1. Rs232 Serial Port

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RS232Data Interfacea Tutorial on Data Interfaceand cablesRS-232 is simple, universal, well understood and supported butit has some serious shortcomings as a data interface. The standards to 256kbps or less and line lengths of 15M (50ft) or less but today we see high speed ports on our home PC running very high speeds andwith high quality cable maxim distance has increased greatly. The rule of thumb for the length a data cable depends on speed of the data, quality of the cable.a TutorialElectronic data communications between elementswill generally fall into two broad categories: single-ended and differential.RS232 (single-ended) was introduced in 1962, and despite rumors for its earlydemise, has remained widely used through the industry.Independent channels are established for two-way(full-duplex) communications. The RS232 signals are represented by voltagelevels with respect to a system common (power / logic ground).

The'idle' state (MARK) has the signal level negative with respect tocommon, and the 'active' state (SPACE) has the signal level positivewith respect to common. RS232 has numerous handshaking lines (primarily usedwith modems), and also specifies a communications protocol.The RS-232 interface presupposes acommon ground between the DTE and DCE. This is a reasonable assumption when a short cableconnects the DTE to the DCE, but with longer lines and connections between devices thatmay be on different electrical busses with different grounds, this may not be true.RS232 data is bi-polar. +3 TO +12 voltsindicates an 'ON or 0-state (SPACE) condition' while A -3 to -12 volts indicatesan 'OFF' 1-state (MARK) condition. Modern computer equipment ignores thenegative level and accepts a zero voltage level as the 'OFF' state. In fact, the'ON' state may be achieved with lesser positive potential.

This means circuitspowered by 5 VDC are capable of driving RS232 circuits directly, however, the overallrange that the RS232 signal may be transmitted/received may be dramatically reduced.The output signal level usually swings between +12V and -12V. The'deadarea' between +3v and -3v is designed to absorb linenoise. In the various RS-232-like definitions this dead area may vary. For instance, thedefinition for V.10 has a dead area from +0.3v to -0.3v.

Many receivers designed forRS-232 are sensitive to differentials of 1v or less.This can cause problems whenusing pin powered widgets - line drivers, converters, modems etc. These type of units needenough voltage & current to power them self's up. Typical URART (the RS-232 I/O chip)allows up to 50ma per output pin - so if the device needs 70ma to run we would need to useat least 2 pins for power. Some devices are very efficient and only require one pin (sometimes the Transmit or DTR pin) to be high - in the 'SPACE' state while idle.An RS-232port can supply only limited power to another device. The number of outputlines, the type of interface driver IC, and the state of the output lines areimportant considerations.The types ofdriver ICs used in serial ports can be divided into three general categories:.

Drivers which requireplus (+) and minus (-) voltage power supplies such as the 1488 series ofinterface integrated circuits. (Most desktop and tower PCs use this type ofdriver.). Low power driverswhich require one +5 volt power supply. This type of driver has an internalcharge pump for voltage conversion.

(Many industrial microprocessor controlsuse this type of driver.). Low voltage (3.3 v)and low power drivers which meet the EIA-562 Standard. (Used on notebooksand laptops.)Data is transmitted and received on pins 2 and 3 respectively.

Data SetReady (DSR) is an indication from the Data Set (i.e., the modem or DSU/CSU) that it is on.Similarly, DTR indicates to the Data Set that the DTE is on. Data Carrier Detect (DCD)indicates that a good carrier is being received from the remote modem.Pins 4 RTS (Request To Send - from the transmitting computer) and 5 CTS (ClearTo Send - from the Data set) are used to control. In most Asynchronous situations, RTS andCTS are constantly on throughout the communication session. However where the DTE isconnected to a multipoint line, RTS is used to turn carrier on the modem on and off. On amultipoint line, it's imperative that only one station is transmitting at a time (becausethey share the return phone pair).

When a station wants to transmit, it raises RTS. Themodem turns on carrier, typically waits a few milliseconds for carrier tostabilize, andthen raises CTS. The DTE transmits when it sees CTS up. When the station has finished itstransmission, it drops RTS and the modem drops CTS and carrier together.Clock signals (pins 15, 17, & 24) are only used forsynchronous communications.

