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FSK to USB interface

A new interface with USB connector

From 2010 to 2018, the RS232 interface has been available. Using this interface, the PC's serial or USB port can be used to monitor POCSAG, FLEX and Mobitex networks in combination with PDW. Since its introduction, several hundreds of interfaces have found their way to satisfied users worldwide.

For some time, I had the desire to develop a USB version of this interface. The availability of a ready-made USB module through ebay made this a lot easier. There was a snag however: these modules appeared to have cloned FTDI chips that are not supported by the latest drivers. Therefore, a new version of the USB interface has been developed.

USB interface

On all FSK to USB interfaces sold through this site, genuine FTDI chips are used from Farnell/Mouser. It ensures that the latest FTDI drivers will work with this interface.

Benefits of this interface

The USB interface is based on the RS232 interface. The USB interface has the following benefits:

  • The interface gets its power from the USB port. No separate power supply needed
  • No separate RS232 to USB cable is necessary
  • The interfaces sold through this site use genuine FTDI chips (FT230XS). Stable drivers are available for most operating systems
  • Run PDW on any Microsoft Windows version
  • Use multiple interfaces on one PC (one user even reported 8 interfaces)
  • Share the interface data to multiple PDW windows. To do this, you need to use serial port splitter that will install multiple virtual COM ports

How does the interface work?

The new serial interface converts the synchronous (continuous) datastream into an asynchronous data signal with a start bit, 8 data bits and one stop bit. The interface works according to the next priciples:
  • The incoming signal is sampled with 5 times the data rate. So e.g. a FLEX-1600 signal is sampled 8000 times per second
  • The transitions from 0 to 1 and from 1 to 0 are used for bit synchronisation. Ideally, when receiving data, these 5 samples will be 11111 or 00000. If the 5 samples received are e.g. 11110 or 10000, resynchronisation needs to take place: one sample to the right resp. one sample to the left
  • The majority of the 5 samples determines whether a 0 or 1 bit has been received
  • When 8 bits have been received, these are sent to the USB module observing the standard RS232 protocol with 19200 bit/s , 8N1 (8 data bits, no parity bit, 1 stop bit)
Bit synchronisation
Apart from the bit synchronization, the interface doesn't contain any intelligence. The only thing the interface does is chopping a synchronous data signal into slices of 8 bits and sending these 8 bits asynchronously, i.e. packing them with start- and stopbits, according to the RS-232 protocol.

Interface hardware

The USB interface [schematic] is based on a PIC16F628 microprocessor. It runs on a frequency of 18.432 MHz. This microprocessor has a built-in comparator ('2 level interface') and a hardware UART. The FT230 IC communicates with a PC's USB port through the mini USB connector.

A double sided circuit board has been designed. One SMD component is mounted at the bottom of the circuit board.

Using two DIP switches, the following bit rates can be selected:

  • 512 bit/s (POCSAG 512)
  • 1200/2400 bit/s (combined setting for POCSAG 1200, 2400 and 1200/2400)
  • 1600 bit/s (FLEX 1600)
  • 8000 bit/s (Mobitex)
Other data rates can be programmed on request.

Top of circuit board
Bottom of circuit board
Component side
Solder side


To use this interface, you should download the latest version of PDW (version 3.x). In the 'Interface' settings in PDW from version 3.0, the user can now choose between 'Slicer' (the 2 or 4 level interface) or 'RS232' (this interface). It takes some getting used to this: changing between different data standards requires the user to change the settings on both the interface and in PDW.


The new version of the USB interface is available since March 2015. You can order a built and tested interface with or without matching enclosure. Prices and 'Buy Now' buttons for instant ordering can be found on the Order page.

The manual can be found here.

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