RS485 USB converter driver polarity
After fixing signal ground issues, you need to confirm signal polarity by sending data packets and checking for confirmation. If a corrupted or messy packet is received, the receiver will request the packet be resent. If this happens continuously then reverse polarity and try again. If polarity is correct then data should flow in both directions per USB protocol.
RS-485 requires a separate signal ground to avoid too much DC offset in the signal, or baseline drift as some call it. Also with LabView you can use NI MAX to configure your Ni-DAQ and communication boards with arbitrary timeouts. This must be a USB issue, as USB does have 1 ms timeouts as it sends packets at a 1 ms rate.
RS-485 only has the timeout restrictions you put in it, but using USB as a source restricts you to USB protocols. RS-485 is a hardware standard, not a software protocol. You will need to make sure events on the RS-485 side return a &quot;ACK&quot; or similar USB response within 1 ms. This means short hops to each RS-485 node, and each node must respond within 1 ms. A saving grace would be if NI-MAX has control over USB functions such as timeouts.
Also with LabView it is easy to decimate data into fixed-rate packets before being sent to a USB port--&gt;RS-485. Also USB high-speed uses 100 us time delay between packets. Check what USB standard is being used, and chose a slower data rate such as 48 mbps. This is where you may have to compromise to make things work - along with adding a signal ground wire (20 ga or 22 ga will work) that hops from node to node. Do NOT Earth ground the signal ground wire.
Try adding the signal ground wire first. Baseline drift can make many low-voltage differential communications work poorly or not at all.
Remember that NI-MAX and LabView are very expensive and powerful software tools. In an hours time you can create diagnostic indicators for polarity match, send inverted data, bit error rate, DC offset in data lines, etc. Build these to take the guess work out of the equation.