any help about Using a different power source?

[Hessam]

Hi. I've programmed an Altera EPM7128 FPGA and I use computer's
parallel port to operate it and it works correctly but the matter is
that I've to use the same computer's 5-Volts power for the FPGA, but I
want to use a different power source. In that case I need to connect
the GND of the different power source and the parallel GND together
for making the FPGA recognizing computer signals. but in this way, the
Mainboard internal Voltage may change and makes problems for the
computer. Any Ideas about it plz? how can I use a different power
source for my circuit. ( I think it would be an easy work because for
example whole PRINTERS use computer signals but they have their own
power source straightly. But I don't know the technic. Help me plz)
THX

 

[Tim Wescott]

Hi. I've programmed an Altera EPM7128 FPGA and I use computer's
parallel port to operate it and it works correctly but the matter is
that I've to use the same computer's 5-Volts power for the FPGA, but I
want to use a different power source. In that case I need to connect
the GND of the different power source and the parallel GND together
for making the FPGA recognizing computer signals. but in this way, the
Mainboard internal Voltage may change and makes problems for the
computer. Any Ideas about it plz? how can I use a different power
source for my circuit. ( I think it would be an easy work because for
example whole PRINTERS use computer signals but they have their own
power source straightly. But I don't know the technic. Help me plz)
THX

Generally the printer and computer both use supplies that are isolated
from the power line, and their grounds are either floating or referenced
to the same ground at the socket.

If you use a wall-wart to power your circuit it'll be isolated;
connecting it's ground to your computer's ground port will provide the
correct reference.

 

[John Fields]

Generally the printer and computer both use supplies that are isolated
from the power line, and their grounds are either floating or referenced
to the same ground at the socket.

If you use a wall-wart to power your circuit it'll be isolated;
connecting it's ground to your computer's ground port will provide the
correct reference.

---
Yes, but he's got the difference in Vcc/Vdd to worry about, and if the
FPGA supply climbs much higher than the computer's, (or vice versa) it
may let the smoke out.

Optoisolation is the sure-fire (sure no-fire!) way to do it, common
ground or not.

 

[Tim Wescott]

---
Yes, but he's got the difference in Vcc/Vdd to worry about, and if the
FPGA supply climbs much higher than the computer's, (or vice versa) it
may let the smoke out.

Optoisolation is the sure-fire (sure no-fire!) way to do it, common
ground or not.

Not to mention what happens when you have two different ground
references that are 80V apart (happened about 10 years ago at work, made
some very interesting ad-hoc modifications to our I/O boards).

None the less if it's done right you can do casual PC hookup without all
the fancy isolation -- I strongly suspect that your $100 HP inkjet
doesn't have optoisolation, for instance.

 

[Hessam]

So you mean I can connect the two GNDs together and it'll work properly?