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Pipsta / Pipsta NFC Python Code Tutorial

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Difficulty Level

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  • The NFC system spans multiple hosts and files, runs as a daemon and uses Regular Expressions which may be unfamiliar to newcomers.

Time to Complete

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  • Many aspects are complex and may require significant time to investigate and understand.

Who Should Read This Document

The complexity of this system means that this document is likely to be beyond the capabilities of beginners in Linux and Python programming. It may, however, be possible for some intermediate users to draw-upon and modify pre-existing techniques.

Prerequisites

It is expected that your Pipsta is fully set-up and configured, that you have completed all aspects of the NFC Printing tutorial.  

How it Works

The NFC server system architecture is as follows:

NFC_server_sys.png

NFC Message (Send to Pi)

In send to Pi mode, the Pipsta app sends a packet of NFC data over to the Pipsta printer. This packet contains:

1) Encoding: (one of two text types)

2) Header: 0x01, 0x02

3) Method: (e.g. pipsta.qr from the drop-down list)

4) Data from the text field

In the current implementation, the system is limited to a total of 64 bytes. It is anticipated that this will be improved in future. In practice, keeping the text field data below 30 characters should suffice, but –if longer payload data is provided—the system will simply truncate this.

The printer decodes the header, and determines that the packet is intended to be returned to the Raspberry Pi and buffers it.

Pi Polls Printer over USB

In nfc.py:

  • get_credentials() sends QUERY_CREDENTIALS to the printer over USB

Printer transfers packet over USB

  • If the printer has NFC data buffered at this time, it will be returned to the Pi

Printer Empties NFC Buffer

  • In response to erase_credentials(), the printer empties the NFC buffer.

Pi Script Processes Intent…

  • nfc.py uses Regular Expressions to validate and group the message components
  • If the ‘method’ component is confirmed as being a ‘pipsta’ method, it is used as a lookup in a Dictionary to determine the module to process the payload. By default, the supported modules are 'banner' and 'qr'
  • The send_to_printer() method in the target module is invoked. This method performs all the processing that is discussed in:

Python Script Sends Print-Job…

  • After generating the appropriate image in Pillow, the appropriate Python module sends a sequence of graphical data to the printer.

Next Steps

The tutorial Adding Bespoke NFC Methods to the Pipsta NFC Server (TBD) will demonstrate how you can extend both the Android app and the Pi host NFC server system to add you own methods.

Shutting Pipsta Down Safely

Whilst the printer is resilient when it comes to powering down, the Raspberry Pi must undergo a strict shutdown process to avoid corrupting the Micro SD card. The most straightforward method of doing this is to double-click the ‘Shutdown’ icon on the desktop.

TIP: If you are already in LXTerminal, type sudo shutdown –h now to shut-down the Raspberry Pi immediately.

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TIP: Always make sure ALL activity on the Raspberry Pi’s green LED (the LED on the right) has stopped before removing the power!

End of Document

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