Serial.read on an ATtiny84

Hello all,
I’m experiencing a little problem with the SoftwareSerial library.
I’m collecting information from a TTL device, which connected on an arduino uno works perfectly.
When I port the code to an ATtiny84 (after adaptation for the pinout), my ATtiny is sending frames, the device responds (I can see the frame with a Serial to TTL adapter) the good information, but the attiny read nothing.
Here is the code of the sketch, which run perfectly on an Arduino Uno. The code is adapted for the ATtiny pinout.

#include <SoftwareSerial.h>

// Pin configuration
#define TxTTL 0
#define RxTTL 1
#define TxPro 5
#define RxPro 6
#define RxZig 9
#define TxZig 10

String buffer;

SoftwareSerial display(RxTTL, TxTTL);
SoftwareSerial programmeur(RxPro, TxPro);
SoftwareSerial zigbee(RxZig, TxZig);

void setup()
{
  zigbee.begin(9600);
  display.begin(9600);
  
}

void loop()
{
  display.println("N1TA*");
  delay(200);
  buffer="";
  while(display.available()>0)
  {
    buffer+=(char)display.read();
  }
  Serial.println("Received value: " + buffer);
  
}

I can create the same problem if I replace the display by an arduino which is sending arbitrary data to the ATtiny: the ATtiny receives nothing (but the electrical signal arrives on the pin). The communication from ATtiny to Arduino is ok.

Thank you very much for your help.

Regards,
TeslaBoy

ATtiny84.PNG

-UPDATE- Hello all. If I port the code on the arduino, same problem. But if I change one of the softwareserial instances to Serial to use the UART of the arduino, it works well. Is there a restriction with the number of instance of SoftwareSerial?

Thanks

I remember reading that softserial can only receive on one instance at a time.