I am doing a small wireless project using the RLP/TLP434A pair from laipac.

Each (TX and RX module) is connected to an 89C51 on a separate board.

The transmitter is working. In the receiver, the 89C51 does not seem to detect the start bit from the RLP434A, however it does work when RLP module is bypassed and the data is sent from the terminal with a max232 circuit.

I programmed the receiver code to turn on an LED when the right packet was received. I checked the hardware and it is working, but not with the RF module.

I also did this same thing with a couple of avr AT90S8515.

With the avr, the receiver is working. With the 89C51, the receiver does not detect the start bit (P3.0 never goes low).

I checked the software by hooking up the receiver board to a terminal using a max232 circuit and echoing the incoming data back to the terminal, and it works fine so I know the uart configuration is correct. I tried the avr chip in the receiver board (a few small mods to the reset circuit) and it works perfectly with the RF module, so I know the hardware is good.

I tried some non-inverting buffers (7407, 74HC7014) I had here, and nothing seems to work. The RX pin will not go low (start bit).

I checked the pin on the RF module (with the 8051 removed) with my scope and it is pulsing high and low. As soon as the 8051 is installed the pin stays high, and nothing works, yet the same circuit will work if I plug in my avr chip.

I tried everything I can think of, including other 8051 devices I have and nothing is working.

I hope some of the pro's can help me figure this out.

I want to post my schematic but i cannot upload the gif file.
(or I don't know how)