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| NEC Protocol |
To my knowledge the protocol I describe here was developed by NEC. I've
seen very similar protocol descriptions on the internet, and there the
protocol is called Japanese Format.
I do admit that I don't know
exactly who developed it. What I do know is that it is used in my ancient
VCR produced by Sharp and was marketed under the name of Fisher. NEC
manufactured the remote control IC.
This description was taken from
the VCR's service manual.
| Features |
- 8 bit address and 8 bit command length
- Complete address and command are transmitted twice for reliability
- Pulse distance modulation
- Carrier frequency of 38kHz
- Bit time of 1.12ms or 2.25ms
| Modulation |
The NEC protocol uses a pulse distance
encoding of the bits. Each pulse is a 560µs long 38kHz carrier burst
(about 21 cycles). A logical "1" takes 2.25ms to transmit, while a logical
"0" is only 1.12ms. The recommended carrier duty-cycle is 1/4 or 1/3.
| Protocol |
The picture above shows a
typical pulse train of the NEC protocol. With this protocol the LSB
transmitted first. In this case Address $16 and Command $59 is
transmitted. A message is started by a 9ms AGC burst, which was used to
set the gain of the earlier IR receivers. This AGC burst is then followed
by a 4.5ms space, which is then followed by the Address and Command.
Address and Command are transmitted twice. The second time all bits are
inverted and can be used for verification of the received message. The
total transmission time is constant because every bit is repeated with its
inverted length. If you're not interested in this reliability you can
ignore the inverted values, or you can expand the Address and Command to
16 bits each! 
A command is transmitted only
once, even when the key on the remote control remains pressed. Every 110ms
a repeat code is transmitted for as long as the key remains down. This
repeat code is simply a 9ms AGC pulse followed by a 2.25ms space and a
560µs burst. 
| Example Commands |
The table below lists the messages sent by the remote control of my senior Fisher 530 VCR.
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http://www.sbprojects.com/