I’ve purchased this 0.56″ 4 digits LED display on Amazon :

which seems to be the clone of a clone. There are very few online resources about it. The only one I’ve found is this one

I have exactly the same screen as they do. It’s linked to an Opta Finder via the A-/B+ RS485 wires. The screen displays 0 by default when powered up but I am unable to display something else (I’ve been using Modbus RTU, not Modbus ASCII). In particular, I tried to follow that :

Display modbus registers are following:

Decimal point:
Slave Addr: 1
Function: 0x06
Start Address: 4

Value:
Slave Addr: 1
Function: 0x06
Start Address: 0

So inserting 2 to the register 4 and 1001 to register 0 and writing the values will show 10.01

but nothing changed. Here is a piece of code I’ve been using :

ModbusRTUClient.holdingRegisterWrite(slave,0x04,2);
delay(2000);
ModbusRTUClient.holdingRegisterWrite(slave,0x00,1001);
delay(2000);

the slave variable is set to 1. So far I’ve used several software serial variants : 8N1, 8N2, 8E1, 8O1…the result is the same.

Does anyone have an idea or at least a link to a kind of datasheet ?