USB RF Modem with Ping Pong CDC

Figure 1 

The project was created to build an RF modem to link two personal computers with the use of frequency hopping technique.

The RF modem is using a very basic RF module which does not contain the built-in MAC layer. A dual CDC + WinUSB interface is being implemented by the modem since the modem interface is USB. The serial port emulation that carries the data is the modem interface while the modem is configured using WinUSB via a custom Win32 application. Error detection and correction is not implemented by the modem but a very basic flow control mechanism is being implemented.

When used with PPP, the RF modem is capable of around 70kbps and for transferring files between PCs, zmodem is used with the use of PPP to create an internet connection. The RF module Semtec DP1205 is the core of the modem. It uses a transceiver and an antenna switch from Semtec XE125 module which is a generic 900MHz FM module without any built-in protocol logic but it has an SPI interface for changing between transmitter and receiver modes. It also sets the RF frequency and FM frequency deviation at 100KHz.

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