I've got a Waveshare PN532 RFID reader/writer. I want to use it with a Pi 4B, but since it is already using a HAT with no free GPIO, I want to use it in UART mode, but not connecting to the GPIO pins 8 and 10, but rather using a TTL-to-USB adapter. Now I want to follow the first step in the wiki, "Quick testing", which is explaining this for a windows PC and "Serial assistant software". It says the connection should be baud rate 115200 and 8N1. The instruction then is:

Select correct serial port and open.

Send data below to wake up FN532 module:

55 55 00 00 00 00 00 00 00 00 00 00 00 00 00 00 FF 03 FD D4 14 01 17 00


The response from PN532 module should be:

00 00 FF 00 FF 00 00 00 FF 02 FE D5 15 16 00

How do I do this on the Pi (Raspbian Buster)? I guess I must somehow send these bytes to /dev/ttyUSB0 and poll the response.

Here is an attempt:

  • terminal 1: cat /dev/ttyUSB0 >out.bin
  • terminal 2: echo -en '\x55\x55\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xFF\x03\xFD\xD4\x14\x01\x17\x00' > /dev/ttyUSB0

But how do I stop cat? If I ctrl-C it, the contents of out.bin is empty. If I use use cat /dev/ttyUSB0, I can see a response of five unprintable characters; my idea was to hexdump -C -n out.bin, but apparently I need to flush and quit cat, or can I pipe it into a "live" hex view without it waiting for linefeed or flush?

photo of ttl-to-usb adapter sitting between Pi and RFID board

  • Not really Pi specific. Perhaps use echo with the -e option to send the bytes.
    – joan
    Dec 14, 2021 at 21:46
  • @joan Can you please advise how to send bytes to ttyUSB0 via echo, and how to hex dump response from ttyUSB0, choosing the given baud rate etc.
    – 0__
    Dec 14, 2021 at 21:49

1 Answer 1


To configure the device

$ stty -F /dev/ttyUSB0 speed 115200 cs8 -cstopb -parenb -echo

To wait for the next fifteen bytes to arrive

$ hexdump -C -n 15 /dev/ttyUSB0

To send bytes (from a second terminal)

$ echo -en '\x55\x55\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xFF\x03\xFD\xD4\x14\x01\x17\x00' > /dev/ttyUSB0

The hexdump terminal then should print

00000000  00 00 ff 00 ff 00 00 00  ff 02 fe d5 15 16 00     |...............|

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