I suspect that getting source level debugging for PLM programs is going to be near to impossible. If I were you I would adapt a strategy that whenever a part of your program requires tracing or debugging that you then convert that particular module to C-51. Over the course of time eventually your program will be in C and then the modern tools, such as Keil C51, can generate full traceability information.

Michael Karas

PS....It is also highly likely that the most modern tool sets in C will produce much tighter object code than PL/M does.
MJK