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A Practical Guide to RS-232 Interfacing
by Lawrence E. Hughes. Mycroft Labs, Inc.
P.O. Box 6045, Tallahassee, FL 32301
The following information is intended to collect together in
one place, and explain in relatively simple terms, enough of
the details of the RS-232 standard to allow a technician to
construct and/or debug interfaces between any two "RS-232
Compatible" devices. A more detailed coverage of the subject
may be found in the book "Technical Aspects of Data
Communication" by John E. McNamara (1977, Digital Press).
This guide is necessary due to the casual way that vendors
implement "RS-232" interfaces, sometimes omitting required sig-
nals, requiring optional ones, or worse, implementing signals
incorrectly. Due to this, and a lack of readily available
information about the real EIA standard, there is often consid-
erable confusion involved in trying to interface two RS-232
devices.
BACKGROUND
RS-232-C is the most recent version of the EIA (Electronics
Industry Association) standard for low speed serial data commu-
nication. It defines a number of parameters concerning voltage
levels, loading characteristics and timing relationships. The
actual connectors which are almost universally used (DB-25P and
DB-25S, sometimes called "EIA connectors") are recommended, but
not mandatory. Typical practice requires mounting the female
(DB-25S) connector on the chassis of communication equipment,
and male (DB-25P) connectors on the cable connecting two such
devices.
There are two main classes of RS-232 devices, namely DTE (Data
Terminal Equipment), such as terminals, and DCE (Data
Communication Equipment), such as modems. Typically, one only
interfaces a DTE to a DCE, as opposed to one DTE to another
DTE, or one DCE to another DCE, although there are ways to do
the later two by building non-standard cables. Rarely if ever
are more than two devices involved in a given interface
(multidrop is not supported). A serial port on a computer may
be implemented as either DTE or DCE, depending on what type of
device it is intended to support.
RS-232 is intended for relatively short (50 feet or less),
relatively low speed (19,200 bits per second or less) serial
(as opposed to parallel) communications. Both asynchronous and
synchronous serial encoding are supported. As 'digital' signals
(switched D.C. voltage, such as square waves) are used, as
opposed to 'analog' signals (continuously varying voltage, such
as sine waves) a very wide bandwidth channel (such as direct
wire) is required. A limited bandwidth channel (such as a phone
circuit) would cause severe and unacceptable distortion and
consequent loss of information.
RS-232 will support simplex, half-duplex, or full-duplex type
channels. In a simplex channel, data will only ever be travel-
ling in one direction, e.g. from DCE to DTE. An example might
be a 'Receive Only' printer. In a half-duplex channel, data may
travel in either direction, but at any given time data will
only be travelling in one direction, and the line must be
'turned around' before data can travel in the other direction.
An example might be a Bell 201 style modem. In a full-duplex
channel, data may travel in both directions simultaneously. An
example might be a Bell 103 style modem. Certain of the RS-232
'hand-shaking' lines are used to resolve problems associated
with these modes, such as which direction data may travel at
any given instant.
If one of the devices involved in an RS-232 interface is a real
modem (especially a half-duplex modem), the 'hand-shaking'
lines must be supported, and the timing relationships between
them are quite important. These lines are typically much easier
to deal with if no modems are involved. In certain cases, these
lines may be used to allow one device (which is receiving data
at a higher rate than it is capable of processing indefinitely)
to cause the other device to pause while the first one 'catches
up'. This use of the hand-shaking lines was not really intended
by the designers of the RS-232 standard, but it is a useful by-
product of the way such interfaces are typically implemented.
Much of the RS-232 standard is concerned with support of
'modems'. These are devices which can convert a serial digital
data signal into an analog signal compatible with a narrow
bandwidth (e.g. 3 kHz) channel such as a switched telephone
circuit, and back into serial digital data on the other end.
