In my [in]experience, 99% of serial communication errors are due to timing mismatches; the other* 99% is due to wiring hoopla.
- It works for short wire but not a 30ft CAT-5 cable
- It works for normal serial wire upto 10ft but not more than that.
CAT-5 cable is twisted pair (also, all of what I have used is also shielded, but apparently this is not common); I assume the "normal serial wire" is two loose wires. As wiring gets longer, your nice digital rise times stretch out, effectively shrinking the available window for sampling incoming data. Twisted pair helps this a bit. What this really means is that your UART clock must be prescaled more accurately to the baud rate being used. System clocks for perfect USART communications should be multiples of 1.8432MHz. Short-range communications can tolerate a few percentage error, but as your sampling window shrinks it must be more accurate. What your prescaler (int), and baud error?
prescaler = (int)(F_CPU / (USART_baudrate * 16) - 1)
actual_baudrate = F_CPU / (prescaler * 16)
baud_error = 100% * (1 - actual_baudrate / USART_baudrate)
- It works for 6V battery but not for 12 volt battery or a 12V adapter.
This sounds fishy! This may be a completely different problem, perhaps causing your widget to reset due to a dirty supply or poor regulation. There are stability issues with LDO linear regulators and varying capacitive loads; there may be dissipation issues from a 12V supply; if one isn't using a LDO regulator, then a 6V battery will really be giving you 4.5V to 5V, and won't be well regulated. Power supply issues are in a class all their own, and must be rectified before peripherals can be effectively debugged.
Other than this, there is always the issue with environmental interference, or EMI. RS-485 is made for this, but should be used with twisted pair[, shielded wire,] like the CAT-5 cable you've used.
**(What, my math is wrong? Do you know who I am!? ;)*
It sounds like what you want is what is known as Bit Banging. This is where you use one or more general purpose IO lines to emulate something like I²C, SPI, RS232 or whatever else you like, purely using software.
While this gives you the ultimate in flexibility, it does tend to tie up the microcontroller while any transfers are going on.
If speed isn't too much of an issue you could tie the transfer of data to a timer interrupt - transferring 1 bit every 10µS or something.
So, transferring a bit with an active low clock would go something like:
- Drive signal line to level of bit to transmit (high or low - 1 or 0)
- Drive clock line low
- Delay
- Drive clock line high
- Delay
- Progress on to next bit until done.
Best Answer
Use a higher transmission voltage (I've seen 12V used successfully with a very similar half-duplex bus in a commercial product with an operating range of several hundred metres) and smaller pullup resistors (2K2-4K7). Also, don't expect high data rates; I'd suggest 4.8kbps as a maximum (the commercial product uses 4kbps).