RS485 has some ?noise? in car

Hello forum. i am in the process of building an RS-485 , full duplex (4 wires) , Bus , in my car.
it is going to have one Master, and multiple (4) slaves. in the meantime it consists of a single master and slave.

at any given time, the "output enable" in the TX line of the slave, is driven low.
The master "asks" for data from the slave, and then the slave drives the "Output enable" of the RS-485 chip (SP485E) high.
it waits 10 mS and then sends 14 characters (32bit each). after that, it waits another 10 mS and drives the output enable LOW.

so, when the car is not working, or at iddle, everything is working fine. but as soon as the RPM rise , the master starts receiving 15 chars, instead of 14.
the extra char is always on the beginning , and is the "-48" (in integer).

in the meantime, i have not set any special filter, in the power supply of the Slave. i am using a simple switching DC-DC converter, (designed for phone charging). So my first guess is that the power line has a lot of noise, when the RPM rise.

any suggestions, as what would be causing the problem and/or how to solve it??

thanks!

RS-485 should be using differential pairs for noise immunity. Are you using proper twisted wire cable for your signal drop?

What voltages are on the two wires for idle, a signal of 0 and a signal of 1? How do they change when the engine is running? (That is 12 separate voltages to measure).

I suspect that you haven't got the correct resistors to pull the signals to the correct idle voltage, so that the start and end of transmission aren't correctly detected.

Diver300, this was the reason. i had not installed any failsafe resistors at all, so when no one was driving the bus, the voltage diference some times would be translated as "1" and hence a start bit... but now its fixed! thanks !