data communication
Equipment (also known as Data Circuit-Terminating Equipment)
or DCE. To be more specific, the device that connects to
the RS-232 interface is called a Data Communications Equipment
(DCE) and the device to which it connects (e.g., the computer)
is called a Data Terminal Equipment (DTE).
It was first introduced by the Electronics Industry Alliance
(EIA) in the early 1960s and is commonly known as RS-232
(Recommended Standard 232). EIA-232 or RS-232 or RS-232
C is a complete serial communication protocol, which specifies
signal voltages, signal timing, signal function, pin wiring,
and the mechanical connections (i.e.: either 25-pin DB-25
or 9-pin DB-9). In 1987, the EIA released a new version
of the standard and changed the name to EIA-232-D. And in
1991, the EIA teamed up with Telecommunications Industry
association (TIA) and issued a new version of the standard
called EIA/TIA-232-E. Many people, however, still refer
the standard name as RS-232C, or just RS-232.
Let us now try to understand about the electrical, mechanical
and functional interface characteristics of this standard.
Mechanical & functional characteristics:
We already know that the RS-232 standard supports two types
of connectors - a 25-pin D-type connector (DB-25) and a
9-pin D-type connector (DB-9). The figure below shows the
pin diagram of the connectors.
Although RS-232 specifies a 25-pin connector, this connector
is often not used. Most applications do not require all
the defined signals, so a 25-pin connector is larger than
necessary. The most popular connector is the 9-pin DB9 connector,
which provides the necessary signals for the serial communication
in modem applications. So let us analyze the functions of
each pins of the 9-pin RS-232 connector in comparison with
that of DB-25. RS-232 signals have a direction (in or out)
depending on whether they are with respect to a DTE or a
|| DB-25 Pin No:
||DCE to DTE
||Determines whether the DCE is connected to a working
phone line or not. (Only used in connection with modem)
||DTE to DCE
||Computer sends information to the DCE.
||DCE to DTE
||Computer receives information sent from the DCE.
||DTE to DCE
||Computer tells the DCE that it is ready to communicate.
Raised by DTE when powered on. In auto-answer mode raised
only when RI arrives from DCE.
||DCE to DTE
||Modem tells the computer that it is ready to talk.
Raised by DCE to indicate ready.
||DTE to DCE
||Computer asks the modem if it can send information.
Raised by DTE when it wishes to send. Expects CTS from
||DCE to DTE
||Modem tells the computer that it can send information.
Raised by DCE in response to RTS from DTE.
||DCE to DTE
||Set when incoming ring detected - used for auto-answer
application. DTE raises DTR to answer. (Only used in
connection with modem)
||12, 13, 14, 16, 19, 24
||Only needed if second channel being used.
||DTE to DCE
||Transmit clock. (Used in Synchronous mode only) as
Transmitter Signal Element Timing.
||DCE to DTE
||Receive clock (used in synchronous mode only) as Receiver
Signal Element Timing.
|| L L
||DTE to DCE
||Local Loop back
||DTE to DCE
||Signal quality detector /Remote Loop back
RS-232 defines the purpose, signal timing and signal levels
for each line. It's an Active LOW voltage driven interface
i.e. it transmits positive voltage for a 0 bit, negative
voltage for a 1. And the output signal level usually swings
between +12 V and -12 V. The high level is defined as between
+5V to +12V, and a low level is defined as between -5V and
-12V. With 2V of noise margin, a high level for the receiver
is defined as between +3V to +12V, and a low level is between
-3V to -12V. The signal voltage between +3 V and -3V, called
"dead area" is designed to absorb line noise.
A low level is defined as logic 1 and is referred to as
"marking." Similarly, a high level is defined
as logic 0 and is referred to as "spacing." The
following figure illustrates the logic level.
In the embedded system, if the device communicates at TTL
level, the connection between the embedded system and external
device is simple. But if the device needs RS232 level signaling,
we will have to insert a RS-232 Line Driver/Receiver between
the processor and the device. Most of the devices used nowadays
need three wires, i.e. Transmit Data, Receive Data and Signal
Ground. No need of hardware flow control signal. This simplifies
the hardware connection as well as the software design.
The figure below shows how the data look like for ASCII-A
Connections & signal flow control:
Flow control is the process of managing the rate of data
transmission between two nodes to prevent a fast sender
from over running a slow receiver. For example, as DTE to
DCE speed is a few times faster than DCE to DCE speed, the
PC can send data to the modem at a higher rate. That means
in this connection sooner or later the data may be lost
because of the buffers overflow, so the control of data
flow should be realized.
Flow control mechanisms can be classified by whether or
not the receiving node sends feedback to the sending node.
That is through a "handshake", an exchange of
characters between a transmitter and a receiver is used
to postpone transmission until the receiver is ready to
receive the data.
This character flow control is of two main types: Hardware
Software Flow Control:
One example is "Xon/Xoff".Two characters Xon and
Xoff are used here control the data flow. Xon is usually
a 17 character, and Xoff is a 19 character. The modem will
only have a small buffer. As when the computer fills it,
the modem sends a Xoff character to inform the computer
about data transfer termination. As soon as the modem empties
the most of the memory for data, it will send the Xon character
to the computer and to start the data transfer again. The
main advantage of this type of data flow control is that
it doesn't need any other wires as the characters are sent
via TD/RD lines. But if the connection is slow, every character
needs 10 bits, which can reduce the connection speed.
