Lego9771 driver for Linux

Phil Jordan, phil@philjordan.eu

About

The Lego DACTA 9771 is an ancient ISA board which connects to a console with
plugs for 4.5V Lego devices - 6 on/off outputs, which double as 3 +/-/off
outputs (for motors) in groups of 2. There are also 2 digital inputs for
optical (or other) sensors.

Driving it

The kit came with some fairly useless DOS software and a booklet with some
printed BASIC source code as an alternative method of driving the hardware.
The crucial parts of this are IN function calls and OUT statements, which all
read/write from I/O port 925. Obviously, the original source code itself isn't
much use on modern OSes, where hardware access must be done from kernel mode;
so I wrote a Linux driver. The board does not appear to raise any interrupts,
so the sensors must be polled.

In theory, it should be possible to grab the port from user space using
ioperm() and read/write with inb()/outb(). This approach seems to segfault,
at least on OpenSUSE 11.2. (the code is in the userspace/ directory, maybe
someone can figure out what's wrong) Even if it did work, it would require
the process to run as root, at least for the ioperm() call. The logical
conclusion is to use a simple kernel module.

Kernel Module

The source (GPL/BSD license) is in kernel_module/ and should build for the
running kernel if the headers are linked from /lib/modules/`uname -r`/build
which is the case on many distros.

It creates a character device on load - currently major/minor numbers are
dumped to the kernel log, so check  dmesg|tail  and then create the device
with
mknod /dev/lego9771 c <major> <minor>

Contribution of a udev script would be much appreciated. :)


The Character Device

Once the device is live on /dev/lego9771, it accepts bytewise reads and
writes. When writing, the low 6 bits of the written byte drive the 6
correspondingly numbered outputs, and the high 2 bits are ignored. When
reading, the low 6 bits return the current state of the outputs, whereas
the high 2 bits represent the states of the sensors connected to ports 6
and 7.

On the command line,
dd if=/dev/zero of=/dev/lego9771 bs=1 count=1
turns off all outputs, whereas
dd if=/dev/urandom of=/dev/lego9771 bs=1 count=1
sets the outputs to random states.

To read out the sensor state, try
dd if=/dev/lego9771 bs=1 count=1 | hexdump -C

For more sophisticated uses, you'll need to write some code.

Have fun!