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Universal Controllers

IR Control

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 late VCR
produced by Sanyo and was marketed under the name of Fisher. NEC
manufactured the remote control IC.
This description was taken from
the VCR's service manual. Those were the days, when service manuals were
fulled with useful information!

Features

8 bit address and 8 bit command length

Address and command are transmitted twice for reliability

Pulse distance modulation

Carrier frequency of 38kHz

Bit time of 1.125ms or 2.25ms

Modulation

The NEC protocol uses 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 half of that, being
1.125ms. 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 is transmitted first. In this case Address $59 and
Command $16 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.

Extended NEC protocol

The NEC protocol is so widely used that soon all possible addresses
were used up. By sacrificing the address redundancy the address range was
extended from 256 possible values to approximately 65000 different values.
This way the address range was extended from 8 bits to 16 bits without
changing any other property of the protocol.
By extending the address
range this way the total message time is no longer constant. It now
depends on the total number of 1's and 0's in the message. If you want to
keep the total message time constant you'll have to make sure the number
1's in the address field is 8 (it automatically means that the number of
0's is also 8). This will reduce the maximum number of different addresses
to just about 13000.

The command redundancy is still preserved. Therefore each address can
still handle 256 different commands.

Keep in mind that 256 address values of the extended protocol are
invalid because they are in fact normal NEC protocol addresses. Whenever
the low byte is the exact inverse of the high byte it is not a valid
extended address.

External Links

NEC
Electronics

Example Commands

The table below lists the messages sent by the remote control of my
late Fisher 530 VCR (it served us well during its 20 years long life).

NEC Message

Key Function

$68-$00

Play

$68-$01

Rec

$68-$02

Audio Dub

$68-$03

Frame Adv

$68-$04

Slow

$68-$05

Quick

$68-$06

Cue

$68-$07

Review

$68-$08

FF

$68-$09

Rew

$68-$0A

Stop

$68-$0B

Pause/Still

$68-$0C

Up key

$68-$1E

Down key

Last Updated: 02 January 2013
© 2001, San Bergmans, Oisterwijk, The
Netherlands
http://www.sbprojects.com/