The first process is called 'MOdulation', and the second pro-
cess is called 'DEModulation', hence the term 'MODEM'. The
actual process used (at data rates of up to 1200 bits per
second) is FSK (Frequency Shift Keying), in which a constant
frequency sine wave (called the 'carrier') is shifted to a
slightly higher or slightly lower frequency to represent a
logic 0 or logic 1, respectively. In a half duplex modem, the
entire available bandwidth is used for one direction. In a full
duplex modem, the available bandwidth is divided into two sub-
bands, hence there is both an 'originate carrier' (e.g. for
data from the terminal to the computer), and an 'answer car-
rier' (e.g. for data from the computer to the terminal). The
actual frequencies (in Hertz) used on the Bell 103A full duplex
modem are:
signal state Originate Answer
logic 0 SPACE 1180 1850
carrier 1080 1750
logic 1 MARK 980 1650
THE STANDARD CIRCUITS AND THEIR DEFINITIONS
For the purposes of the RS-232 standard, a 'circuit' is defined
to be a continuous wire from one device to the other. There are
25 circuits in the full specification, less than half of which
are at all likely to be found in a given interface. In the sim-
plest case, a full-duplex interface may be implemented with as
few as 3 circuits. There is a certain amount of confusion asso-
ciated with the names of these circuits, partly because there
are three different naming conventions (common name, EIA cir-
cuit name, and CCITT circuit name). The table below lists all
three names, along with the circuit number (which is also the
connector pin with which that circuit is normally associated on
both ends). Note that the signal names are from the viewpoint
of the DTE (e.g. Transmit Data is data being sent by the DTE,
but received by the DCE).
PIN NAME EIA CCITT DTE-DCE FUNCTION
1 CG AA 101 --- Chassis Ground
2 TD BA 103 --> Transmit Data
3 RD BB 104 <-- Receive Data
4 RTS CA 105 --> Request To Send
5 CTS CB 106 <-- Clear To Send
6 DSR CC 107 <-- Data Set Ready
7 SG AB 102 --- Signal Ground
8 DCD CF 109 <-- Data Carrier Detect
9* <-- Pos. Test Voltage
10* <-- Neg. Test Voltage
11 (usually not used)
12+ SCDC SCF 122 <-- Sec. Data Car. Detect
13+ SCTS SCB 121 <-- Sec. Clear To Send
14+ STD SBA 118 --> Sec. Transmit Data
15# TC DB 114 <-- Transmit Clock
16+ SRD SBB 119 <-- Sec. Receive Data
17# RC DD 115 <-- Receive Clock
18 (not usally used)
19+ SRTS SCA 120 --> Sec. Request To Send
20 DTR CD 108.2 --> Data Terminal Ready
21* SQ CG 110 <-- Signal Quality
22 RI CE 125 <-- Ring Indicator
23* CH 111 --> Data Rate Selector
CI 112 <-- Data Rate Selector
24* XTC DA 113 --> Ext. Transmit Clock
25* --> Busy
In the above, the character following the pin number means:
* rarely used
+ used only if secondary channel implemented
# used only on synchronous interfaces
Also, arrow direction indicates which end (DTE or DCE)
originates each signal, except for the ground lines (---). For
example, circuit 2 (TD) is originated by the DTE, and received
by the DCE. Certain of the above circuits (11, 14, 16, and 18)
are used only, or in a different way, by Bell 208A modems.
A secondary channel is sometimes used to provide a very slow (5
to 10 bits per second) path for return information (such as ACK
or NAK characters) on a primarily half duplex channel. If the
modem used suppports this feature, it is possible for the
receiver to accept or reject a message without having to 'turn
the line around', a process that usally takes 100 to 200
milliseconds.
On the above circuits, all voltages are with respect to the
Signal Ground (SG) line. The following conventions are used:
Voltage Signal Logic Control
+3 to +25 SPACE 0 On
-3 to -25 MARK 1 Off
Note that the voltage values are inverted from the logic values
(e.g. the more positive logic value corresponds to the more
negative voltage). Note also that a logic 0 corresponds to the
signal name being 'true' (e.g. if the DTR line is at logic 0,
that is, in the +3 to +25 voltage range, then the Data Terminal
IS Ready).
ELECTRICAL CHARACTERISTICS OF EACH CIRCUIT
The following criteria apply to the electrical characteristics
of each of the above lines:
1) The magnitude of an open circuit voltage shall not exceed
25V.
2) The driver shall be able to sustain a short to any other
wire in the cable without damage to itself or to the other
equipment, and the short circuit current shall not exceed
0.5 ampere.
