ASCII 255 inserted in response after powerup

[Joel Moore]

I've noticed recently that the first time I communicate with a T100MD888+ controller after it's been powered up, the response is prepended by an ASCII 255 character.

Has this always been the case and my old communications code was just able to ignore it or is this something new?

And is it intentional?  Can I rely on it to detect a powerup condition?

[support]

Is the other device powered up before the PLC? When power is applied to the PLC there may be transition of signal on the TX line which is more like a noise. The fact that you receive a 0xFF which means the Tx line goes to high and stays there so this is most likely a result of noise. I don't think you can rely upon it to detect a power on condition.

[Joel Moore]

Well, it's not like it arrives on its own when I power up the Triplc.  It only see it when I first attempt to communicate with the controller.  But maybe it's sitting in one of the buffers. I'll do some experimenting to see if that's the case.