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).

Inductive and Capacitive reactance are exactly opposite in their effects in AC circuity and will cancel each other respectively. Inductive Reactance is the electrical resistance caused by the build-up of electric or magnetic fields and increases as the frequency gets HIGHER. Capacitance Reactance is the electrical resistance to a signal’s ability to charge or discharge to a voltage state and increases as the frequency gets LOWER. Thus, if a circuit contains 150 ohms of inductive reactance and 125 ohms of capacitive reactance in series, the net reactance, is 150 – 125, or 25 ohms of inductive reactance.

The key truth of all communication lines is that the source impedance must ideally be equal to the load impedance in order to achieve maximum power (signaling) transfer and minimum signal reflection at the destination. Serial communication operates most efficientially when the source and load impedance are matched to 120 ohms. Unfortunately, when higher baud rates and/or longer transmission lines are implemented, the effective load impedance tends to change causing a mismatch. Keep in mind that the ideal impedance of 120 ohms only describes the intrinsic impedance of the transmission line and it is not a function of the line length. As a result, the transmitted signal will not be completely absorbed by the load and a portion is reflected back into the transmission line, causing a condition called “cross-talk”. As the source and load impedance become more equal these reflections are eliminated.

There are several different ways to counter cross-talk, however, the most practical solution is to create an additional load to absorb the excessive reflections. This can be done by simply placing a “termination resistor” in parallel, with the communication medium. In RS485, a termination resistor may be added across the receiver’s “+” and “-” lines in order to bring the load impedance back into compliance. Keep in mind that resistor termination can potentially add heavy DC loading to a system and overload the source. Therefore it is best to confer with the hardware manufacturer before adding any line termination. That being said, a terminating resistor of less than 90 ohms should not be used and termination resistors should only be placed at the trunk ends of the transmission line. No more than two terminations should be placed in any system that does not use repeaters.

Many manufacturers suggest utilizing a termination resistor valued between 100 and 150 ohms; however, they fail to mention that if proper communication cabling is utilized and the baud rate is not excessively high, a load termination resistor may not be needed at all in shorter cable runs. Remember, if improper communication cabling is utilized, all rules are thrown out and it is only a “hit-n-miss” if the application works at all. Belden 9841 or 9842 is EIA-485 cabling is made specifically for paired, half-duplex, multi-point RS485 serial communication applications

In our power plant we are having problems to establish Modbus RS485 connection between Microscada and Beckwith M-3425 generator protection relay.We use CAT5 cable and in beckwith module i have inserted 200ohm termination resistor.Does anyone done this job before?

All are Rs 485 2 wire,Modbus,9600,shiled twised pair cable out of 24 Energy meter the following meter are not communicating Meter ID 11,14,20&23.

We are using Terminating resistor also (120ohm 0.25W) for both ends.We are using Rs 485 to Rs 232 Converter.

I am check individually for Energy meter ID 11,14,20 & 23 separately cable that only one meter that time these meter are working.

Please help me.

Thanks
Sri.V

this kind of bus with voltage dividers is fine for just a point to point RS485 connection because there must be just two terminators in the bus, but what you propose should be done in every node.

The transformer idea can’t work because RS485 isn’t a zero average voltage standard: no one warrants that the total duration of zeros is equal to the total duration of ones, leading to a DC voltage component, which in turn saturates any transformer. Even if the transformer is not saturated because of the line impedance limiting the current which can flow through it, in case several zeroes or ones form a packet, they would be lost because the transformer works on voltage variations.

first, both wire must be protected against transients with diode clamps in both directions, VCC and ground. And then they should be forced to remain in the required point (half of VCC) with resitor voltage dividers. These will also work as terminators to avoid reflections.

unless noise is extremely high it should work fine.

if full isolation is required 1:1 transformer can be used for that purpose.