I'm trying to interface an Arduino Mega2560 to a Modbus RTU device using an SN75176B transceiver, on a solderless breadboard at this stage. When I'm driving DE high and writing serial data out of the Arduino, the highs are nice and clean but the lows are full of 3MHz noise. It looks like this:
The blue line is the arduino output, the yellow is the transceiver output (A to B). The arduino's UART is running at 9600 baud. Obviously the device connected has trouble understanding this.
According to the data sheet, all this device needs is a bypass capacitor between VCC and GND. Admittedly the one I'm using is a bit overspecced, but would that cause this?
(Note: I have 120ohm termination resistors at both ends)