smps noise to the mcu circuit

Dear Friends ,

I have a circuit which works as following :

Mcu board :

reads temperature via pt100 sensor , shows temp on lcd and it has to print the data on a thermal printer .
Mcu board is working on linear power supply which powers voltage to pt100 circuit , mcu , lcd etc.

The thermal printer requires 5V , 2A of current so i am using a pc smps to power it on.Now to communicate to the printer i am using max232 on the mcu board to connect uart tx,rx & mcu gnd(linear power supply) to the printer....So i have to connect the mcu gnd to the smps ground in order to communicate.

By doing this i see a lot of jump on the temperature reading . The temp reads jump of upto 10 deg plus or minus , so the noise is coming in the circuit.
The moment i disconnect the tx,rx,gnd connector which is connecting to the thermal printer, the noise goes away . Please suggest what could be done . Can we use smps to power the thermal printer & onnect to mcu gnd both.What should be done to minimise this problem. Any past experience or suggestion is welcome .




View attachment EPM - 207.pdf

Just use another make (Other company) of 5volt supply. Cheap ones make great noise. Use better ones.
Cheers