On Sun, 22 Apr 2018, Gene Heskett wrote:
Date: Sun, 22 Apr 2018 11:10:03 -0400
From: Gene Heskett <[email protected]>
Reply-To: "Enhanced Machine Controller (EMC)"
<[email protected]>
To: [email protected]
Subject: Re: [Emc-users] Mesa choices
On Saturday 21 April 2018 23:50:09 jeremy youngs wrote:
So, got the 7i90 today, been swimming through the manuals.
I haven't yet connected the board , that will likely happen mid to
late week as I seem to have all of the automotive work i want right
now ( life could be worse) . I downloaded the support software and
manuals. I am speculating that the softdmc libraries are preloaded in
the backup fpga directory on the card ? Am I correct in presuming this
?
ISTR they come unprogrammed, and set to be connected to a parport for
programming the correct config for your use into it with mesaflash. It
has a backup recovery that must be used long enough to reset it to
parport communications IF its been set to talk SPI, which is faster on
the pi's but not available on *86 machines. Except for the missing spi
capability on x86 machines, the parport on the x86 motherboard s/b fast
enough.
But note, and I emphasize this strongly, the 50 pin connectors hook
directly into the fpga, and noises above 4 volts may blow the fpga's
buffers, so the surge and noise protection of a 7i42TA, which also gives
you a handy mechanical terminal arrangement to connect it to the outside
world, and will protect it from the noise pickup from the stepper motors
etc. I destroyed several before I understood the importance of that, and
the one driving my lathe has had functions moved to different card
outputs because of blown buffers. I actually wound up putting the pi,
the 7i90, and the 3 7i42TA's in a separate box, and mounted that box to
the lid of the old rusty box the power stuffs was in. Then my noise
problems disappeared.
Just to clarify a few things, the 7I90 I/O pins are safe up to about +7V peak
(they are fully 5V signal tolerant) but since the bus switches and FPGA pins
have protection diodes to ground, any signal less than about -0.7V with
unlimited current _will_ damage the protection diodes often causing them to
short to ground.
Unfortunately with VFDs and step drives or other high frequency noise sources
its very easy to have low impedance high frequency ground offsets of greater
than 0.7V amplitude and with enough current to exceed the 50 mA rating of the
input diodes, so a mistake as simple as touching a 7I90 I/O pin to chassis
ground can destroy the input protection diode. If you use a breakout/daughter
card this is usually not an issue but if you use the bare I/O pins you have to
be very careful about ground differences. If you want to use bare I/O pins for
things like limit switches, a 220 Ohm series resistor makes it much more
resistant to excessive current from ground noise.
Peter Wallace
Mesa Electronics
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