POCSAG Decoder V2.00
At discriminator output, we get a stream of bits with about ±1 V width. PC can't process this signal. Using the serial port (RS232), we need signal with ±12 V width. This interface makes the signal compatible with RS232 specifications, so the PC can use it.
Here is the diagram, which is in fact the HAMCOM receiver circuit diagram:

Fig. 4 - HAMCOM Receiver
| Component | Value |
|---|---|
| IC1 | LM 741 |
| R1 - R2 | 100 K |
| D1 .. D4 | 1N 4148 |
| C1 | 0,1 µF |
| C2 - C3 | 10 µF 16V |
Please, refer to HAMCOM documents for informations about this circuit.
In HAMCOM interface, the signal is directed to DSR. You can direct it to CTS. You can place a switch to select the output between DSR or CTS.
The table below shows the SUB-D connections.
| Signal | 25 Points | 9 Points |
|---|---|---|
| TxD | 2 | 3 |
| RTS | 4 | 7 |
| CTS | 5 | 8 |
| DSR | 6 | 6 |
| GND | 7 | 5 |
| DTR | 20 | 4 |
This simple circuit is the only one you need to receive data. To send data, you need to complete this diagram with at least the FSK diagram. You can make a complete HAMCOM interface if you place the Transmit Audio and PTT diagram.
To receive, the easiest is to make a board you can put in a SUB-D 25 case. Here is an example:

Fig. 5 Printed Circuit - Copper side
Use the LM741 size to get the good size if you print this board.
Components are placed like this:

Fig. 6 - Components
Notice that I use a Diode Bridge instead of the four diodes.
Here we are. You get all you need to decode POCSAG messages. I hope this program will be useful for you.
galak@gulliver.fr