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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.

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I'm pretty sure it does not depend explicitly on the type of device or hub depth on the other end.

The reader sends out an electromgnetic field, which is used to power the chip inside the RFID card, and also to transmit data back to the device. (the card switches a load on and off, which results in variation of field strength over time. The reader detects this.) And there is the USB communication circuit, which is logically independent from the reader circuit.

It is possible that the field strength heavily depends on the supply voltage of the reader. Your laptop is quite strong, but when you attach a hub without power supply, it might lower the voltage.

The embedded Linux box might deliver less voltage since it might be powered by USB itself (like a raspberrypi), and if you attach a display including a hub, which might also be powered over USB, the voltage for the reader can be pretty low. And that causesan cause a weak field an so short reading distance.

Finally, can you measure the voltage on the supply line at the reader? USB allows something in the range 4.7V-5.xV. If you find a relation between voltage and distance, you found the reason.

Or put a hub with own power supply between reader and next device and check if it's better.

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