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|#68164 - RE: ULN2803 Output droping at 3KHz.|
Responding to: Prahlad J. Purohit's previous message
You have described the motor drive as being equipped with + and - inputs for each of its control signals. From the previous postings you have made as to how you have hooked it up with the + inputs tied off to +5volts and then driving the - inputs with the ULN2803 it seems to me that your including an additional opto coupler is not necessary at all.
You should look closely at the data book for the motor drive unit and most likely you will find that the unit already has opto-coupled inputs. This is very typical for units that support NPN and PNP type input drive configurations wherein a + and - pins are provided. It will also be very important for you to check into if the motor drive input circuit includes its own internal resistor that is in series with the + input to opto input to - input loop. Most opto input drivers I have seen include these resistors, but if your unit does not have them then you must provide then externally. The picture below shows how you can connect if the resistors are onboard the motor drive.
Now all that said, you should also be aware that there are some drives that are designed to be driven over a long cable. In some of these cases the +/- input pairs to the motor drive are actually designed to be driven from your controller board using a differential line driver chip such as a 26LS31. If your motor drive is of this type then you should use the 26LS31 or similar other type driver chip on your board. (Note that common RS422 type driver chips from various vendors can be a substitute for the 26LS31). In any case, even if your drive has these differential type inputs, I do not see the need to include additional opto couplers on your controller board.