Hello,

Recently I designed a prototype which gets data from USB and send it to RS485. Device uses Ft232R as the USB interface. Device is fully opto-isolated and the USB side is DC/DC powered. 89s52 is used to convert software protocols and etc.

The prototype works well but the biggest handicap is the way FT232 chip reacts to environmental noises. It can crash and reset the USB connection even with minimal noise radiated from a near-by device such as an adapter being plugged in and out an outlet.

I believe that I followed every rule to immune my PCB from EMI. As I noted above, the USB side is fully isolated. Signals are passed through usiing 4N28 opto-couplers and a NME0505 is used to supply isolated powers. Well, I know that optical isolation doesn't guarantee EMI isolation but what else can be done? It is not likely to mount such a device in metal case. A normal plastic case is what most of similar type of devices use. And I don't think is about grounding. The top surface is almost fully filled with ground plane and bottom has a ground plane too. ( Board is double sided )

Is it something wrong with the FT chips or is it my design lacking from some bad design? Should I put the FT chip in a faraday cage? Moreover, the 80s52 which takes place on same board and is placed on non-isolated side of the board doesn't crash at all.