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RS-232 Interface

PDW and the RS232 port

There is an increasing lack of PC's with serial ports these days. Decoding data networks such as Flex, POCSAG and Mobitex on a computer without RS232 port is only possible using the PC's soundcard. A USB to serial converter in combination with a 2 level interface won't work.

PDW doesn't use the normal RS-232 TxD/RxD lines, but uses the status lines (CTS, DSR, DTR). The 'abuse' of these lines for PDW purposes does not comply with the standard RS-232 protocol. If you have direct control over the serial port (i.e. through a physical port) you can sample these lines without a problem. If you use a USB to RS-232 converter, all serial communications goes through the converter's driver and PDW doesn't have direct control.

Data communication protocols like Flex, POCSAG and Mobitex use synchronous communication: a steady flow of ones and zeros. An RS-232 port is made for asynchronous communication: start bit, data bits, stop bit. The serial interface presented here takes care of conversion.

Using standard RS232 communication enables the user to

  • Use a serial to USB converter of choice
  • Use a serial PCMCIA or PCI card
  • Run PDW on any Microsoft Windows version, including 10 and 11
  • 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.
  • Use a serial to Bluetooth adapter for wireless monitoring
  • Monitor remotely by using a serial to Ethernet converter
  • Run PDW in Wine under Linux (who's going to develop a Linux version of PDW?)

How does the interface work?

The 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 serial port 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 RS-232 interface [schematic] [partlist] is based on a cheap and common PIC16F627 or 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. To convert the output signal to RS232 levels (-10V / +10V), a MAX232 is used.

A standard RS-232 output is implemented. The circuit presented here only uses DIL (dual inline) components, making it easier to assemble the circuit board. Further to that, the circuit board is single sided, facilitating DIY production.

Since October 2014, there is also a USB version of this interface available.

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.
Circuit board design
Component placement

Changes to PDW

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 'standard' 2 or 4 level interface) or 'RS232' (the new serial 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 manual of the interface can be found here.


Since its introduction in September 2010, several hundreds of interfaces have found their way to satisfied users worldwide. Early 2018, this interface has been discontinued. Its successor, the USB interface, remains available.

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