MCP2551 CAN Bus Signal Quality Issues at 125kbps With Peugeot 308 Unit

Hi All

I am connected to a CAN 125kbps network via MCP2551 microchip transistor but signal looks pretty bad as in the attached picture (slow fall + overshoot + ringing)

I am looking for ways to improve it

The network is basically two nodes, one is my board and the other is a peugeot 308 unit (disconnected from the rest of the car).
The Peugeot unit communicates well with my board with no errors but non the less it looks bad and i would like to improve
Original netwrok does not have terminating resistors at all and when i try to connect (even high resistance) bus is completly disabled (Peugeot unit not responding)

Thanks &
Best Regards
Job

Hi Job

Can you show us how you connected your circuit? Also if I may ask, what are we exactly looking for? What is this graph showing?

Thanks,
Kevin

Hi Kevin

2 different circuits
1. Black box, peugeot parking assistance - 12V power input + CAN bus
2. My PCB - 12V input + CAN using MCP2551 microchip transceiver + dspic30f5011 microcontroller

Two circuits connected to the same 12V power source (stable) and together only (apart from the rest of the car network) via CAN and the picture is showing osciloscope output while probe is connected to CANL + CANH (differential)

As you can see the signal is not square as it should i.e. slow drop and rising is with slight overshoot and ringing

Questions is: what can i try to improve it?

As said original CAN network does not have terminating resistors + wires are very short and baud rate is only 125kbps. Also adding even a 50kohm resistor (not to mention recommended 120ohm) between CANH & CANL result in complete break down of the bus i.e. signal is almost streight

Thanks again
Job