Why IrDA is not suitable for receiving IR remote control commands. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / infrared technology

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Well, actually, you can certainly use IrDA to receive commands from conventional IR remotes, but with very large restrictions. Doesn't work with all remotes. Command recognition stability is very low. If you use IrDA built into the motherboard, then you need to conjure with the drivers, if external, then you need to remove the drivers or periodically plug the receiver into another COM port. USB IrDA cannot be used at all, since it cannot be accessed directly as a COM port (do not confuse it with a virtual COM port).

By popular demand, a plugin for COM IrDA was written, for more information about all the limitations and difficulties of use, see slydiman.narod.ru/scr/plugins/ir210.htm

And now in more detail

Through IrDA, data is transmitted in the same way as through the COM port with minor differences. The presence of a pulse is a logical 0, the duration of the pulse is 3/16 bit time. Typically 8 bit mode is used, no parity and 1 stop bit. The first pulse is considered as a start pulse, then, depending on the selected transmission rate (usually 115200), the presence or absence of a pulse at a given time determines the value of the next bit (0 or 1). A byte is considered successfully received if the stop bit is received correctly, i.e. if at the right moment there is no impulse. The picture shows the signal when transmitting data through the COM port (UART) and through IrDA.

You can access IrDA like a regular COM port only if the device is connected to the COM port or to the IrDA connector on the motherboard. In the second case, you will have to edit the INF files with your hands so that Windows does not guess that this is an infrared port. Using, for example, a USB IrDA device to work with a remote control will not work at all.

Most importantly, each information pulse sent from a remote control is, in fact, an IR background of a given duration with a frequency of 30 to 56 kHz.

Theory

Let's say that everything is in order with the stop bit, then everything will be as in the picture ( BUT ). An IR background appeared, after 86.8 µs (at a speed of 115200) the first byte was received, an RX CHAR EVENT event was generated. After waiting for the end of the packet reception, we count the number of bytes and the number of single least significant bits in the last byte, thus we find out the pulse duration (T2) with an accuracy of 9 μs. After waiting for the next RX CHAR EVENT and stopping between them, we will find out T1. Subtracting T2 from T1, we find out the duration of the pause. It would seem that there is enough information to decode the command (the durations of pulses and pauses between them are known), but ...

Practice

If at the time of reading the stop bit in the IR background there is a pulse, the byte will not be accepted. see picture (B). Thus, in the event of incorrect reception of one or more bytes, an RX CHAR EVENT may occur at point (1), (2), or (3). Moreover, RX CHAR EVENT may occur several times during one information pulse from the remote control, for example, at points (1) and (3). The most likely is the error-free reception of a byte that overlaps the end of the information pulse from the remote control (no pulse will get to the stop bit).

All this applies to the case when the DCD plugin, slydiman.narod.ru/scr/plugins/dcd.htm or IR210, slydiman.narod.ru/scr/plugins/ir210.htm is used to analyze a command from a remote location (the principle of operation is similar to WinLIRC ). The UIR plugin, slydiman.narod.ru/scr/plugins/uir.htm, has no idea about the encodings of various remotes at all, it stupidly analyzes the sequence of received bytes. The probability that for the same remote control command IrDA will give the same bytes is very small. The result, for example, may vary depending on the distance between the IrDA receiver and the remote control. Conversely, the same bytes can be received for different console commands.

Conclusion: at a certain frequency of the IR background (i.e. with a certain model of remote control) with a big stretch, IrDA can be used to receive remote control commands with pulse-distance modulation and with relatively short pulses, while focusing on the time between RX CHAR EVENT (DCD plugin). IrDA cannot be used to receive IR commands from remotes with a different type of modulation, as well as if the commands contain a long first information pulse, which is quite common.

Generating IR remote control commands via IrDA

Here the situation is slightly better. If you know the exact command format for a given remote, you can form several packets and send them through IrDA after a given time. In this case, you need to use a baud rate of 38400 (the closest to the frequency of most remotes). You will get a signal very close to the original, but it will not be perfect. Every 9 pulses there will be a dip (stop bit). In addition, the duty cycle will be approximately 19% (should be 50%). The receiving equipment may perceive the stop bit as a short pause between information pulses and incorrectly decode commands.

Publication: cxem.net

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