SLC 503 Com Problem

For about a year I have been using an HMI program written in VB.net that uses the Tomantron DF1 driver through the RS232 port to read and write data to an Allen Bradley SLC-503 processor (Channel O).

Everything works very well. I purchased a newer SLC-503 to build a second unit and cannot seem to write to the PLC files. The older SLC-503 has a 1747-OS302 Series C FRN 6 and the newer SLC-503 is 1747-OS302 Series C FRN 11. The program runs fine and the HMI computer will read the PLC files. I just cannot write to the PLC. The SLC program is exactly the same and I have been over the com settings and found no differences.

I have tried more than one new SLC-503 with no better results. Can someone tell me what has changed.

Thanks
Kevin