My 3150-MCM is configured to have no Write Data, only Read Data. Occasionally, I am seeing my data overwritten with what appears to be configuration data.

3150-MCM write block count and write block start are both set to 0. The write block start parameter must be set to a value which does not overlap with the read data area. A write start value of 75 will generally take care of this problem. In the sample ladder for the 3150-MCM, register N7:28 is the write block start parameter.