On Fri, 1 Mar 2013, Klemen Dovrtel wrote:
> Date: Fri, 1 Mar 2013 23:23:48 -0800 (PST)
> From: Klemen Dovrtel <klemen_dovr...@yahoo.com>
> To: "Enhanced Machine Controller (EMC)" <emc-users@lists.sourceforge.net>
> Subject: [Emc-users] fpga epp data transfer - strange behaviour
>
> Hello everybody,
I am using fpga to monitor the incremental encoder and transfer the axis
absolute position to pc using parallel port and epp communication. When I run
the Axis GUI I am getting some strange following errors. I found out that
occasionally i get some bad axis position data transfer from the fpga. But
what is very strange that this happens only when the axis moves in negative
direction and never when it moves positive direction or stays still (please
see the image attached)). So i thought there must be some bug in pfga encoder,
not in epp communication, but i have no idea what could possibly be wrong -
the code is very simple (i pasted it below).
I increase the servo-thread period, but the result was the same. I also tried
to locate a certain position of axis when the data is bad, and i could not do
this - it seems this is happening randomly.
Does anybody have some idea where to look for the error?
Regards,
Klemen
My first guess is that you are not sampling the 32 bit count atomically, That
is, with the EPP transfers you are reading the count as 4 individual bytes so
you must latch all 32 bits of the count at the beginning of the 4 byte
transfer so that you dont have scrambled data in your 32 bit count if the
count happens to change when reading.
Peter Wallace
Mesa Electronics
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