DIY-Multiprotocol-TX-Module/docs/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.
1. 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
1. Wire
1. Shrink tube to isolate the resistor
1. For an external trainer, a 3.5mm mono jack plug
1. Soldering iron and solder

## Trainer jack

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

![Image](/docs/images/CPPM_BG_IRX4_Jack.jpg)

### Here is the modification on a Jumper 4-in-1:
   
![Image](/docs/images/CPPM_JP4IN1_Jack.jpg)

## Heart beat within the module bay

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

![Image](/docs/images/CPPM_BG_IRX4_Mark.jpg)
![Image](/docs/images/CPPM_BG_IRX4_Soldered.jpg)

### Here is the modification on a Jumper 4-in-1:
   
![Image](/docs/images/CPPM_JP4IN1_Mark.jpg)
![Image](/docs/images/CPPM_JP4IN1_Soldered.jpg)
![Image](/docs/images/CPPM_JP4IN1_Soldered2.jpg)