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DTE & DCE
| The original purpose of RS232 is to allow large central computers to exchange data with terminals in remote buildings across a campus or across the world. To perform this task the computers - known as Data Terminal Equipment (DTE) use Data Communications Equipment (DCE). DCEs use whatever communications medium they are designed for. Very often the lowest cost communications medium is an ordinary phone line, so the DCE is a modem - but in principle it might be a digital copper communications cable such as X21, fibre optic or a wireless link. In an ideal world whatever medium the link is using would be hidden behind the DCE - which presents the computer engineer with a standardised interface with two data wires and a set of modem control lines. RS232 was intended as the control interface between DTE and DCE rather than as a communications medium in its own right. As the computer industry evolved, however, the need for a dense network of local connections emerged. RS232 cables were pressed into service as a communications medium to support terminals and printers. |
Serial
Data is communicated serially by devices such as modems. RS232 requires data to be in serial form - so if the DTE internal operations are parallel it is responsible for conversion between the two forms.
RS232 does not impose a particular data rate on the transmit and receive circuits - More or less any rate can be chosen - the lowest used are 75 baud (certain kinds of teleprinter) and the highest are about 150,000 baud - ISDN terminal adapters. RS232 tends to speak of baudrate. Baudrates run from 300 in multiples of 2 - so 600, 1200, 2400, 4800, 9600 etc.
The highest RS232 rates are not good enough for local networks and Internet connections. ADSL modems tend to be USB or Ethernet - a typical PC serial port cannot handle much greater throughput.
Handshakes
The hardware handshakes bear some resemblance to what a human operator would need to set up a modem call. Older modems often had buttons that technicians could use to set things up if the software was to dumb to do things itself.
The amount of intelligence that could be incorporated in electronic devices grew, so that terminals (DTE) could talk to communications equipment such as modems (DCE) without the need for any human intervention. In the early 1970s equipment commonly used dedicated logic for the control circuits.
By the late 70's microprocessors and UARTs operated by software were coming into use. The modem receives explicit instructions using the Hayes AT command set and may not need hardware handshakes at all.
RS232 has proved to be a very flexible standard. Unfortunately the flexibility sometimes goes to far and make it difficult to interconnect equipment. Physical problems such as incompatible connector types and sexes are easy enough to deal with. It is not uncommon to find 5-pin DIN plugs used as connectors and every circuit except ground and the two data pins missed out.
Real difficulties arise when handshake circuits need to be interconnected. so the definitions given below might be helpful.
The circuit names are not only used with RS232 cables themselves, but are also seen in the diagnostic lights on the front of communication equipment.
The following paragraphs describe the function of RS232 signals seen from the perspective of the DTE.
Circuits
| Protective Ground | PG | Pin 1 | Circuit AA | CCITT 101 |
The protective ground ensures that the chassis of equipment, plug metalwork and other parts of the circuit are all held at ground potential so that users are not exposed to shock and that electrical interference is deflected from the circuitry and bled to ground.
Most equipment is already supplied with an electrical earth path through its connection to the electricity supply, but there may be a requirement for equipment connected to telecommunications lines to connect to an additional telecommunications earth.
Many RS232 installations use shielded cable and the protective ground will then be connected to the cable shield so that equipment chassis and the cable form a "Faraday cage" around the active circuitry protecting it from electromagnetic interference.
Connecting the protective ground to equipment at both ends of a circuit can form a ground-loop. If there is a difference in the electrical potential of the earth wires in the two places then a current will flow along the protective ground, and this may interfere with communication or in extreme circumstances damage interface circuits and blow fuses. Extreme cases need to be investigated from the point of view of electrical safety.
To avoid earth-loop phenomena engineers commonly connect only one end of the protective ground. Different companies follow different policies some connect only at the terminal, others only at the central system. Connecting at the central system is probably best as this will avoid a collection of different ground-potentials being present at concentrations of wiring such as patch-panels and hubs.
| Signal Ground or Common Return | GND | Pin 7 | Circuit AB | CCITT 102 |
The signal ground is the zero-volts or common return line for all other circuits in the interface. The quality of the signal ground is very important because noise will prevent a proper distinction between the voltage determined signal levels.
