The 7i90 is designed to be programmed from an epp parport, and I don't
believe it can be programmed again once an spi interface has been put in
it with out doing some jumper moving to reset it to parport drive before
it can be reprogrammed again. Its explained in the docs for it. It is
also an unbuffered 3.3 volt device, and any noise below ground or above
3.4 volts will destroy it, so make sure you have enough 7i42TA's to
buffer and protect it. A single bolt star ground is very important
around the 7i90. Each 7i42TA buffers 1 50 pin, 24 i/o lines, so all 72
needs 3 of them and short 50 pin to 50 pin ribbon jumpers. Substitute
7i33TA's for the 7i42TA's in your case so you'll need two 7i33TA's and
one 7i42TA for the gpio stuffs since each 7i33TA only handles 4 servo's
including their encoder returns. The TA versions are worth their price
in bottled beer because of the built in screw terminals.
Cheers, Gene Heskett
Thats not quite true, the 7I90 I/O pins will all take voltages from about
-0.5V to +7V. That said, the I/O pins are bare IC pins and are easy to
damage with negative inputs unless there is some current limiting. This is
because all I/O pins have ESD protection diodes with anode to ground and
cathode to the I/O pin. These diodes are tiny and can be damaged by current
10 mA. How do you get a negative input? Its very easy, simply connecting
an I/O pin to frame ground will do it if there is any high frequency, high
current device in the immediate area (Step drives, servo drives, VFDs etc)
Also connecting any external device to the FPGA card when a common ground
has not been established between the device and the FPGA card will do
it as well. If you are using bare FPGA pins and you have noisy grounds
you can protect the I/O pins by adding a series resistor (say 470 Ohms)
Peter Wallace
Mesa Electronics
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