Simple USB to RS232 adapter on pic18f2455
This adapter allows you to interface with your car’s OBD-II bus. It provides you a USB interface using the ELM327 command set and supports all major OBD-II standards:
- SAE J1850 PWM
- SAE J1850 VPW
- ISO 9141-2
- ISO 14230-4
- ISO 15765-4 CAN
You do not have to be familiar with all the details of OBD-II protocol as the adapter and PC desktop software will do it for you. The adapter kit schematic is shown here. It is built around Microchip’s PIC18F2455 microprocessor with USB support. The ISO9141/14230 interface is implemented on MCP2021-500, the J1850 VPW output bus is controlled by MCP1407 driver and the pair of P-channel and N-channel MOSFETs (Q1, Q2) is driving PWM bus and PIC18F2455 internal comparator together with resistors R4, R5 producing the differential PWM input. The circuit is powered from USB bus. Transistor Q3 is for legacy L-line initialization in ISO9141 protocol. The adapter has two connectors on the board DB9-M and USB type B connector. The board fits SERPAC A20 enclosure, Mouser part reference 635-A-20-A or DigiKey part SRA20A-ND. For ordering kit see the Products page. The kit is including the PCB, preprogrammed PIC chip and all the components required to assemble the device.
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