What is the effect of RS-485 in communication cables?
RS-485 signal attenuation in communication cables
The second factor that affects signal transmission is signal attenuation during cable transmission. A transmission cable can be seen as an equivalent circuit composed of distributed capacitance, distributed inductance and resistance.
The distributed capacitance C of the cable is mainly generated by two parallel conductors of the twisted pair. The resistance of the wire here has little effect on the signal and can be ignored. The loss of the signal is mainly due to the LC low-pass filter composed of the distributed capacitance and distributed inductance of the cable. LAN standard two-core inductor for PROFIBUS (Siemens standard cable for DP bus), attenuation coefficient at different baud rates.
Pure resistance load in communication cable
The third factor that affects communication performance is the size of a purely resistive load (also called DC load). The purely resistive load referred to here is mainly composed of terminating resistor, bias resistor and RS-485 transceiver.
When describing the EIARS-485 specification, it was mentioned that the RS-485 driver can output at least 1.5V of differential voltage with 32 nodes and a 150Ω termination resistor. The input resistance of a receiver is 12kΩ, and the equivalent circuit of the entire network is shown in Figure 5. According to this calculation, the load capacity of the RS-485 driver is: RL=32 input resistors in parallel||2 terminal resistances=((12000/32)×(150/2))/(12000/32)+(150/ 2))≈51.7Ω
The most commonly used RS-485 drivers are MAX485, DS3695, MAX1488/1489, and SN75176A/D used by Hollysys. Some of the RS-485 drivers have a load capacity of 20Ω. Without considering many other factors, the maximum number of nodes that can be driven by a drive is much greater than 32, based on the relationship between drive capacity and load.
When the communication baud rate is relatively high, a bias resistor on the line is necessary. Connection method of bias resistor. Its function is to pull the level away from 0 level when there is no data on the bus (idle mode) after the line enters the idle state. In this way, even if a relatively small reflected signal or interference occurs on the line, the data receiver attached to the bus will not cause a malfunction due to the arrival of these signals. Through the following example, the size of the bias resistor can be calculated: termination resistance Rt1=Rr2=120Ω;
Assuming the maximum peak-to-peak value of the reflected signal Vref≤0.3Vp-p, the negative half-cycle voltage Vref≤0.15V; the reflected current Iref≤0.15/(120||120)=2.5mA caused by the reflected signal on the resistance of the terminal. The hysteresis value of a general RS-485 transceiver (including SN75176) is 50mV, that is:
(Ibias-Iref)×(Rt1||Rt2)≥50mV
Therefore, the bias current Ibias ≥ 3.33 mA generated by the bias resistor can be calculated
+5V=Ibias (R pull-up + R pull-down + (Rt1||Rt2)) (2)
Equation 2 can be used to calculate R pull-up = R pull-down = 720Ω
In practical applications, there are two methods for adding bias resistors to the RS-485 bus:
(1) Allocate the bias resistance to each transceiver on the bus. This method adds a bias resistor to each transceiver connected to the RS-485 bus, and adds a bias voltage to each transceiver.
(2) Only one pair of bias resistors is used on a section of bus. This method is more effective for the presence of large reflected signals or interference signals on the bus. It is worth noting that the addition of bias resistors increases the load on the bus.
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