I have a question about using a AI channel that has been configured for TTL logic as a digital input? what we have done is used a USB-6211 device as a controller for a machine but are in need of a couple more input channels. so Our programmer configured two of the AI channels as DI to get more inputs. the issue is I have had to add an external resistor to get the channels to function properly as an input. I was wondering if anyone has done this and what if any potential pitfalls I need to look out for?
I have just wired digital lines straight into the Analog Input with no problems. The only real issue is that you have to read the voltage and compare it to some threshold in order to get into a boolean. If you find you need a bunch more DIO, look into getting something like the USB-6509 (96 DIO).
Okay, question 1: do you actually mean TTL? 1970's transistor-transistor logic, where current flows INTO the output terminal during the low state? Because any NI DAC input will be high-impedance, and incapable of sourcing the needed current. Hence the resistors.
If you mean simple 0-5V voltage levels, then you should be fine. Be careful about edge speeds and ringing, but the termination resistor you mention should see to that.
Of course, you could go completely old-school, and us a 74LS151 as an input multiplexer, and get 8 inputs while consuming 3 outputs and 1 input on your 6211, or 16 inputs while consuming 4 outputs and 1 input with an 74LS150. Obviously both will have much overhead in scanning & software.
Thank you for your response. I will look into the other USB device. The plan was to use this device as a proof of concept of using NI products for machine control. ultimately the goal is to get to a FPGA device and make it totally stand alone. this way we will not be susceptible to windows crashes.
I like the USB 6509 but there is a side function that we use and require an analog output for basically a signal generator. we use this sine wave that is generated to manipulate a tension controller and give us a cyclic tension test that is very accurate.