Servo - July 2008

Figure 2. Bi-Phase encoding.

but there the similarity ends. The packet format looks like that in Packet 2. By ~Device ID and ~Command, I mean that it is inverted, or 1’s complement of its respective Device ID and Command. To check for a proper reception, all you need to do is AND the byte with its respective complement; if it comes up all zeros, then you have a good reception. When you hold down a button on the NEC remote, it does not send out the same command over and over; it sends out a special signal called the Repeat Code.

The NEC protocol has four timing values that we care about, and one we don’t (the Lead Out). Table 2 shows the ones that we pay attention to.

Phillips RC- 5 Format

Some useful information on the Phillips RC- 5 format

Figure 3. RC- 5 packet.

can be found here:

www.hifi-remote.com/infrared/IR-bi-phase.shtml

and here:

http://home1.stofanet.dk/hvaba/fprc5rx/index.html

The last of the big three formats that I’m going to talk about is the Phillips RC- 5 protocol. Rather than using PPM encoding like SONY and NEC, the RC- 5 format uses Bi-Phase encoding. This means that the transition from a logic ‘1’ to a logic ‘0’ and vice-versa is shown by a change in the bit phase. If you are like most of us, your eyes just glazed over at that description. This is one time that a picture is pretty much needed to explain what I mean (see Figure 2).

Note that it isn’t the timing that shows what the bit is, but rather the phase of the timing. But how — you ask — do you know what a ‘1’ is and what a ‘0’ is? You know because the bit stream of RC- 5 starts out with two 1 bits and then a toggle bit, which on the first press of the button is a 0. Figure 3 shows how a transmission is formatted. An RC- 5 packet consists of the preamble of 1 1 0, then a five-bit address and then a six-bit control. RC- 5 packets are encoded with the MSB (Most Significant Bit) first. Because you know that the first bit is a 1, then any bit transition from that point onward you can track to know what the current bit is — a 1 or a 0.

Figure 3 may look a little odd because I put both same and switch phases in every bit cell except for the start bits. Just to make it more confusing though, I’ve read that the start bits can be either 1 1 or 1 0. I hope not; it’d make it hard to figure out the 1 startup. The RC- 5 protocol will repeat the button press every 113.8 ms, but every packet after the first one will have the Toggle bit toggled differently than the last start bit.