How to Solve RS-485 Auto Transceiver Circuit Application Issues?

Principle of Automatic Transceiver Circuits

Common RS-485 automatic transceiver circuits are shown in Figure 1, and the circuit’s logic truth table is shown in Table 1.

• When TXD is low, DE and RE are high, and the RS-485 transceiver is in transmit mode, AB is low, which means sending a low signal to other nodes;

• When TXD changes from low to high, DE and RE become low, and the RS-485 transceiver is in receive mode. At this time, the AB pin is in a high-impedance state, R3 pulls the B pin to GND, and R4 pulls the A pin to VCC, making AB high, which means sending a high signal to other nodes.

Since TXD is high, the RS-485 transceiver is in receive mode, so TXD only needs to be kept high to receive data, meaning that the automatic transceiver circuit is in receive mode while sending a high signal.

Figure 1 RS-485 Automatic Transceiver Schematic

Table 1 Automatic Transceiver Circuit Schematic

The reasons for the slow communication speed of RS-485 automatic transceivers are mainly two:

1. If using the automatic transceiver circuit shown in Figure 1, the turn-off time of the transistor is relatively long (mainly due to the long storage time when the transistor is turned off). The turn-on delay time of the transistor in the inverting circuit shown in Figure 1 is 10.2ns, and the turn-off delay time is 1.14μs. Therefore, if TXD sends a low signal, the DE & RE pins will take a long time to rise to high, resulting in a long delay in sending the low signal, as shown in Figure 4, which takes 1.468μs.

Figure 2 Turn-On Delay Time

Figure 3 Turn-Off Delay Time

Figure 4 Low Signal Sending Delay

2. The automatic transceiver circuit sends high signals through external pull-up and pull-down resistors, resulting in slow rising edges, as shown in Figure 5. It can be seen that the rising edge of the high signal is slow, limiting high-speed communication applications.

Figure 5 Rising Time of Sending High Signal

As shown in Figure 6, when TXD becomes high, DE & RE pins drop to low, and the AB differential voltage rises slowly. At this time, the RS-485 transceiver is already in receive mode, and before the AB differential voltage reaches the threshold level of the RS-485 transceiver, the RXD pin will output a low signal for a period. For example, for a transceiver with a threshold level of -200mV to -50mV, the RS-485 transceiver will output a low signal until the AB differential voltage rises to -50mV. This low signal duration is related to the rise time of the AB differential voltage and the receive delay of the RS-485 transceiver.

Since the serial port generally divides each bit into 16 parts and detects the level signal of the middle 3 parts to determine the high or low of this bit, if this low signal is maintained during the signal detection of each bit, it will cause the MCU to receive a start bit, thus receiving erroneous data. Therefore, this issue also limits high-speed communication applications and reduces communication reliability.

Figure 6 TXD Sends High Signal, RXD Receives Low Signal for a Period

To address these issues, you can choose the ZLG SC4450S wide-voltage input surface-mount isolated RS-485 automatic transceiver module (as shown in Figure 7). This module supports communication speeds up to 500kbps and will not output a low signal for a period when sending a high signal, as shown in Figure 8, greatly improving communication reliability.

Figure 7 Surface-Mount Isolated RS-485 Transceiver SC4450S

Figure 8 SC4450S Communication Waveform

In high electrostatic or surge environments, additional peripheral protection circuits are needed to protect the RS-485 transceiver. If the junction capacitance of the protection circuit added to the RS-485 bus is large, it will affect the quality of the communication waveform and even lead to communication anomalies. The communication waveform using high junction capacitance protection circuits is shown in Figure 9, where the waveform is severely distorted, affecting communication quality.

Figure 9 Waveform Affected by High Junction Capacitance

To address these issues, you can choose ZLG SC4450S, and according to the protection circuit provided in the data sheet, as shown in Figure 10, it can comprehensively enhance the protection capability of the RS-485 bus. The EMC protection capability can reach ±8kV for electrostatic contact, ±4kV for surge common mode, and ±2kV for differential mode, meeting the needs of most industrial applications.

Figure 10 Low Junction Capacitance Protection Circuit

Figure 11 Communication Waveform After Adding Protection Circuit