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The X-10 and IR Node

This node actually combines four functions in one very busy box:

I call it my node0 because it is the first and probably most important box in my system. Each function is actually a logically seperate software module. In this manner, it is possible to split it away from this physical node, and put it into another one. All messages are sent equally to each module below.

The serial interface

The PIC16C74 has one hardware UART. This is assigned to the 485 port on this box. A software UART is implemented and connected to the 232 port. All characters are echoed from one port to the other, and then used internally to decode network messages.

The LCD panel

I use a 2x16 line LCD panel from EPSON. This uses the industry standard Sharp 44780 Controller, and code examples exists in several places. It was difficult at first getting the timing right, but the result have been successful. The network address of the LCD module is 'L'. The 'data' field of the network message will be displayed in the LCD window, allowing any node to display a message on the window. For example, sending the following message over the network:

LC_Hello[ret]

This displays "Hello" on the LCD screen, then the LCD sends an ACK back.
By the way, the address of the sender of the message above is 'C',
which is the central controller or PC.

The Infra Red interface

The PIC controls a oscillator and driver that flashes an IR led via a jack on the box's rear panel. There is also a Sharp IR cube that demodulates the raw IR signal to TTL level pulses. The software module has been written to send and receive the Sony SIRCS protocol.  The IR module's address is 'I', and it can accept commands or requests for sampled IR data. For example, the network command:
IC_QQ[ret]

This causes the software module to respond with the most recently
sampled IR command. The network command:
IC_[1][17][ret]
Causes the module to send a function code 17 to device code 1, and then
return an ACK. This command turns up the volume on my TV.

Sony protocol
Scope shot of Sony TV "Volume Up" command.

Analysis of several protocol formats shows a pattern that is very common.  In the section below 'high' means presence of light (modulated to 38 - 40 kHz), and 'low' means no light.  A '1' means a logic high for the transmitted data, and '0' is a logic low.
  • First, the packet starts with a long high period and a low period.  This is what I call the start bit.  
  • Then follows the data bits.  A '1' is a long period (high or low), while a '0' is a short period.
    • For example, in the Sony scope shot above, the 'low' periods are all the same and the data is transmitted via the high periods.  A long high period is a '1', and short high is a '0'.
    • Some manufacturers do the opposite, such as for example from my ViewEra TV.  Here, the data is transmitted during the low periods as they have different lengths.  The highs are all the same duration.
  • The data bits are then sent serially.  For example, for the Sony command above, there are 12 bits.  For the ViewEra example, there are 32 bits.  For the Sony command above, the bits are "010010010000".  From others we know (see link below) that the bits need to be reversed to understand the pattern, and that produces: 00001 0010010.  The first five bits are the device code (1 is the Sony TV), and the rest (18 decimal) is the button code, which is "Volume Up".
It seems possible to develop a PIC based IR decoder that will take any command and reduce it to about a dozen bytes that has all these decoded (not just a raw sample). 

Useful links:
  • Sony SIRCS protocol
  • A really big depository of IR data

    • The X-10 interface

    This software module is connected to an X-10 TW523 module; its address is 'X'. For example, the following network command:

    XC_QQ[ret]
    May cause the software module to return:

    CX_A1[ret]
    This means that the oldest received X-10 command is A-1. The software
    module buffers up 30 X-10 commands in a fifo, sending the oldest one first.
    This way the main controller can query the X-10 interface at its
    convenience, and messages will not be lost.


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