Hardware Flow Control:
Most serial communications uses software Flow Control, but
there is an alternative: hardware handshaking. Hardware
flow control is also known as RTS/CTS flow control. To realize
this control two additional wires (RTS & CTS) in the
sequential cable are used. This results in increasing the
data transmission rate, as no time is spent for Xon-Xoff
characters transmission. The flow control mode (whether
Xon-Xoff or Hardware) can be selected as indicated below:
Here the transmission activates the Request To Send (RTS)
line when the computer is ready to send data .If the modem
has a free buffer for this data, it activates the Clear
to Send (CTS) line in response and the computer starts sending
data. If the modem lacks free memory, it won't activate
One of the main drawbacks of hardware handshaking is that
some modems cannot deal correctly with binary data streams
that contain characters, which look like DC1/DC3. Hardware
handshaking is also known as Out-of-Band flow control, because
the signals are generated and observed outside the flow
of the data.
Care must be taken when enabling hardware handshaking. Both
parties have to agree and be compatibly configured before
you start. Software flow-control is re-active. Things proceed
normally until one party says, "Stop!" But the
hardware flow-control is pro-active. You do not start transmitting
until you receive the Go signal. If you never get it, you
never start. Fortunately, with most modems the hardware
handshake rules are not turned ON until a connection has
Let us now try to understand how a serial port can be connected
to another one.
Null Modem connection:
The serial communication standards show the use of DTE/DCE
communication, the way a computer should communicate with
a peripheral device like a modem. But in Null modem connection,
the PCs are connected back-to-back with cables, each acting
as a DTE, which means there is no DCE in this case. This
type of connection finds many uses nowadays. The null modem
can be configured in many ways using the number of signal
lines available. In most situations, the original modem
signal lines are reused to perform some sort of handshaking.
Handshaking has many advantages. It can increase the maximum
allowed communication speed because then the computer will
be able to control the flow of information. In null modem
connection without "flow control", the communication
may be possible only at the speed at which the receiving
side can handle the amount of data.
Before we talk about the Null-modem connections, let us
try to refresh our knowledge about the different types of
flow control signals used in RS-232. The first two flow
control pins are known as RTS (request to send), an output
signal from the DTE, which obviously comes as the input
for the DCE and CTS (clear to send), which come as the answering
signal from the DCE side. Before sending a character, the
DTE asks permission by setting its RTS output. No information
will be sent until the DCE grants permission by making the
CTS line high.
The other two flow control signals, DTR, data terminal ready
and DSR, data set ready are used to signal the status of
one communication side to the other. The DTE uses the DTR
signal to signal that it is ready to accept information,
whereas the DCE uses the DSR signal for the same purpose.
The last flow control signal present in DTE/DCE communication
is the CD carrier detect. It is not used directly for flow
control, but indicates the existence of a communication
link between two modem devices.
a) Without handshaking:
This is the simplest and commonly used way of connection,
which is shown in the following figure. As you can see,
only the data lines and signal ground are cross-connected.
All other pins have no connection, which means there is
This type of connection can be used to communicate with
devices, which do not have modem control signals.
b) Loop back handshaking:
Before going to talk about loop-back handshaking, let us
try to point-out the issues associated with the simple null
modem without handshaking.
1. Suppose the software on both sides of the communication
is well structured. What happens if either the DCE or DTE
checks for the DSR or CD inputs? These signal levels will
never go high, as it is kept not connected, which may
2. Same thing may happen for the RTS/CTS handshaking sequence
also. The RTS output is set high by the DTE and then
waits for a ready signal to be received on the CTS line.
This may enhance the possibility for the software to hang
because no physical connection is present to either CTS
To overcome this problem and still be able to use a cheap
null modem we can use the connection layout shown above
termed as Null modem with loop back handshaking, which make
the well defined & structured software to think that
there is some handshaking available (even though it is a
c) Partial handshaking:
The problems associated with the loop back handshaking are
1. The DSR signal input indicates that the other side is
ready for communication. But the line is connected back
to the DTR output of the same
side, which means, that the software doesn't see the ready
signal of the other device, but its
own. This may create problem for the proper communication.
The same thing happens in the case of the CD input.
2. Again this also has no functional enhancements over the
simple connection. There is no way both devices can control
data flow, other than by using XON/XOFF handshaking.
3. If the software is designed for using hardware flow control
(in the case of loop-back handshaking) there is a chance
for data loss. When data speeds reach the limit the receivers
can handle, communication may stop immediately
without any reason.
In the case of CTS/RTS of this type connection, the software
may not hang-up because the CTS input on the same connector
side is receiving clearance immediately when the RTS is
Both the simple null modem connection and the null modem
with loop back handshaking have no provisions for hardware
flow control. If a condition comes that the hardware flow
control is necessary the null modem with partial handshaking
can be used.
d) Full handshaking:
The software, which uses only the RTS/CTS protocol for
the flow control, cannot go for the partial handshaking
null modem connection. The solution for this is the use
of "full handshaking". This is the most expensive
null modem connection. In this mode of connection, all the
pins, except RI and CD are used. The main advantage of this
connection is that there are two signaling lines in each
direction. Both the RTS and DTR are there to send flow control
information to the other device. This makes it possible
to achieve very high communication speeds, provided that
the software has been designed for it.
Virtual null modem
This is also a type of communication method to connect two
computer applications directly using virtual serial port.
Unlike null modem cable, virtual null modem is a software
solution, which emulates hardware null modem in computer.
All features of hardware null modem can be made available
in virtual null modem as well. Some of the main advantages
of this are:
1. No serial cable is needed.
2. Serial port of computer is not used.
3. Unlimited number of virtual connections is possible.
4. Transmission speed of serial data is limited only by
5. Virtual connection over network or Internet is possible.
So Unlimited distance of communication can be achieved.
Module - 9 (CAN interface)
Module - 7(Serial communications basics)
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