Controlling Fischertechnik with Java
Department of Computer Science
Rochester Institute of Technology
http://www.cs.rit.edu/~ats/talks/ft/ [ pdf ] [ code ] [ code.zip ]
Fischertechnik offered a parallel interface in 1984 and a serial interface in 1997. The interfaces control four digital outputs and eight digital and two analog inputs. There is an extension box which doubles the number of digital connections. By now another company provides a USB interface.
This talk describes Java classes to access the serial interface using the Java COMM API and a driver for Windows to access the parallel interface.
Bytecodes
The serial interface has a byte protocol:
c1 mm |
tt |
|
c5 mm |
tt x1 x2 |
|
c9 mm |
tt y1 y2 |
The Java Communications API javax.comm can be used to access serial ports. Sun provides an implementation for Windows and Solaris and there is an implementation for Linux .
ft.Interface
ft.Interface contains a thread that tries to keep an up-to-date copy of the interface state by polling over javax.comm .
ft.properties has to be in the current directory. It contains lots of properties to configure ft.Interface , e.g., it decides if the analog inputs are monitored.
C> cd ft C> java -classpath .. ft.Interface COM1 100 8 true 1 100 in 20050 msec, 4 Hz
javax.comm
Only a single driver can be entered in the file javax.comm.properties .
ft.comm
ft.comm implements a very rudimentary serial driver for Windows.
ft.comm.Driver implements javax.comm.CommDriver
initialize() is called by CommPortIdentifier and loads from ft.properties among other things names like COM1 which are used as port names and must be usable to get to the device.
getCommPort() is later called by CommPortIdentifier and has to deliver a port object for a name.
ft.comm.SerialPort extends javax.comm.SerialPort
SerialPort() gets the port name and creates the streams which getInputStream() and getOutputStream() deliver.
All other methods are blanked out — as far as possible. The speed improvement probably results from not implementing listeners.
ft.comm.SerialInputStream extends java.io.InputStream
SerialInputStream() gets the port name and connects to a Windows handle.
ft.comm.SerialOutputStream extends java.io.OutputStream
SerialOutputStream() gets the port name and connects to a Windows handle.
javah creates the headers for the native methods which use Windows functions. The library is loaded by ft.comm.Driver at first access.
Speed
C> cd ft\comm C> nmake -f nmakefile -t install C> nmake -f nmakefile install C> cd ft C> java -classpath .. ft.Interface COM1 100
Parallel Interface
The parallel interface originally had many outgoing and very few incoming wires. Fischertechnik defined a way to transmit serial bits. The different wires can be influenced over time by writing the hardware port of the interface.
Input is read from the busy wire while the clock wire is toggled:
Parallel Driver
The serial byte protocol can be implemented by a fake serial driver which accesses the parallel port:
C> cd ft C> java -classpath .. ft.Interface 3bc 10000
The speed can be as high as 4500 Hz.
ft.comm.ParallelOutputStream extends java.io.OutputStream
ParallelOutputStream() is called first with the port name and records an index to access the device.
ft.comm.ParallelInputStream extends java.io.InputStream
ParallelInputStream() is called first with the port name and records an index to access the device.
The native methods are implemented in C as part of ft.comm.dll .
write has to send two bytes and calls ftOutput() with the port address and the second byte. This function toggles the data lines.
read can receive one or three bytes, depending on the last write. ftInput() and ftAnalog() toggle the data lines and watch the busy line.
Controllers and Views
ft.Interface can be viewed as a model encapsulating the state of a Fischertechnik interface. The package ft contains numerous other classes which can be used as observers and building blocks for graphical interfaces.
C> java ft.Diagnose COM1 $ java ft.Diagnose /dev/ttyS0
