SDC2160 and SDC2130 slightly garbled serial data through DB15 tx/rx

Hi Folks,
I'm trying to solve comm stability issues by switching from the on-board USB<->RS232 interface to the rx/tx on the DB15 connector. I'm connecting using a Prolific Technologies USB<->RS232 interface.

I appear to have the correct pins and serial setup, as I can query the board and get responses. But the responses are 80% garbled. For instance, the response to the ?VAR query is generally something like:

V�R=3;4:p:0��:8:0:0:1>8:�:0;2:0z0:0

You can see the resemblance to the correct response, but dang is it noisy.

I've checked that the serial settings are 115200 8n1, no flow. I've tried both of the grounds on the DB15 connector to rule out a floating ground, on the odd chance the grounds are not identical. I've googled the RS232 interface and chipset for known problems. The test setup has good solder joints. The cables are not longer than 50cm total and only unshielded at their solder point, which is about 8cm.

An interesting thing to note is that the request transmitted to the board seems to always work. It's just the response that's garbled.

Any suggestions out there about what I can try next? I'm out of hypotheses for now.

We use the usb to serial from usconverters.

www.usconverters.com/usb-rs232-adapter-xs8801

I've not seen garbled data on our controllers from our PC using this. It also supports windows 8. From the other converter we were using from Gigaware, their chipset didn't support windows 8 so we stopped using them.

Perhaps try another converter and see if you get the same results.

From their link from the one I posted above.

This link they provided, explains why sometimes the usb to serial interface might cause the garbled data and other things. It may not be the case with yours, but some information that may be of some use.

www.usconverters.com/index.php?main_page=page&id=56&chapter=0