I have a RFID reader that sends the TAG information over USB serial. I have had some troubles with it in a product where the detection success rate changes alot depending on what the USB RFID tag is connected to, even between devices of the same kind but larger variations in different types devices it is connected to. To name a few examples of the detection distances
-Connected to laptop PC: 5cm
-Connected to USB hub which is connected to PC: 4.5cm
-Connected to an embedded linux box: 3cm
-Connected to a display unit (which has an internal hub) which is connected to the linux box: 1.5cm
-Connected to a USB hub (same hub as PC case which had 4.5cm detection distance), which is connected to the display unit which is connected to the linux box: 1.7 cm.
These connection arrangements all result in various usb hub depths of 1-3, but there isnt a straight relationship here between hub depth or device and the detection distance. In all these cases the RFID reader was plugged to a USB extension cable and taped down to be measured in the same spot with a plastic ruler.
Is this a common phenomenon? Detection distance being so reliant on whats on the other end of the chord. There doesn't seem to be a strong correlation with a circuit/hardware issue. It feels like a software/system issue. There is some traffic on the hubs in the situations where the detection distance drops but you would think that the actual detection distance would be entirely dependent on the near surrounding of the RFID reader, not what device it is connected to 30cm away.