Digital I/O needs a ground reference, so keep all grounds tied to your cards DGND. The floating signal source terminology only applies to analog inputs.
Caution- digital I/O typically only accept 5V max, check the specification listed in the manual of your card. Just bringing this up because you mentioned a 9V DC power supply.
Could I do it like this?
It seems that you want to simulate the arrangement of switches shown in the diagram by a digital I/O board.
(This is the conclusion, but could you explain the blue text, i believe that way may make me draw the conclusion)
It is correct that all the input lines of your circuitry must be connected to digital outputs of your board.
Anyhow, to work properly, DGND of your digital I/O board MUST be connected to GND of your circuitry. Current always needs a complete circuit to flow. You have to complete the circuit by not only connecting the signal lines but the GND connection of your system, why not connecting to the computer ground? Could you pls explain? I don't understand here.
It seems as if your set up does not have a GND terminal or connector, my setup has a negative terminal, the source of 9V DC power supply is without earth but you definitively have to connect GND of the excitation board (dig I/O board) to GND of your existing system. why? DO NOT rely on earth connections between systems. (Did yo mean the fixture with eath and the measurement system with the earth? ) Your setup MAY work without a separate GND line between the dig I/O board and the circuitry, since in most PCs GND is connected to power supply earth, and if the designers of the circuitry (you mean my test fixture here?) have used the same grounding scheme (you mean both my fixture and the computer use the earth ground here?) , this might work. But this is far from a proper grounding scheme and may cause severe hazzle.
As long as you do not have a very noisy environment and/or very long cables, in most cases you can also use the Power GND pin of your dig I/O board for GND connection.
Attached are two setup. I don't understand why I could not set up this way. Could you please explain?
I think this is very important
The last one is the correct setting, could you help me to see if it is what you are talking about?
On the Original setting you simply connected the digital input you wanted to test to BOTH dig output AND DGND. This, of course, will short the output (short the output mans it is only wire without connecting to ground, is that right? How about computer ground, DGND wire is not also connected to ground if my meaning first correct) , and no test signal is generated. (I believe if you answer me the first part, I will understand. If not, would you pls describe a little bit?
The 'Setting is right' seems to be correct. However, I would suggest to connect DGND directly to the neg terminal of the 9VDC power supply. This will avoid noise generated by excessive long and winding GND connections. (Did you mean the lead cable wire from the groud of the push buttons in the fixture to the DGND will contain noise, how come the wire put in the negative terminal of the 9V DC using the same wire will have no noise, am my understanding correct first, if so, would you pls explain?
The third setting will only work (and probably not very good) if there is parasitic GND connection via the mains supply earth (here you meant computer ground connected to the earth, right?). Similar to a supply circuit (supply circuit means a signal source with a power supply and a resistor in a close loop circuit, right) , a signal circuit must always be closed, i.e. both need a return path. In this setting the return path is open (in that case, the return path isn't using the computer ground like connecting the ground of the fixture, could you pls explain and I don't understand), and you very probably will never get proper test signals.
4) According the right setting in the second paragraph, why the "reset" light on the test fixture has a dimm ligh even the power switch is turn off when connecting to DGND from test fixture to the CB68LP. I know computer has the electricity. Even I make a program with a push buttons, then I turn on and off, the dim light still there, why? Could you pls explain? Without DGND connected, the test fixture is normal without any light on in no electricity provided.
5) As you can see this setup, I use DMM to test the gournd of the fixture and DGND in the CB68LP, they all go "beep". That means they are on the same ground wire. I have a BNC cable (hard ground) connected from the fixture to NI5112 board, everthing perfer to the hard ground, why digital signal does not flow to the hard ground, is that because when using DGND, everything in Digitla I/O lines and DGND will flow to the connection point which is the ground of the push button of the fixture, so it does not affect the operation. am i correct?When using AI+ and AI- on the CB68LP board with the Digital I/O and DGND using at the same time. They are not affected each other. Is that because Digital part is doing digital part and analog part is doing analog part. Analog part will not go over to the digital I/O and digtal I/O will not go over to the analog I/O except they have common ground. THat is why I use diff mode. Am I correct?
Pls answer the blue text:
Anyhow, to work properly, DGND of your digital I/O board MUST be connected to GND of your circuitry. Current always needs a complete circuit to flow. You have to complete the circuit by not only connecting the signal lines but the GND connection of your system, too.
How come you don't afraid of the ground loop issue?
It is much better (and probably will avoid some trouble) to connect the signals as directly to the dig I/O board as possible, this involves using the DGND terminal of the board.
Are you saying itis much better to setup like the diagram you verify in the previous mail? If so, what other methods could I do? I could not think of.
Yes. Using the 'parasitic' earth connection is VERY unsuitable in this case. You should always use a direct GND connection. Also, when both systems (computer and test fixture) are earthed, you should connect them to the same mains outlet if possible, there may be parasitic voltages between earth terminals of distant mains outlets.
You are talling about different ground (earth) here. How come the digital circuit components can not solve this problem? Even the fixture has no earth connected, only digital I/O lines connected to earth. If the the earth (computer) has a very large ground 4.5V and the fixture has 0.3V. How come the digital components impendance can not solve this problem