What's means of the drone interfaces?

PIXHAWK Cub Orange flight control

There are various devices on the VTOL fixed-wing drone, and there are many types of interfaces. The main ones are: PWM, PPM, SBUS, serial port (including TTL, 232, 422, 485), SPI, I2C, CAN, USB, Relay (GPIO) ), AD, SDIO, HDMI, today let's take a look at the meaning of these interfaces.

PWM: An interface that all models and drones are inseparable from. A single-line signal, sending positive pulses periodically, and changing the pulse width as a way to transmit information, one pin transmits one channel, often with a ground wire and a power wire to control a servo or an ESC, it is the first entry for drones or model aircraft. Interface to know. The advantage is simple, stable transmission of a variable signal, the disadvantage is low speed. The current common standard is 50Hz, and there are also 300Hz to 400Hz.

PPM: It is an upgraded version of PWM, that is, each signal cycle becomes a combination of a group of multiple pulse widths to transmit the change information of multiple channels at the same time. In the early days, it was also used for the radio signal of the remote control and the signal of the model aircraft simulator. Now it is mostly used for the connection between the receiver and the flight control. There are many receivers with PPM signal output, which are necessary for the entry of drones. The advantage is the stable transmission of multiple channels, the disadvantage is that it is slower.

S.BUS: A single-channel digital transmission protocol designed by the remote control manufacturer FUTABA. There is only one signal pin and one ground wire. It supports HUB expansion of multiple servos and ESCs connected to a signal source, so S.BUS is actually a signal source. It is a kind of bus, and its principle is actually a changed serial port protocol. Its advantage is pure digital signal, very reliable, with bus function. The defect is that there are fewer technical compatible devices belonging to the manufacturer. At present, it is the best choice for the flight control to connect to the receiver. It is said that foreign users use PPM receivers more, and domestic users use SBUS receivers more. This may be related to the extensive use of FUTABA remote controllers in China.

Serial port: Serial port is the most common device interface in the current control field, and the hardware forms include TTL, 232, 422, and 485. TTL is the basic signal, and three pins are commonly used. One pin is used for signal input called RX, the other pin is used for signal output called TX, and the other pin is the signal reference ground (two TTL interface devices must be connected to the same ground!). Generally, 0 volts and 3-5 volts represent 0 and 1. The flight control chip comes with this kind of serial port, and it will bring many of them to connect multiple devices (such as GPS module, data transmission, WIFI module, etc.) , There are 5 PIXHAWK flight controllers. TTL signals have lower voltages, experience

It is not suitable for long-distance transmission of more than 1 meter, so the 232 interface was developed, which uses positive and negative levels to indicate 0 and 1. Others are the same as TTL, which greatly extends the transmission distance, but the speed is still insufficient. Therefore, the 422 interface was developed. Each pin of RX and TX becomes a pair of signal lines with positive and negative voltages jumping at the same time, so that the interference signal is cleverly offset, and the transmission distance and speed are double leaps, but 5 lines are needed for debugging. cause a lot of trouble. The 485 serial port is half-duplex, which combines the advantages of 232 and 422. It uses a pair of positive and negative voltage signal lines to send and receive, but an additional signal is required to control the transceiver.

Conversion, the 485 bus has bus characteristics, and can connect multiple devices on two wires and the ground wire, but the work of transceiver, chip selection, arbitration, verification, etc. requires you to write programs to coordinate, and the workload is heavy.

SPI: This is a high-speed interface for on-board communication. It uses a master-slave design and a dedicated clock line. Each SPI has 4 pins: master-in-slave-out MISO, master-out-slave-in MOSI, clock CLK and ground wire . The master device is responsible for managing information, clock synchronization and communication with all devices. One SPI can connect multiple devices, but each must have a chip select. Because of the clock line, it is easy to achieve synchronous communication of all devices. Due to its stability and high-speed characteristics, it is used on the flight control board to connect all sensors and the main microcontroller.

I2C: is the bus used to connect high-speed devices on the board. Has three signal wires, signal, clock and ground. It is also a master-slave design with a synchronous design of the clock, but the signal line needs to undertake the input and output of multiple devices, which is managed by the master device. All devices on I2C come with an address, or label, that the master device uses to identify the device. In flight control, it is used to connect many devices that are not of high importance, such as indicator lights, magnetic compass, airspeed, ultrasonic wave, laser ranging and so on. There are actually manufacturers who use this bus to connect multiple ESCs in order to save trouble. This is very dangerous, because the 3-5v level, It is susceptible to interference in long-distance transmission.

CAN: The earliest bus interface designed for automobiles. It transmits differential signals and has excellent anti-interference performance. It is specially used for multiple long-distance transmissions between devices in large interference environments. There are only two signal lines, H and L, on which all devices are connected, and the bus chip is responsible for arbitration. This is actually the equipment on the drone, and it is the best choice for the ESC of the multi-rotor drone. At that time, the CAN interface is more complicated, and the cost of the CAN transceiver chip is not low. PIXHAWK
Flight controllers and ESC32 ESCs have been equipped with this interface for many years, and the CAN interface is also used on DJI flight controllers. It is believed that there will be more and more flight controllers using CAN interface in the future.

USB: Civil bus interface, can connect multiple devices through HUB, it can be said to be a perfect interface. However, the protocol is too complicated, the programming workload is huge, and the interface connection form is easy to cause problems. Special wire must be used, and it cannot exceed 2 meters. On the flight control, it is mainly used for ground debugging, reading and writing parameters and other ground operations.

Relay: It is a GPIO signal with only two states of 0 and 1. It exists in some pins of the flight control to automatically control the camera shutter and pesticide nozzle. The advantage is that it is reliable, but the disadvantage is that the amount of information is extremely small, resources are wasted, and there is no verification and other functions.

AD: Analog-to-digital conversion interface, the way of use is to measure voltage. At present, the flight controller is used to measure voltage and current. This method has a low cost and can also be used on small drones. Other distance detection devices are still in use, such as airspeed and ultrasonics.

SDIO: Used to connect SD card or TF card for flight data recording. Due to the common application of TF card, low cost and satisfactory capacity and speed, it has gradually become an essential equipment for advanced flight control, used for flight recording, accident analysis, fault diagnosis and so on.

HDMI: High-Definition Multimedia Interface, is a digital video/audio interface technology. It is a dedicated digital interface suitable for image transmission. It can transmit audio and video signals at the same time. The maximum data transmission speed is 5Gbps. Perform digital/analog or analog/digital conversion. Features of HDMI interface:
(1) It can transmit uncompressed audio signals and high-resolution video signals, digital signals, and high quality.
(2) Increase the data transmission bandwidth up to 5Gbps.
(3) It can support 1080P video (the latest version 2.0b, up to 8K).
(4) Theoretical 20m, the actual is generally about 3m.
(5) Simultaneous transmission of audio, video, and copyright protection is very popular in the field of consumer electronics.
(6) HDMI is an external interface, and its ability to improve video resolution and color depth is limited.
(7) HDMI compatibility is not good. At present, the HDMI interface is widely used in image transmission and cameras. It is very important to choose a high-quality HDMI adapter cable.