I have done a lot of work with RS232 input in my time and I have encountered several pitfalls that I did not expect to stumble across.
You are not specifying if you are using a "real" COM port or a USB COM port device. The USB COM port devices are great, but they do not work 100% as a native COM port and depending on the manufacturer (most of them seem to use an FTDI (www.ftdichip.com
) driver) you can run into issues with buffer sizes and read/write timing.
The '0' characters you describe is something I see quite a bit - usually in groups of four '0's. Are you sure they only appear in the second matrix? You should check to see if the characters are in fact read from the COM port and appear in the first matrix as well.