PS/2 Keyboard IR Receiver
Project overview
A few years ago, someone gave me a small wireless infra-red keyboard, part no. RC-KB2, designed
to be used with a Kenwood audio system. As soon as I saw it, I wanted to discover it's transmission
format. Why? Because I would then be able to build a small receiver which would allow me
to use this keyboard on a PC with a PS/2 port.
Two years later, I finally found (or took?) the time to do it. This page's purpose is to
document the project. Maybe someone else would like to use a keboard like this one with their PC?
In any case, the receiving part of the code could be rewritten for other keyboards or even
regular infra-red remotes.
Pictures
Pictures of the Kenwood RC-KB2 keyboard:
Infra-red led
The keyboard
The keyboard again.
Pictures I took during development:
Development setup
Infra-red receive close-up. Part no: tsop1238
Pictures of the receiver I built:
Inside the enclosure
Done!
Technical Details
When the keyboard emits infra-red, it is not continuously. In fact, it modulates the infra-reds at a frequency between 35 and 38 Khz. I did not measure the exact frequency, but I assume it is in the 35-38 khz range because an infra-red receiver salvaged from a VHS Vcr receives the signal correctly at a pretty good distance.
After looking at the receiver's output with a digital oscilloscope, I understood how the data was transmitted. As my infra-red receiver had an active low output, a low level in the following diagrams indicates that an infrared wave was present.
When the keyboard is sending a key press event, it starts by generating the following signal which I called 'Attention':
4.5ms
----+ +----
|________|
9.1ms
Right after the attention signal, the keyboard sends the data bits. The time elapsed
between active pulses (560us) varies depending on the bit value being transmitted (1 or 0). A long time
(approx. 1640us) represents a 1 and a short time (approx. 560us) represents a 0.
-----+ +-+ +-----+ +-----+ +-+ +--...
|_| |_| |_| |_| |_|
0 1 1 0 ...
When a key is held down, the keyboard sends a short signal which I called 'repeat':
2.2ms
----+ +--+ +----
|_________| |_|
9.1ms ^
600us
The keyboard sends 32 data bits, in other words, 4 bytes. Here is the meaning of each byte:
| Byte | Description |
|---|---|
| First | Peripheral Identification. Constant value. |
| Second | Keycode high byte. |
| Third | Keycode low byte. |
| Fourth | Bitwise NOT of the keycode low byte. Adding the third and fourth bytes will always result in 255. Useful for error detection. |
keycodes_rc-kb2.txt
Keymap
The keyboard
The biggest usability problem I faced with this keyboard is the fact that there are only a limited number of possible key combinations. And when a combination works, for instance shift + a, a different keycode is sent. This is very different from a PC keyboard which would transmit the following sequence instead: 'Shift key pressed, A key pressed'...
Here is an overview of the possible key combinations. See keycodes_rc-kb2.txt for the full list:
- Shift keys may be combined with all letters keys, numbers keys and the following character keys: ;',./-+
- Room B keys may be combined with a limited set of keys, typically application specific keys such as: Confirm, Repeat, Random, Display, +10, +100, les chiffres 0-9, set, disc skip down, disc skip up and the stop/play/prev/next/rew/ff keys.
- After the CONT PLAY key has been pressed, the next key I send to the PC is preceeded by the CTRL pressed event and followed by a CTRL released event.
- After the Sub Out Monit. key has been pressed, the next key I send to the PC is preceeded by the ALT pressed event and followed by a ALT released event.
- It is possible to press CONT PLAY followed by Sub Out Monit. to obtain a CTRL + ALT + somekey event.
Here is a table which describes which keyboard keys generate which standard PC keyboard key events. Obvious keys, such as numbers, letters and ponctuation are omitted from this table. Of course, some keys are missing. But I dont plan to use this keyboard for programming or playing games so it should be fine. Anyway, if I ever need other keys, I'll be able to implement them.
| Kenwood key | PS/2 Key | Comments |
|---|---|---|
| Power | ESC | |
| Confirm to +100 | F1 to F9 | |
| Delete | Backspace | The delete key is located where the backspace key normally is. |
| Disc.Sel. | Tab | |
| P.mode | \ | |
| Check | insert | |
| Clear | Delete | |
| Set | Home | |
| Mode | End | |
| Cursor L | Left arrow | Absolutely necessary! |
| Cursor R | Right arrow | |
| Charac. Srch. Down | Down arrow | |
| Charac. Srch. Up | Up arrow | |
| Disk Skip Down | Page Down | |
| Disk Skip Up | Page Up | |
| Room B + Disc skip Down | Shift + Page down | Useful for scrolling in xterms and the Linux console |
| Room B + Disc skip Up | Shift + Page up | |
| Cont Play | Ctrl + ???? | Magic key to send the next key between CTRL down and CTRL up events. |
| Sub Out Monit. | Alt + ???? | Matic key to send the next key between ALT down and ALT up events. |
Firmware
Here's the hex file which must be programmed into the Atmega8: ir2ps2.hexFuses bytes for this project: high byte = 0xc9, low byte = 0x9f
For details about how to program an AVR, visit my AVR programming page.
Source code:
The source code is available under the terms of the GPL license. Consult the COPYING file inside the tarball for more information.
ir2ps2-1.0.tar.gz
References
The PS/2 Keyboard Interface: http://www.computer-engineering.org/ps2keyboard/Interfacing the AT keyboard: http://www.beyondlogic.org/keyboard/keybrd.htm
Petr Simandl - PC keyboard communication protocol: http://www.simandl.cz/stranky/elektro/keyboard/keyboard_a.htm
Disclaimer
I cannot be held responsible for any damages that could occur to you or your equipment while following the procedures present on this page. Also, I GIVE ABSOLUTELY NO WARRANTY on the correctness and usability of the informations on this page. Please note, however, that the procedures above have worked in my case without any damages or problems.Now you cannot say that I did not warn you :)