3) Signals shall be considered in the MARK (logic 1) state when
the voltage is more negative than -3V with respect to the
Signal Ground. Signals shall be considered in the SPACE
(logic 0) state when the voltage is more positive that 3V
with respect to the Signal Ground. The range between -3V and
3V is defined as the transition region, within which the
signal state is not defined.
4) The load impedance shall have a DC resistance of less than
7000 ohms when measured with an applied voltage of from 3V
to 25V but more than 3000 ohms when measured with a voltage
of less than 25V.
5) When the terminator load resistance meets the requirements
of Rule 4 above, and the terminator open circuit voltage is
0V, the magnitude of the potential of that circuit with
respect to Signal Ground will be in the 5V to 15V range.
6) The driver shall assert a voltage between -5V and -15V
relative to the signal ground to represent a MARK signal
condition. The driver shall assert a voltage between 5V and
15V relative to the Signal Ground to represent a SPACE sig-
nal condition. Note that this rule in conjunction with Rule
3 above allows for 2V of noise margin. Note also that in
practice, -12V and 12V are typically used.
7) The driver shall change the output voltage at a rate not
exceeding 30 volts per microsecond, but the time required
for the signal to pass through the -3V to +3V transition
region shall not exceed 1 millisecond, or 4 percent of a bit
time, whichever is smaller.
8) Lower capacitance cable allows longer runs.
9) The impedance of the driver circuit under power-off condi-
tions shall be greater than 300 ohms.
Note that two widely available integrated circuit chips (1488
and 1489) implement TTL to RS232 drivers (4 per chip), and
RS232 receivers to TTL (also 4 per chip), in a manner consis-
tent with all of the above rules.
DEFINITION OF THE MOST COMMON CIRCUITS
1 CG Chassis Ground
This circuit (also called Frame Ground) is a mechanism to
insure that the chassis of the two devices are at the same
potential, to prevent electrical shock to the operator. Note
that this circuit is not used as the reference for any of the
other voltages. This circuit is optional. If it is used, care
should be taken to not set up ground loops.
2 TD Transmit Data
This circuit is the path whereby serial data is sent from the
DTE to the DCE. This circuit must be present if data is to
travel in that direction at any time.
3 RD Receive Data
This circuit is the path whereby serial data is sent from the
DCE to the DTE. This circuit must be present if data is to
travel in that direction at any time.
4 RTS Request To Send
This circuit is the signal that indicates that the DTE wishes
to send data to the DCE (note that no such line is available
for the opposite direction, hence the DTE must always be ready
to accept data). In normal operation, the RTS line will be OFF
(logic 1 / MARK). Once the DTE has data to send, and has deter-
mined that the channel is not busy, it will set RTS to ON
(logic 0 / SPACE), and await an ON condition on CTS from the
DCE, at which time it may then begin sending. Once the DTE is
through sending, it will reset RTS to OFF (logic 1 / MARK). On
a full-duplex or simplex channel, this signal may be set to ON
once at initialization and left in that state. Note that some
DCEs must have an incoming RTS in order to transmit (although
this is not strictly according to the standard). In this case,
this signal must either be brought across from the DTE, or pro-
vided by a wraparound (e.g. from DSR) locally at the DCE end of
the cable.
5 CTS Clear To Send
This circuit is the signal that indicates that the DCE is ready
to accept data from the DTE. In normal operation, the CTS line
will be in the OFF state. When the DTE asserts RTS, the DCE
will do whatever is necessary to allow data to be sent (e.g. a
modem would raise carrier, and wait until it stabilized). At
this time, the DCE would set CTS to the ON state, which would
then allow the DTE to send data. When the RTS from the DTE
returns to the OFF state, the DCE releases the channel (e.g. a
modem would drop carrier), and then set CTS back to the OFF
state. Note that a typical DTE must have an incoming CTS before
it can transmit. This signal must either be brought over from
the DCE, or provided by a wraparound (e.g. from DTR) locally at
the DTE end of the cable.
6 DSR Data Set Ready
This circuit is the signal that informs the DTE that the DCE is
alive and well. It is normally set to the ON state by the DCE
upon power-up and left there. Note that a typical DTE must have
an incoming DSR in order to function normally. This line must
either be brought over from the DCE, or provided by a
wraparound (e.g. from DTR) locally at the DTE end of the cable.
