What is the matching method of RS-422 and RS-485 transmission lines?

Last Update Time: 2021-12-30 10:59:47

Termination resistors are generally used to match RS-422 and RS-485 bus networks. However, in short distances and low speeds, terminal matching can be disregarded. Under what circumstances does matching need not be considered? Theoretically, when sampling at the midpoint of each received data signal, as long as the reflected signal attenuates sufficiently low at the beginning of the sampling, matching may not be considered. But this is actually difficult to grasp. An article by MAXIM in the United States mentioned an empirical principle that can be used to determine what data rate and cable length need to be matched: when the signal conversion time (rise or fall time) When the electric signal is more than 3 times the time required for unidirectional transmission along the bus, no matching is required. For example, the MAX483 RS-485 interface with limited slope characteristics has a minimum rise or fall time of 250ns. The signal transmission rate of a typical twisted pair is about 0.2m/ns (24AWG PVC cable), as long as the data rate is 250kb/s Within, the cable length does not exceed 16 meters, when using MAX483 as the RS-485 interface, you can not add terminal matching.

Generally, the termination matching method uses the termination resistance method. As mentioned earlier, RS-422 is connected in parallel with the resistor at the far end of the bus cable, and RS-485 should be connected in parallel with the termination resistor at the beginning and end of the bus cable. The termination resistance is generally 100Ω in the RS-422 network and 120Ω in the RS-485 network. Resistance equivalent to the characteristic impedance of the cable, because the characteristic impedance of most twisted pair cables is about 100 ~ 120Ω. This matching method is simple and effective, but has a disadvantage. The matching resistor consumes more power, which is not suitable for systems with strict power consumption restrictions.

Another relatively power-saving matching method is RC matching. Using a capacitor C to cut off the DC component can save most of the power. However, the value of the capacitor C is a difficult point, and a trade-off between power consumption and matching quality is required.

There is also a matching method using diodes. Although this scheme does not achieve a true "matching", it uses the clamping effect of the diode to quickly weaken the reflected signal to achieve the purpose of improving signal quality. The energy saving effect is remarkable.

RS-422 and RS-485 network failure protection

Both the RS-422 and RS-485 standards specify a receiver threshold of ±200mV. This stipulation can provide a relatively high noise suppression capability. As mentioned above, when the receiver A level is higher than the B level by +200mV or more, the output is positive logic, otherwise, the output is negative logic. However, due to the existence of the third state, that is, after the host sends a message data at the sending end, the bus is placed in the third state, that is, there is no signal to drive the bus when the bus is idle, so that the voltage between AB is -200 ~ +200mV Until it tends to 0V, this poses a problem: the output state of the receiver is uncertain. If the output of the receiver is 0V, the slave in the network will interpret it as a new start bit and try to read the subsequent bytes. Since there will never be a stop bit, a frame error result is generated and there is no more device request Bus, network paralyzed. In addition to the above-mentioned bus idle will cause the voltage difference between the two lines is less than 200mV, this situation will also occur when open or short circuit. Therefore, certain measures should be taken to prevent the receiver from being in an uncertain state.

Usually bias is added on the bus. When the bus is idle or open, the bias resistor is used to bias the bus to a certain state (differential voltage -200mV). Figure 1. Pull A up to ground and B down to 5V. The typical value of the resistance is 1kΩ. The specific value changes with the capacitance of the cable.

The above method is a more classic method, but it still cannot solve the problem of bus short circuit. Some manufacturers move the reception threshold to -200mV/-50mV to solve this problem.


If you want to know more, our website has product specifications for RS-422 and RS-485 transmission lines, you can go to ALLICDATA ELECTRONICS LIMITED to get more information