USB Digital Setting Circles

The serial version of the DSC circuit described in these pages is readily adapted to use with a USB-serial conversion cable. In fact, FTDI makes the TTL-232R-5V cable that converts TTL-level serial data coming straight from the PIC into USB for the PC. Using this cable allows us to eliminate the MAX232 chip (and several capacitors) used in the ser
USB Digital Setting Circles - schematic

ial circuit that converts TTL to RS-232 and back. At also allows us to eliminate the external power supply and the components making up the voltage regulator for the board. Instead, we draw power from the USB port itself. While updating the original circuit for USB, it made sense to make a couple of other changes. Most noticeable is the elimination of the pull-up resistors on the unused PIC I/O pins. Less obvious is the change from the four-pin oscillator to a simple crystal for clocking the PIC. Finally, the original PIC16F84 was replaced by an equivalent but cheaper part, the PIC16F628 (requiring updated hex code ). And, of course, there`s no longer a DB9 connector on the board for a serial port. Instead, 6-pin connector JP4 is the connection point for the TTL-232R-5V cable. The result is that the USB version of the circuit is considerably simpler than the serial version. FAR Circuits sells a kit for this circuit that includes a PC board and all board-mounted parts and connectors, but not the TTL-232R-5V cable. You`ll need to purchase the TTL-232R-5V cable separately. Both Mouser and DigiKey sell this cable for about $20 plus shipping. Go here for information on how to order the kit from FAR Circuits.

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