Sorry, I am slow today and still can't follow. Spend to many hours yesterday. Thx for your patiance.
Setup is a connection of a Jeti device to the TX2 and RX2 Pin on the Arduino Mega 2560. Since the device uses half-duplex on a one line transmission, both pins are connected via 2.4k Resistor while the line is connected to the TX pin (www.jetimodel.com/en/show-file/26/). Even during sending the receiving pin is high.
CrossRoads: They are not pulled up in the '2560 - they appear to be high as a result of being connected to the USB/Serial adapter chip thru 1K resistors. See RN4A and RN4B on the Mega R3 schematic at the lower left.
Ok, I found them and missinterpreted them as pull-ups.
Grumpy_Mike: Those are RN4A and RN4B - they are series resistors they are not pull up resistors.]Grumpy_Mike: Those are RN4A and RN4B - they are series resistors they are not pull up resistors.
But I can not understand how they affect the TX2 and RX2 pin.
Grumpy_Mike: All pull up resistors are not internal.
http://arduino.cc/de/Main/ArduinoBoardMega: Each pin can provide or receive a maximum of 40 mA and has an internal pull-up resistor (disconnected by default) of 20-50 kOhms
There are no pullups for Serial2.
ok, understood. Is that true for all TX and RX Pins and also others? I misinterpred the documentation as far as the the internal resistor can be activated for each pin.
I'm pretty sure if you look at Serial.h or Serial.cpp, whatever file is called out by Serial.begin(), you will not find the pullups enabled.
Now I found the corresponding section in the datasheet (example is for the wrong pin, but the others work as well)
• TXD – Port D, Bit 1
TXD, Transmit Data (Data output pin for the USART). When the USART
enabled, this pin is configured as an output regardless of the value of
• RXD – Port D, Bit 0
RXD, Receive Data (Data input pin for the USART). When the USART
enabled this pin is configured as an input regardless of the value of
DDD0. When the
USART forces this pin to be an input, the pull-up can still be
controlled by the PORTD0
Thank you for your replies