Sandro,
I recently solved a similar problem that had plagued me for a long time. While trying to DNC to a Meldas M50, I would receive intermittent P460 TAPE I/O ERROR. I tried different DNC software, but I experienced the same problem with all of them. So I decided to write my own DNC software, and that's when I realized what the problem was.
The way the M50 handles the DNC is this (and I think the M64 is no different): When it's receive buffer is low or empty, it sends the XON character to tell the PC to start sending. And when it's receive buffer is almost full, it sends the XOFF character to tell the PC to stop sending. So far, this is nothing unusual. What I found was that the M50 doesn't send the XOFF character until the receive buffer is very close to being full. Something in the area of 8 characters. This in itself doesn't cause problems, it's actually the DNC software on the PC that causes the problem. I found that most DNC software WILL NOT STOP SENDING IN THE MIDDLE OF A BLOCK. So even though the PC received the XOFF character, transmitting continued until the block was finished. And with the M50's receive buffer almost full, this would cause the P460 error if the block was long enough.
So my DNC software was written specifically to avoid this problem, and works great on the Mitsubishi controls. If you want to try it, it's included with NCPlot beta 20:
www.ncplot.com
Regards,
Scott