Command Station for DCC Adapter for Xpressnet

Posted on Feb 6, 2014

Xpressnet is a bus system for model railways to interconnect input devices, computer interfaces and stationary decoder or feedback detectors. Many commercial input devices (including the multi-mouse from Roco) are based on this bus. The following adapter enables OpenDCC to interface to Xpressnet devices. Xpressnet devices need a power supply of 12V. This voltage is created with an linear regulato

Command Station for DCC Adapter for Xpressnet
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from the supply of OpenDCC. On large Xpressnet installations, however, this power supply may be overloaded. In this case, the 12V should be generated with a powerful external power supply. The fuse F1 must be left out in this case. SJ1 should be left open. Xpressnet uses a half-duplex RS485 connection. The baud rate is 62. 5 kBaud, 9 bit are used, no parity, one stop bit. The chip Max485 (or Max481) takes the data from the serial interface of the processor and converts it into the R485 differential signal. While not required, it is strongly recommended that slew-rate limited transmitters be used to minimize high frequencies on the bus, and thus help avoid possible termination/reflection issues. A LED displays the transmission direction (normally not needed). By testing different transcievers I found that the differential lines of RS485 may come to an undefined state, if neither master nor slave is sending. A pullup resitor of 1k5 to 5V on line A and a pulldown of 1k5 to GND on line B is recommended. The adapter could be plugged on the pin headers for JP1 and JP2 boat and on the interface for ISP. Only the power supply (+15 V) for regulator for Xpressnet must be connect with a separate wire from the power supply plug of the command station. I`m using long wire-wrap pin header to get a distance of 20mm between the PCBs. The processor Atmega32 of OpenDCC must be replaced by an Atmega644P, to support the second USART....

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