On the DCE end of the interface, this signal is almost always
present, and may be wrapped back around (to DTR and/or RTS) to
satisfy required signals whose normal function is not required.
7 SG Signal Ground
This circuit is the ground to which all other voltages are
relative. It must be present in any RS-232 interface.
8 DCD Data Carrier Detect
This circuit is the signal whereby the DCE informs the DTE that
it has an incoming carrier. It may be used by the DTE to deter-
mine if the channel is idle, so that the DTE can request it
with RTS. Note that some DTEs must have an incoming DCD before
they will operate. In this case, this signal must either be
brought over from the DCE, or provided locally by a wraparound
(e.g. from DTR) locally at the DTE end of the cable.
15 TC Transmit Clock
This circuit provides the clock for the transmitter section of
a synchronous DTE. It may or may not be running at the same
rate as the receiver clock. This circuit must be present on
synchronous interfaces.
17 RC Receiver Clock
This circuit provides the clock for the receiver section of a
synchronous DTE. It may of may not be running at the same rate
as the transmitter clock. Note that both TC and RC are sourced
by the DCE. This circuit must be present on synchronous
interfaces.
20 DTR Data Terminal Ready
This circuit provides the signal that informs the DCE that the
DTE is alive and well. It is normally set to the ON state by
the DTE at power-up and left there. Note that a typical DCE
must have an incoming DTR before it will function normally.
This signal must either be brought over from the DTE, or pro-
vided by a wraparound (e.g. from DSR) locally at the DCE end of
the cable. On the DTE side of the interface, this signal is
almost always present, and may be wrapped back around to other
circuits (e.g. DSR, CTS and/or DCD) to satisfy required hand-
shaking signals if their normal function is not required.
Note that in an asynchronous channel, both ends provide their
own internal timing, which (as long as they are within 5% of
each other) is sufficient for them to agree when the bits occur
within a single character. In this case, no timing information
need be sent over the interface between the two devices. In a
synchronous channel, however, both ends must agree when the
bits occur over possibly thousands of characters. In this case,
both devices must use the same clocks. Note that the transmit-
ter and receiver may be running at different rates. Note also
that BOTH clocks are provided by the DCE. When one has a syn-
chronous terminal tied into a synchronous port on a computer
via two synchronous modems, for example, and the terminal is
transmitting, the terminal's modem supplies the Transmit Clock,
which is brought directly out to the terminal at its end, and
encodes the clock with the data, sends it to the computer's
modem, which recovers the clock and brings it out as the
Receive Clock to the computer. When the computer is transmit-
ting, the same thing happens in the other direction. Hence,
whichever modem is transmitting must supply the clock for that
direction, but on each end, the DCE device supplies both clocks
to the DTE device.
All of the above applies to interfacing a DTE device to a DCE
device. In order to interface two DTE devices, it is usually
sufficient to provide a 'flipped' cable, in which the pairs
(TD, RD), (RTS,CTS) and (DTR,DSR) have been flipped. Hence, the
TD of one DTE is connected to the RD of the other DTE, and vica
versa. It may be necessary to wrap various of the hand-shaking
lines back around from the DTR on each end in order to have
both ends work. In a similar manner, two DCE devices can be
interfaced to each other.
An RS-232 'break-out box' is particularly useful in solving
interfacing problems. This is a device which is inserted
between the DTE and DCE. Firstly, it allows you to monitor the
state of the various hand-shaking lines (light on = signal ON /
logic 0), and watch the serial data flicker on TD and/or RD.
Secondly, it allows you to break the connection on one or more
of the lines (with dip-switches), and make any kind of cross-
connections and/or wraparounds (with jumper wires). Using this,
it is fairly easy to determine which line(s) are not function-
ing as required, and quickly build a prototype of a cable that
will serve to interface the two devices. At this point, the
break-out box can be removed and a real cable built that per-
forms the same function. An example of this kind of device is
the International Data Sciences, Inc. Model 60 'Modem and
Terminal Interface Pocket Analyzer' (also called a 'bluebox').
Care should be taken with this type of device to connect the
correct end of it to the DTE device, or the lights and switches
do not correspond to the actual signals.