CPPM_HW_Mod.md

CPPM Multi module hardware modification

By default Multi uses the telemetry line to send the received channels using a RX protocol (FrSky, DSM, AFHDS2A, Bayang) to the radio. But this does not work on FrSky radios since the telemetry lines of the internal and external modules are shared. On a STM32 module and with a simple hardware modification, you can go around this hardware limitation by using CPPM to send the trainer information to the radio.

There are 2 ways to connect the module CPPM trainer signal available on the STM32 USART1.TX pin (BOOT0 programming TX pin) to the radio:

1. Use the trainer jack input which is supported by all radios.
2. Use the heart beat module bay pin 2 which is currently supported by erskyTX but not yet by OpenTX. FrSky ACCESS and X10/X12 radios do not support this mod.

For the hardware modification you need:

1. 1K resistor
2. Wire
3. Shrink tube to isolate the resistor
4. For an external trainer, a 3.5mm mono jack plug
5. Soldering iron and solder

Trainer jack

Here is the modification on a Banggood 4-in-1 or an iRangeX IRX4 / IRX4+:

Here is the modification on a Jumper 4-in-1:

Heart beat within the module bay

Here is the modification on a Banggood 4-in-1 or an iRangeX IRX4 / IRX4+:

Here is the modification on a Jumper 4-in-1: