Multiple serial devices need interface!

This is continuation of Multiple devices on RS232 / serial line.
RS232 serial TX lines of combined 3 ATtinys draw too many current, so signal was degraded and ATmega didn't received what ATtinys sent. I figured out, that I can use other ATmega328 w/ Arduino bootloader as serial interface (first thing that came to my mind was multiplexers for eg. 4051, unfortunately I didn't have one, but I have spare ATmegas).
For testing purposes you don't need C3:C8 caps, but they ensure stability, also you don't need xtal and it's caps, but you need to burn bootloader for noncrystal ATmega328. If it have bootloader or fuses set to work with crystal you have to attach crystal for burning.
Here's example entry for boards.txt (which is located in:C:\Users\You\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.6.15) paste it in the end, user: You and version: 1.6.15, may be different:
##############################################################
m328p8MHzInternalRC.name=ATmega328 w/internal RC 8MHZ
m328p8MHzInternalRC.upload.tool=avrdude
m328p8MHzInternalRC.upload.protocol=arduino
m328p8MHzInternalRC.bootloader.tool=avrdude
m328p8MHzInternalRC.bootloader.unlock_bits=0x3F
m328p8MHzInternalRC.bootloader.lock_bits=0x0F
m328p8MHzInternalRC.bootloader.extended_fuses=0xFF
m328p8MHzInternalRC.bootloader.high_fuses=0xDA
m328p8MHzInternalRC.bootloader.low_fuses=0xE2
m328p8MHzInternalRC.bootloader.file=optiboot/optiboot_atmega328.hex
m328p8MHzInternalRC.upload.maximum_size=28672
m328p8MHzInternalRC.upload.maximum_data_size=2048
m328p8MHzInternalRC.upload.speed=115200
m328p8MHzInternalRC.build.f_cpu=8000000L
m328p8MHzInternalRC.build.board=AVR_MINI
m328p8MHzInternalRC.build.core=arduino
m328p8MHzInternalRC.build.variant=standard
m328p8MHzInternalRC.build.mcu=atmega328p
##############################################################


ATtinys work with sketches similar to those I used before. When they receive specified character (ASCII: 'a','b','c') they send 2 bytes: 1st is encoder value, 2nd is button state ASCII 'a' for nonpressed,  'c' for pressed.
ATmega (helper) sends "addresses" over software serials one for each tiny, receives data and stores it in arrays. When it receives 'a' over hardware serial it sends data combined in array to Arduino (master). Arduino recievs data over sw serial on pins 2,3 and sends them over hw serial  to PC serial monitor/terminal.

This time I tested all setup before publishing :)

Codes:
ATtiny's (remember to change letters: line 25),
ATmega - helper,
Arduino - master.

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