How to Improve Stability for Output Signal RS485 Pressure Sensor

Pressure sensors using RS485 output signal are getting more and more applications in practical work, which is due to the development of pressure sensors in the direction of intelligence and digitization. At the same time, due to the simple design, convenient implementation and low price of the RS485 bus output circuit, it has a wide range of applications in pressure sensors and differential pressure sensors. However, due to various interferences at the engineering site and failures of communication nodes, stability problems in data transmission and reception will occur. At the same time, in the case of multi-machine communication, the failure of one node will cause the entire bus communication to collapse. Therefore, how to ensure the stability in the communication of the RS-485 bus output becomes a problem that needs to be focused on.
In order to ensure the stability of the signal transmission for the pressure sensor, this paper will improve the stability of the RS-485 pressure sensor from the software and hardware design.

• RS485 Pressure sensor software

In terms of software, since the 485 bus is an asynchronous half-duplex communication bus, the bus can only present one state at a certain moment. Therefore, in 485 communication, we should pay attention to the software programming for the control terminal DE of the 485 chip. In order to work reliably, it is necessary to make an appropriate delay when switching the 485 bus state, and then send and receive data. The specific method is to put the control end into the sending state in the data transmission state, delay about 1ms, and then send the data. After one packet of data transmission is end, delay about 1ms, and then put the control terminal into the receiving state. It makes the bus to have a stable working process when the state is switched.

• RS485 Pressure sensor hardware

In terms of hardware, it is generally necessary to pay attention to the following points.

1. Generally 485 chip has a DE control terminal. When designing the system, ensure that the RS-485 interface is in the receiving data state when each communication unit is powered on reset.
2. Using isolation technology: Using photoelectric isolation technology can greatly improve the anti-interference ability of the communication unit. When the communication speed of the 485 bus is high, high-speed optocoupler is needed.
3. In order to achieve the impedance matching of the anti-interference characteristics and line characteristics for the 485 bus, a TVS transient clutter suppression device that can resist surge is usually selected at the transmission end of the 485 bus, or directly select the 485 chip that can resist lightning strike. In the line design, a matching resistor (resistance value is determined by experiment) is connected to the beginning and end of the RS-485 network transmission line to reduce the reflection of the transmitted signal on the line.
4. Taking into account the special conditions of the line (such as 485 bus short circuit), in order to prevent the communication of other extensions in the bus from being affected, two resistors with small resistance are connected in series at the 485 signal output, so that the faulty unit will not affect the communication of the entire bus.
5. Due to the characteristics of the RS-485 chip, the receiver's detection sensitivity is ±200mV. When the absolute value of the potential difference at the 485 chip differential port is less than 200 mV, the output electrical level of 485 is an indeterminate level. Therefore, according to the actual situation, appropriate pull-up and pull-down resistors should be connected at the differential terminals to ensure that the output electrical level of 485 is determined.

Through the above hardware and software measures, the stability of the pressure sensor with RS-485 output can be greatly improved, meeting the requirements for most engineering applications.