Technical Article #11 Page 1/1 - B&B Electronics

1 Technical Article #11 Page 1/1 cable SELECTIONFOR RS-422 AND RS-485 SYSTEMSM aking sense of the specificationsSelecting data cable for an RS-422 or RS-485 system isn tdifficult, but often gets lost in the shuffle of larger system should be taken, however, because intermittent problemscaused by marginal cable can be very difficult to the obvious traits such as number of conductorsand wire gauge, cable specifications include a handful of lessintuitive Impedance (Ohms): A value basedon the inherent conductance, resistance, capacitanceand inductance of a cable that represents theimpedance of an infinitely long cable . When the cableis cut to any length and terminated with thisCharacteristic Impedance, measurements of the cablewill be identical to values obtained from the infinitelength cable . That is to say that the termination of thecable with this impedance gives the cable theappearance of being infinite length, allowing noreflections of the transmitted signal.

2 If termination isrequired in a system, the termination impedance valueshould match the Characteristic Impedance of Capacitance (pF/ft): The amount of equivalentcapacitive load of the cable , typically listed in a perfoot basis. One of the factors limiting total cable lengthis the capacitive load. Systems with long lengths benefitfrom using low capacitance velocity (% of c): The speed at whichan electrical signal travels in the cable . The value giventypically must be multiplied by the speed of light (c) toobtain units of meters per second. For example, a cablethat lists a propagation velocity of 78% gives a velocityof 300 106 = 234 106 meters per second."..intermittent problems caused by marginal cablecan be very difficult to troubleshoot."Plenum cablePlenum rated cable is fire resistant and less toxic whenburning than non-plenum rated cable . Check building andfire codes for requirements. Plenum cable is generally moreexpensive due to the sheathing material RS-422 specification recommends 24 AWG twistedpair cable with a shunt capacitance of 16 pF per foot and100 ohm characteristic impedance.

3 While the RS-485specification does not specify cabling, these recommendationsshould be used for RS-485 systems as can be difficult to quantify whether shielding is requiredin a particular system or not, until problems arise. Werecommend erring on the safe side and using shielded cable is only slightly more expensive than are many cables available meeting therecommendations of RS-422 and RS-485, made specificallyfor that application. Another choice is the same cablecommonly used in the twisted pair Ethernet cabling. Thiscable, commonly referred to as Category 5 cable , is definedby the EIA/TIA/ANSI 568 specification. The extremelyhigh volume of Category 5 cable used makes it widelyavailable and very inexpensive, often less than half the priceof specialty RS-422/485 cabling. The cable has a maximumcapacitance of 17 pF/ft ( pF typical) and characteristicimpedance of 100 5 cable is available as shielded twisted pair(STP) as well as unshielded twisted pair (UTP) and generallyexceeds the recommendations for RS-422 making it anexcellent choice for RS-422 and RS-485 systems.