PLC problem

Helle all,
I've a labeller machine controlled by (sematic S7-200 PLC + Pro-face HMI panel),
a communication problem occured between the PLC & HMI panel!.
when start the machine, the HMI panel showing an alarm says"Target node not found (PLC1)" !, making the machine stop !.
The communication cable consist of 2 wires (Tx & Rx) only and i've found that both wires are shorted with the machine body!, then ,i've replaced it with a suitable external cable connecting the PLC with the HMI panel, but the alarm stay appear and the machine still stop!
The Plc in run mode and all inputs are ready while no output unless one !
Please advice me for this problem.
Waiting for kind help...

If the 2 wires were "shorted" with the machine body, then the serial port was probably damaged...

Thank you..
Now , i want to connect the PLC to PC via Mpi adapter .
PLC model: Simatic S7-200 CPU 224XP
MPI adapter: Simatic S7 PC adapter USB(6ES7 972-0CB20-0X0A).
Please , advice me if this MPI adapter is suitable to be use with this PLC ?
Waiting for kind help..

Which Proface panel?

I use a length of profibus 2 core cable and the profibus 9 pin plugs with the terminating resistors set to "on".

The proface panel should have an adaptor plugged into it part number CA3 ADPCOM-01

Use this: USB/PPI Multi-Master: 6ES7 901-3DB30-0XA0

I have used it to connect the S7-224XP to HMIs and PCs and it works just fine.

Thanks 2 all,
But a new problem facing me with the loader software, i've Simatic Manager & Step7 Lite, but both can not be online with the PLC!.
Please advice me i've the above loaders are suitable for such PLC model or may should i use S7- MicroWin with S7-200 PLC?
Help me plz...

With S7-200 you need to use the Step 7 Microwin

Hello friends,
I'm got Microwin v4 software ,and used MPI cable(pc adapter usb) with the s7-200 PLC.
There is a communication error when online the PLC but i've successed to upload 3 networks of the project from the PLC while the other networks are empty.
please, i need your's kind comment about this status?

post some screens with the error...