What about the load capacity and transmission performance of the RS-485 bus?

Last Update Time: 2021-12-27 11:07:24

When designing the network configuration (bus length and number of loads) composed of the RS-485 bus, three parameters should be considered: purely resistive load, signal attenuation, and noise tolerance. The two parameters, purely resistive load and signal attenuation, have already been discussed before, and now we are talking about noise margin (NoiseMargin). The noise tolerance of the RS-485 bus receiver should be at least greater than 200mV. The previous discussion was carried out on the assumption that the noise margin is zero.

In practical applications, in order to improve the anti-interference ability of the bus, the noise tolerance of the system is always better than that specified in the EIARS-485 standard. The relationship between the amount of bus load and the length of the communication cable can be seen from the following formula: Vend=0.8 (Vdriver-Vloss-Vnoise-Vbias) (3)

Among them: Vend is the signal voltage at the end of the bus, which is specified as 0.2V during standard measurement; Vdriver is the output voltage of the driver (related to the number of loads. The number of loads is between 5 and 35, Vdriver=2.4V; when the number of loads is less than 5. Vdriver=2.5V; when the number of loads is greater than 35, Vdriver2.3V); Vloss is the loss of the signal during transmission in the bus (related to the specifications and length of the communication cable), the standard cable provided by Table 1 Attenuation coefficient, according to the formula attenuation coefficient b=20lg (Vout/Vin), Vloss=Vin-Vout=0.6V can be calculated (Note: the communication baud rate is 9.6kbps, cable length is 1km, if the special rate increases, Vloss will increase accordingly Large); Vnoise is the noise tolerance, which is specified as 0.1V in the standard measurement; Vbias is the bias voltage provided by the bias resistor (typical value is 0.4V).

Multiplying 0.8 in formula (3) is to prevent the communication cable from entering the full load state. It can be seen from equation (3) that the size of Vdriver is inversely proportional to the number of loads on the bus, and the size of Vloss is inversely proportional to the length of the bus. The other parameters are only related to the type of driver used. Therefore, on the RS-495 bus where the driver is selected, the number of loads with a certain baud rate is directly related to the maximum distance that the signal can be transmitted. The specific relationship is:

Within the allowable range of the bus, the more the number of loads, the smaller the distance that the signal can transmit; the less the data with load, the longer the distance that the signal can transmit.

17. The influence of distributed capacitance on the transmission performance of RS-485 bus

The distributed capacitance of the cable is mainly generated by two parallel conductors of twisted pairs. In addition, there is also distributed capacitance between the wire and the ground. Although it is small, it cannot be ignored in the analysis. The influence of distributed capacitance on the transmission performance of the bus is mainly because the fundamental signal is transmitted on the bus, and the signal expression is only "1" and "0". In a special byte, such as 0x01, the signal "0" makes the distributed capacitor have sufficient charging time, and when the signal "1" comes, due to the charge in the distributed capacitor, it is too late to discharge, (Vin+)-(Vin-)- It is also greater than 200mV. As a result, the connection is mistaken for "0", which eventually results in a CRC check error and an entire data frame transmission error.

Due to the influence of the distribution on the bus, data transmission errors are caused, which reduces the performance of the entire network. There are two ways to solve this problem:

(1) Reduce the baud rate of data transmission;

(2) Use cables with small distributed capacitance to improve the quality of the transmission line.

18. Definition of Simplex, Half Duplex and Full Duplex

1. If the information can only be transmitted from one party A to the other party B at any time during the communication process, it is called simplex.

2. If at any moment, information can be transmitted from A to B, and B can also transmit A, but only exists in one direction of transmission, called half-duplex transmission.

3. If at any time, there are two-way signal transmission from A to B and B to A on the line, it is called full duplex.

The telephone line is a two-wire full-duplex channel. Due to the use of echo cancellation technology, the two-way transmission signal will not be confused. Duplex channels sometimes separate the receive and transmit channels and use separate lines or frequency bands to transmit signals in opposite directions, such as loopback transmission.


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