OBD-II ELM327 compatible AllPro adapter using 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.

AllPro adapter.JPG

The OBD-II Cable for kit

The adapter uses 9 pin D type female connector to link up to vehicle’s OBD-II J1962 connector. The pinout was chosen to match many of the commercially available cables. The pin connections required of the cable are as follows:

J1962 Pin Description DB9F Pin
2 J1850 Bus + 7
5 Signal Ground 1
6 CAN High 3
7 K Line 4
10 J1850 Bus – 6
14 CAN Low 5
15 L Line 8
16 Vehicle Battery Positive 9
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The Software

Here is the list the software I have tested so far with adapter:


For more detail: OBD-II ELM327 compatible AllPro adapter using PIC18F2455

Current Project / Post can also be found using:

  • pic18f25k80 elm327 schematic
  • elm327 pic18f hex
  • elm327 project
  • obd pic hex

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