Laptop RS-485 advice wanted


Thread Starter


I need occasional access for my field techs to a Modbus device. They can barely speak CAN (well some better than others...)

Used to get "COM1" Serial RS-232 ports integrated in old WinXP laptops, (then converted to RS-485) but not anymore. Having trouble getting new USB RS-232 ports to consistently work for my field techs across a Win7 32b/64b environment (Windoze drvr/ port issues.

Any advice would be much appreciated.
I have had excellent results with Keyspan's USA-19HS for a USB/RS-232 adapter, but I'm still on XP.

I have been tempted to buy the unit at the link below, because it has a signal ground terminal; but their statement, "It has a surge and static protection of up to 600W (sic)" makes me wonder what they know about voltage isolation.

My observation is that the virtual COM port that USB devices install to is dependent upon the USB port they are plugged into. Plugging into the same port minimizes confusion when the COM port has to be specified.
I should note that I'm not using the adapter with Modbus so I don't know if it would work for that protocol. It has automatic transmit control (rather than using the RTS line).

Lynn August Linse

I hate to say "Buy Expensive/Avoid Cheap", as I'm sure some cheap USB/serial devices work ... the problem is which one?

As someone suggested, I'd probably go with B&B, even paying for galvanic isolation & external power since normal USB 'serves' your notebook's digital ground out, shorting it to whatever surge/noise exists on your external device.

I've seen PC 'reboot' when a large USB device is hot-plugged into it.