RE: Futaba R127DP Rx: PCM1024 *AND* PCM512?
Look at the last letter in the Rx designator....127DF. The "F" signifies FM.
(conversely, a 129DP, 138DP, 149DP, etc all denote PCM receivers)
The Futaba 127DF is an FM (PPM) receiver. It doesn't decode any PCM signal. 512, 1024, 2048 or any other.
By selecting the correct mode of modulation at the TX (PPM), your PCM TX will then work fine with the FM Rx, but it will NOT function as a PCM system one iota if the 127DF receiver is involved.