The modem or DSU extracts the clock from the datastream and provides a steady clock signal to the DTE. Note that the transmit and receiveclock signals do not have to be the same, or even at the same baud rate.Note: Transmit and receive leads (2 or 3) can bereversed depending on the use of the equipment -DCE Data Communications Equipment or a DTE Data Terminal Equipment.Glossary of Abbreviationsetc. CTSClear To Send DCE - DTEDCDData Carrier Detected (Tone from a modem) DCE - DTEDCEData Communications Equipment eg. ModemDSRData Set Ready DCE - DTEDSRS Data Signal RateSelector DCE - DTE (Not commonlyused)DTEData Terminal Equipment eg. X.21 interface on a DB 15connectoralso seealso see end of page for more info X.21GeneralVoltages:+/- 0.3VdcSpeeds:Max. 100Kbps (X.26)Max.

10Mbps (X.27)The X.21 interface was recommended by the CCITT in 1976. It is defined as adigital signaling interface between customers (DTE) equipment and carrier'sequipment (DCE). And thus primarily used for telecom equipment.All signals are balanced. Meaning there is always a pair (+/-) for eachsignal, like used in RS422.

The X.21 signals are the same as RS422, so pleaserefer to RS422 for the exact details.Pinning according to ISO 4903Sub-D15 MaleSub-D15 FemalePinSignalabbr.DTEDCE1Shield-2Transmit (A)OutIn3Control (A)OutIn4Receive (A)InOut5Indication (A)InOut6Signal Timing (A)InOut7Unassigned8Ground-9Transmit (B)OutIn10Control (B)OutIn11Receive (B)InOut12Indication (B)InOut13Signal Timing (B)InOut14Unassigned15UnassignedFunctional DescriptionAs can be seen from the pinning specifications, the Signal Element Timing(clock) is provided by the DCE. This means that your provider (local telcooffice) is responsible for the correct clocking and that X.21 is a synchronousinterface.

Hardware handshaking is done by the Control and Indication lines. TheControl is used by the DTE and the Indication is the DCE one.Cross-cable pinningX.21 Cross CableX.21X.15RS232D uses RJ45 type connectors(similarto telephone connectors)Pin No.Signal DescriptionAbbr.DTEDCE1DCE Ready, Ring IndicatorDSR/RI2Received Line Signal DetectorDCD3DTE ReadyDTR4Signal GroundSG5Received DataRxD6Transmitted DataTxD7Clear To SendCTS8Request To SendRTS. SPECIFICATIONSRS232RS423Mode of OperationSINGLE-ENDEDSINGLE-ENDEDTotal Number of Drivers and Receivers on One Line1 DRIVER1 RECVR1 DRIVER10 RECVRMaximum Cable Length50 FT.4000 FT.Maximum Data Rate20kb/s100kb/sMaximum Driver Output Voltage+/-25V+/-6VDriver Output Signal Level (Loaded Min.)Loaded+/-5V to +/-15V+/-3.6VDriver Output Signal Level (Unloaded Max)Unloaded+/-25V+/-6VDriver Load Impedance (Ohms)3k to 7k=450Max. Driver Current in High Z StatePower OnN/AN/AMax.

Driver Current in High Z StatePower Off+/-6mA @ +/-2v+/-100uASlew Rate (Max.)30V/uSAdjustableReceiver Input Voltage Range+/-15V+/-12VReceiver Input Sensitivity+/-3V+/-200mVReceiver Input Resistance (Ohms)3k to 7k4k min.One byte of async dataCabling considerations - you should use cabling made for RS-232 data but I haveseen low speed data go over 250' on 2 pair phone cable. Level 5 cable can also be used butfor best distance use a low capacitance data grade cable.The standard maxim length is 50' but if data is async you can increase thatdistance to as much as 500' with a good grade of cable.The RS-232 signal on a single cable is impossible to screen effectively fornoise. By screening the entire cable we can reduce the influence of outside noise, butinternally generated noise remains a problem. As the baud rate and line length increase,the effect of capacitance between the different lines introduces serious crosstalk (thisespecially true on synchronous data - because of the clocklines) until a point is reached where the data itself is unreadable. SignalCrosstalk can be reduced by using low capacitance cable and shielding each pairUsing a high grade cable (individually shield low capacitance pairs) thedistance can be extended to 4000'At higher frequencies a new problem comes to light. The high frequencycomponent of the data signal is lost as the cable gets longer resulting in arounded, rather than square wave signal.The maxim distance will depend on the speed and noise level around the cablerun.On longer runs a line driver is needed.

