Monday, May 09, 2005

Vik's found a rather neat PIC project that does RS232 with 2 resistors and a diode. It might also make a good basis for a front control panel for the RepRap machine.

P.S. (12-V-2005): Tried the simple circuit on the Tx and Rx pins of an old 16F73 at a miserable 4800 baud. Didn't work, so back to the good old MAX232.

Nice design. There are some PICs that have built-in uarts that would make the serial terminal code much simpler, though. I've used the 16f876 and 16f877 before, both of which have uarts. Not much more expensive than the 16f84 and they have a lot more features. More I/O pins too. There's some uart code here that works with these devices. It's written for c2c but should be adaptable to picc.
The most likely reason that the 16F73 test didn't work is that the on-board uart doesn't have a way to invert the TX and RX signals. Bob Blick's example inverts the signals programmatically, neatly avoiding the need for an external inverter. There are some interesting alternatives on the PicList site, some inverting, some not. Me, I prefer a MAX233.
Yes - but it would be nice to do it with just 2 resistors and a diode. The +/- nonsense of the RS232 has got to be the most tedious legacy albatross that computing is saddled with (if one can be saddled with an albatross)...
Thanks, Steve. I'll be looking at the UART code later. Meanwhiles TI have kindly sent me a few sample SOIC MAX232s and ECONZ have donated some unwanted SOIC components to the cause including some handly E2PROM I2C chips. I'll start looking into methods of developing a PCB that takes SOICs when I've got my side of the X-Y table sorted out.
I note the MAX233 requires no external capacitors. Might make assembly easier and saves on baord space.

Vik :v)
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