Summary of Simple USB to RS232 adapter on pic18f2455
This article describes a DIY OBD-II adapter kit that interfaces with a car's OBD-II bus via USB using the ELM327 command set. It supports major standards including SAE J1850 PWM/VPW, ISO 9141-2, ISO 14230-4, and ISO 15765-4 CAN. The device is built around a PIC18F2455 microcontroller and includes various interface chips and transistors to handle different communication protocols.
Parts used in the OBD-II Adapter Kit:
- PIC18F2455 microprocessor
- MCP2021-500 for ISO9141/14230 interface
- MCP1407 driver for J1850 VPW output bus
- P-channel MOSFET (Q1)
- N-channel MOSFET (Q2)
- Resistors R4 and R5
- Transistor Q3 for legacy L-line initialization
- DB9-M connector
- USB type B connector
- SERPAC A20 enclosure
- Preprogrammed PIC chip
- PCB
Overview
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
Hardware
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.
For more Detail: Simple USB to RS232 adapter on pic18f2455
-
What communication standards does this adapter support?
The adapter supports SAE J1850 PWM, SAE J1850 VPW, ISO 9141-2, ISO 14230-4, and ISO 15765-4 CAN. -
How is the circuit powered?
The circuit is powered directly from the USB bus. -
Which microprocessor is the adapter built around?
The adapter is built around Microchip’s PIC18F2455 microprocessor which has USB support. -
What component handles the ISO9141/14230 interface?
The MCP2021-500 implements the ISO9141/14230 interface. -
What is the function of Transistor Q3?
Transistor Q3 is used for legacy L-line initialization in the ISO9141 protocol. -
Can I use this adapter without knowing OBD-II protocol details?
No, you do not have to be familiar with all the details as the adapter and PC desktop software handle them for you. -
What connectors are found on the board?
The board features a DB9-M connector and a USB type B connector. -
Which enclosure fits the board?
The board fits the SERPAC A20 enclosure.