This is a simple modem used to increasethe maxim distance you can run RS-232 data.Making sense of the specificationsSelecting data cable isn't difficult, but often gets lost in the shuffle of larger system issues. Care should be taken. However, because intermittent problems caused by marginal cable can be very difficult to troubleshoot.Beyond the obvious traits such as number of conductors and wire gauge, cable specifications include a handful of less intuitive terms.Characteristic Impedance (Ohms): A value based on the inherent conductance, resistance, capacitance and inductance of a cable that represents the impedance of an infinitely long cable.

When the cable is out to any length and terminated with this Characteristic Impedance, measurements of the cable will be identical to values obtained from the infinite length cable. That is to say that the termination of the cable with this impedance gives the cable the appearance of being infinite length, allowing no reflections of the transmitted signal. If termination is required in a system, the termination impedance value should match the Characteristic Impedance of the cable.Shunt Capacitance (pF/ft): The amount of equivalent capacitive load of the cable, typically listed in a per foot basis One of the factors limiting total cable length is the capacitive load. Systems with long lengthsbenefits from using low capacitance cable.Propagation velocity (% of c): The speed at which an electrical signal travels in the cable. The value given typically must be multiplied by the speed of light (c) to obtain units of meters per second.

For example, a cable that lists a propagation velocity of 78% gives a velocity of 0.78 X 300 X 106 - 234 X 106 meters per second.Plenum cablePlenum rated cable is fire resistant and less toxic when burning than non-plenum rated cable. Check building and fire codes for requirements. Plenum cable is generally more expensive due to the sheathing material used.The specification recommends 24AWG twisted pair cable with a shunt capacitance of 16 pF per foot and 100 ohm characteristic impedance.It can be difficult to qualify whether shielding is required in a particular system ornot, until problems arise. We recommend erring on the safe side and using shielded cable. Shielded cable is only slightly more expensive than unshielded.There are many cables available meeting the recommendations of RS-422 and RS-485, made specifically for that application.

Another choice is the same cable commonly used in theTwisted pair Ethernet cabling. This cable, commonly referred to as Category 5 cable, is defined by the ElA/TIA/ANSI 568 specification The extremely high volume of Category 5 cable used makes it widely available and very inexpensive, often less than half the price of specialty RS422/485 cabling. The cable has a maximum capacitance of 17 pF/ft (14.5 pF typical) and characteristic impedance of 100 ohms.Category 5 cable is available as shielded twisted pair (STP) as well as unshieldedtwisted pair (UTP) and generally exceeds the recommendationsmaking it an excellent choice for RS232 systems.RS232 - V.24/V.28 - IS2110 - X.20 bis (for Async) -X.21 bis (for Sync)GeneralIn this document the term RS232 will be used when refered to this serialinterface. The description of RS232 is an EIA/TIA norm and is identical to CCITTV.24/V.28, X.20bis/X.21bis and ISO IS2110. The only difference is that CCITT hassplit the interface into its electrical description (V.28) and a mechanical part(V.24) or Asynchronous (X.20 bis) and Synchronous (X.21 bis) where the EIA/TIAdescribes everything under RS232.As said before RS232 is a serial interface.

Rs232 Serial Port

It can be found in many differentapplications where the most common ones are modems and Personal Computers. Allpinning specifications are writen for the DTE side.All DTE-DCE cables are straight through meaning the pins are connected one onone. DTE-DTE and DCE-DCE cables are cross cables. To make a destiction betweenall different types of cables we have to use a naming convention.DTE - DCE: Straight CableDTE - DTE: Null-Modem CableDCE - DCE: Tail Circuit CableInterface MechanicalRS232 can be found on different connectors. There are special specificationsfor this. The CCITT only defines a Sub-D 25 pins version where the EIA/TIA hastwo versions RS232C and RS232D which are resp.

On a Sub-D25 and a RJ45.