...a look at the gluion's hardware is necessary, as its design differs
fundamentally from most other sensor interfaces. While the latter
are usually based on Microcontrollers (MCU), the gluion is built around
a Field Programmable Gate Array (FPGA).
Now, when you design a system around an MCU you typically write
a program (usually C or Assembler) that determines the program flow
of the internal CPU, much like on a PC. However, in addition, an
MCU provides a number of modules that perform tasks like communication
(e.g. RS232 to a host), counting (e.g. pulses from a rotary encoder),
or timing (e.g. the lag of an ultrasound pulse). Integrating those
into the system usually requires proper interrupt handling, a task
that can become tricky as the number of modules increases. Yet,
for many projects that number is actually too low, so modules have
to be shared among different functions which increases the overall
complexity and can impact the accuracy of the measurements. Finally,
each MCU offers a different set of modules so you can never be sure
that your design will survive the migration to a new chip.
In contrast, an FPGA lets you define your own modules and how many
you want of each. This is because an FPGA is essentially reconfigurable
hardware where you don't program a given architecture but design
one to your own liking. While this low-level approach may seem rather
complicated where many wheels might have to be reinvented, it actually
fits the requirements of a sensor interface well, where results
from the measurement modules simply have to be forwarded to the
host - no calculations are done, no CPU is needed.
For more details have a look at this paper: "the
gluion - advantages of an FPGA-based sensor interface"
(NIME 2006)
Now, coming back to the question, a configuration defines how the FPGA's
elementary building blocks, logic gates, are wired to each other, thereby
forming higher level modules which in turn can be connected to implement
a complete system. On the gluion there is a combination of fixed and flexible
modules. Fixed ones are present in every configuration as they handle
common tasks, like the OSC/Ethernet connection towards the host computer.
The flexible modules in turn implement various schemes for handling digital
sensors. They can be mixed and matched depending on the current requirements
- control for 60 motors in one project, a huge switch matrix in the next.