Within apparatus the signal ground may be connected to the frame ground there is often a link for this purpose. Ground connections should go to a single point in the apparatus.
| Transmitted Data | TD | Pin 2 | Circuit BA | CCITT 103 | DTE to DCE |
Transmitted data goes from the Data Terminal Equipment (DTE) to the Data Communications Equipment (DCE). In the DCE the serial data will be encoded for onward transmission.
The DTE should not transmit on DT unless it has asserted DTR and RTS and verified that DSR and CTS are On.
TD should be held in marking condition (binary 1) when it is not actively operating. The advantage of this in a communications circuit is that it helps distinguish dead circuits from live but idle circuits. The tone or current represented by "1" will be present continually if the circuit is idle, will be modulated if it is operating and will only be absent if it is dead.
| Received Data | RD | Pin 3 | Circuit BB | CCITT 104 | DCE to DTE |
Received Data enters the Data Terminal Equipment (DTE) from the Data Communications Equipment (DCE). The DCE takes data from a communications medium and passes it to the DTE.
The device driving RD should hold it in marking condition (binary 1) when it is not actively operating.
If DCD is off then RD should not be active (there is no carrier so there cannot be data)
If the channel is half-duplex then RD should not be active when the DTE is asserting RTS, and for a brief interval afterwards to allow line reflections to decay.
| Request to Send | RTS | Pin 4 | Circuit CA | CCITT 105 | DTE to DCE |
Informs the Data Communications Equipment (DCE) that transmission will be required, and controls the direction of transmission on a half-duplex channel, maintaining the DCE in a transmit mode.
When RTS changes from off to on the Data Communications Equipment (DCE) takes what action is necessary to allow transmission and indicates completion by turning on Clear to Send (CTS).
The DTE then sends data to the DCE, which forwards it, holding CTS on whilst it does so.
When RTS changes from on to off the DCE completes transmission of any material transferred to it.
When the DCE has returned to an idle condition and is in a position to respond to a further RTS it may turn off CTS, ready for the next RTS.
| Clear to Send | CTS | Pin 5 | Circuit CB | CCITT 106 | DCE to DTE |
Indicates to the Data Terminal Equipment (DTE) that the Data Communications Equipment (DCE) is ready to accept Transmitted Data (TD) which will be placed on the communications medium for onward transmission.
Whilst the DCE is forwarding data conditions on the communication channel might change, in which case the DCE will turn CTS off and the DTE should cease sending.
When the DCE has completed forwarding data and returned to an idle condition it may turn off CTS to indicate this. In some cases the DCE will leave CTS on whilst the communications channel remains connected.
| Data Set Ready | DSR | Pin 6 | Circuit CC | CCITT 107 | DCE to DTE |
The local Data Communication Equipment is connected to a communication channel (of hook, not in test mode, any switching functions needed to establish a call done.
The signal indicates the status of the local data set (DCE) not the status of the channel.
When DSR is not on the Data Terminal Equipment (DTE) can disregard the status of other signals except Ring Indicator (RI).
If DSR is dropped during a call the DTE should interpret this as a lost or aborted connection and terminate the call. A subsequent DSR-on is a new call.
| Data Terminal Ready | DTR | Pin 20 | Circuit CD | CCITT 108.2 | DTE to DCE |
Indicates to the Data Communications Equipment (DCE) that the Data Terminal Equipment (DTE) is in a state where it will begin communicating.
Switches the DCE to the communications channel or medium when on. The DCE indicates when it has connected by Data Set Ready (DSR) turning on.
