Belden Rs485 Cable Price

RS485 and Line Termination Explained

RS485 is a bidirectional, half-duplex, communication bus structure consisting of a single Master and at least one Slave.  The maximum number of slaves can vary widely from system to system however; most manufacturers cap the max number of slaves between 16 and 32.  Most RS485 signals operate ideally on a DC bias of 5 volts.  The signals are driven alternately from each other.  That is, each line operates inversely from the other and each is also referenced to the other from an electrical standpoint.  The receiver looks at the difference, not the absolute voltage value, of the two signals.  This is referred to as the “Line Bias” and it is critical in RS-485 applications. A bias difference of higher than 0.3v is generally accepted as valid but can be as high as 0.7v, depending on the system.  Absolute values below this are considered “undefined” or “gray” and may result in either a high or low reading by the receiver.  In many non-isolated applications, a “ground” or “shield” connection is provided in addition to the 2 data lines however, this is not necessary as the signals are referenced to each other and not to absolute ground.  It is only provided as a ground point for the communication cable shielding.  It is important to note that the shield ground should only be connected at a single trunk end in non-isolated applications.  If grounds are tied together in non-isolated systems, even a slight voltage difference, between the absolute ground points, will create a “ground loop” condition which can cause serious damage to the equipment.  Note that in isolated equipment, the above does not apply as there is no direct connection between the ground points however, in all applications, proper cable shield grounding should be practiced to eliminate and/or reduce electrical interference.

All devices, including wire, have a resistance to electricity.  Although resistance changes slightly with temperature and age, it remains the same at all times, no matter what electricity is doing.  As stated earlier, RS485 communication utilizes fluctuating signaling across a pair of conductors which creates an RF frequency.  The baud rate determines the RF frequency at which the signals operate and the lower the frequency, the higher the impedance.  Impedance is a measure of the overall opposition to current flow and only exists where there is AC, or fluctuating DC signals. In other words, Impedance is the measure of how much the circuit impedes the flow of alternating current. It is like “ohmite” resistance, but it also takes into account an additional property called Reactance.  Reactance is the opposition to alternating current due to either inductance (inductive reactance) or capacitance (capacitive reactance).

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