The circuit make good use of the on-chip comparator in a single 12F675 PIC to detect the low level data and clock signals from the scales.
The firmware reads both 24 bit binary and 7 digit BCD output scales types, can switch the scale to fast or slow reading, zero the scale display and outputs the data in an easily readable format at 115,200 Baud.
Circuit description:
The Vcc(+1v5) , Clock and Data signals from the scale are connected to the PIC via 3k3 current limiting resisters, red LEDs are used to limit the voltage to about 1.2 volt.
The circuit connects to a PC comport, stealing power from the RTS output and a negative bias from the DTR output. A series diode and 470R resister with a 4v7 zener is used to limit the power supply to less than 5v for the PIC. A universal PNP transistor inverts the serial output from the PIC its collector is biased negative from the PC’s DTR output.
Note:
The scale’s Vcc (+1.5 volt) is connected to the vernier/scale body so care should be taken not to short circuit the PC ground to the vernier body.