When DTR goes off the DCE completes its task and disconnects from the channel, indicating when it has done so by turning DSR off
If the DCE can Auto Answer then it will do so if this signal is on when a ringing indication is received
| Ring Indicator | RI | Pin 22 | Circuit CE | CCITT125 | DCE to DTE |
The Data Communications Equipment (DCE) detects an incoming call on the medium and informs the Data Terminal Equipment (DTE) so that it may arrange for data to be received.
| Data Carrier Detect | DCD | Pin 8 | Circuit CF | CCITT 109 | DCE to DTE |
Data Carrier Detect is also known as the Received Line Signal Detector. If this line is on the Data Communications Equipment (DCE) is receiving a signal that meets the suitability criteria established by its manufacturer.
If DCD is off the signal is not suitable for demodulation. The Receive Data (RD) circuit is not carrying valid information and should be clamped to binary 1 marking - possibly after an appropriate delay to allow data to clear.
On half-duplex channels the DCD circuit should turn off whenever Request To Send (RTS) is on. The carrier for a received signal should cease so that the outgoing carrier can take it's place.
| Signal Quality Detector | Pin 21 | Circuit CG | CCITT 110 | DCE to DTE |
On indicates a good quality signal with low probability of error
Off indicates a high probability that an error has occurred, and may be used to call automatically for re-transmission of a signal.
SQD frequently isn't implemented.
| Data Signal Rate Selector | Pin 23 | Circuit CH | CCITT 111 | DTE to DCE |
Selects the data signalling rate or range of rates when on.
| Data Signal Rate Selector | Pin 23 | Circuit CI | CCITT 112 | DCE to DTE |
As for CCITT 111, but the signal is driven by the Data Communications Equipment.
| Transmitter Signal Element Timing | Pin 24 | Circuit DA | CCITT 113 | DTE to DCE |
If the circuit is implemented then the Data Terminal Equipment provides this signal continually whilst it is powered on. The On/Off transition on this circuit should occur in the centre of each signal element on the transmitted data circuit
| Transmitter Signal Element Timing | Pin 15 | Circuit DB | CCITT 114 | DCE to DTE |
If the circuit is implemented then the Data Communications Equipment provides this signal continually whilst it is powered on. The Off/On transitions of the timing signal nominally coincide with the transitions between signal elements on the transmitted data circuit.
| Receiver Signal Element Timing | Pin 17 | Circuit DD | CCITT 115 | DCE to DTE |
If the circuit is implemented then the Data Communications Equipment provides this signal continually whilst it is powered on. The On / Off transitions on this circuit should occur in the centre of each signal element on the Received Data circuit.
| Secondary Transmitted Data | Pin 14 | Circuit SBA | CCITT 118 | DTE to DCE |
Equivalent to Transmitted Data (TD) and obeying the same rules except that the handshakes used are the secondary handshakes.
The secondary circuit may be used as a slower back-channel, for primary circuit assurance or to provide an interrupt for the primary channel. The interrupt role can also be served by the Secondary Request to Send circuit.
| Secondary Received Data | Pin 16 | Circuit SBB | CCITT 119 | DCE to DTE |
Equivalent to Received Data (RD) and obeying the same rules except that the handshakes used are the secondary handshakes.
| Secondary Request To Send | Pin 19 | Circuit SCA | CCITT 120 | DTE to DCE |
Equivalent to RTS, except that it requests establishment of the secondary channel.
If the secondary channel is used as a backward channel then when the primary RTS is on the secondary should not be on. Alternatively the secondary can be locked on, and channels may be switched using the primary RTS
| Secondary Clear To Send | Pin 13 | Circuit SCB | CCITT 121 | DCE to DTE |
Equivalent to CTS, except that it indicates availability of the secondary channel. May not be provided if the secondary channel is used only for circuit assurance or as an interrupt channel.
| Secondary Data Carrier Detect | Pin 12 | Circuit SCF | CCITT 122 | DCE to DTE |
Equivalent to DCD, except that it indicates carrier detected on the secondary channel. If the secondary channel is used only for circuit assurance then an On condition indicates this, and Off indicates no Assurance / Interrupt.