I've been playing around with RTNet (realtime ethernet). I've been looking
at it as an alternative to parallel ports, and internal cards.
I've built an embedded slave using the Atmel NGW100 board. I have a test
application that runs on RTAI linux (the master), which reads the encoder
and IO statuses, does PID calculations, then sends PWM and IO ouputs. The
unoptimised code can run at 200Hz. I'm hoping to get this up to 1kHz. This
is driving my dinky robot arm (6v 30mA motors, encoder, 485:1 gearbox).
On top of RTNet runs the TDMA protocl, time division multiple access. The
master sends out a sync frame at the start of every period (current every
5ms - 200Hz). The 5ms period is divided up into time slots, and the master,
and each slave are allocated one or more slots to send.
Now I'm trying to integrate it into EMC and I have a couple of issues.
1) RTNET wants to run the rtai timer in one-shot mode. So, I can either set
the EMC base thread to run at a high rate so RTNET can get close to when it
wants the timer to be fired, or I can modify EMC to use the one-shot timer
instead of the periodic timer. (RTNET has the option of not starting the
rtai timer and use whatever is active). How will the one-shot timer effect
EMC threads? I'm guessing this will introduce jitter. Is this a problem
for a servo system?
2) As described above, the time on the ethernet wire will be divided up into
5ms periods, with slots defined as...
0ms | 0.1ms | .3ms
Sync | Slot 0 | Slot 1
| Slave sends encoders | Master sends PWM info
In order for this to work, I need to be able to synchronise the Sync frame
of RTNet (the start of the period), with the servo thread in EMC (with
offset).
Ideally, when the EMC servo thread starts processing, RTNet has already
received the data from wire, and it is ready to be processed. So I need to
set the Rtnet Sync frame to begin at "servo thread time - (0.1ms +
transmission delay)". Rtnet currently doesn't have a way to do this. Does
EMC? If not, I need to change either EMC or RTNET to support this. Does
EMC need a facility like this? On the Rtnet mailing lists, I was asked if
EMC supports realtime events like Fieldbus, but I have no idea what it is?
3) And finally, when I create my realtime hal component to do this, should I
be using the function interface of EMC, or just use RTAI events?
If I use hal functions, then I rely on getting question 2) solved. I need a
read() function to be called at the start of the cycle, and a write() at the
end. Then I need to ensure they are called at the correct times.
The other way I was thinking of handling it would be to use RTAI directly,
setting up a read task and a write tasks that fired independently, and just
kept the hal pins and paramaters up to date. This would have the problem of
possibly being 0.5 - 1.0 cycles out of sync, but then there wouldn't be
timing issues of missing read/write time slots.
Any thoughts?
Thanks,
Frank
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