NI Multifunction I/O Device
Selecting a DAQ Board
NIMH MonkeyLogic works with any NI board that is supported by the NI-DAQmx driver. There is no need to buy a new board, if you already have one from your old systems. If you buy a new one, pick up the one that can handle all your I/O needs in one board. It will save the overall cost greatly. Usually new products using the PCIe bus are cheaper than old PCI-type boards. The following table lists some NI boards that are suitable for general I/O requirements.
|Model||Analog Input (SE / DIff)||Analog Output||Digital I/O|
|PCI-6221||16 / 8||2||24 (P0: 8, P1: 8, P2: 8)|
|PCI-6229||32 / 16||4||48 (P0: 32, P1: 8, P2: 8)|
|PCIe-6320||16 / 8||0||24 (P0: 8, P1: 8, P2: 8)|
|PCIe-6321||16 / 8||2||24 (P0: 8, P1: 8, P2: 8)|
|PCIe-6323||32 / 16||4||48 (P0: 32, P1: 8, P2: 8)|
To connect external devices to the NI board, you need to know which pin is mapped to which signal. You can find this information in the product's datasheet (PDFs on the web) or by right-clicking on installed devices in the NI Measurement & Automation Explorer (NI MAX) software (Windows help file).
Inputs from external devices can be connected to NI boards via NI terminal blocks. Unshielded screw terminal blocks, such as CB-68LP and CB-68LPR, are low-cost and good for designing a custom interface box of your own. Or you can choose BNC terminal blocks, like BNC-2090A, if you want some ready-made solution. For the details, please refer to the NI DAQ Multifunction I/O Accessory Guide.
AI Ground Configuration
To measure analog voltage signals accurately, you need to know whether the signals are ground-referenced or floating and configure the data acquisition device (i.e., NI board) accordingly. NI boards require different wiring schemes for different ground configuration, so you should be aware how the signal sources are connected to your board. There are typically three ground configuration modes available for NI devices; differential (DIFF), referenced single-ended (RSE) and nonreferenced single-ended (NRSE).
If your signal sources are referred to an absolute voltage reference, such as earth or building ground, you can say your signals are ground-referenced and use the RSE mode. However, in a typical lab environment where many devices run on custom power supplies or batteries, it is likely that your signals are floating sources which are not tied to a fixed reference. Then, you should use DIFF or NRSE. For more information, please refer to this online document, Field Wiring and Noise Considerations for Analog Signals.
Below are the instructions how you should connect the leads from your signal sources to NI devices, according to the ground configuration of your choice. You also need to change the [AI configuration] option on the MonkeyLogic main menu.
Differential mode (DIFF)
In the differential mode, measuring a signal requires connecting one lead of the signal source to an AI channel pin (AI 0, AI 1, ...) and the other lead to a channel larger by 8. For example, if we measure a signal on channnel 0, one lead goes to AI 0 (Ch 0+) and the other to AI 8 (Ch 0-). This mode can deliver more accurate measurements with less noise but takes twice as many channels of the NI board as the other modes.
Referenced Single-Ended mode
In this mode, the measurement is made with respect to the common-mode voltage, for example, the earth ground of the wall outlets. For this configuration, one lead of each signal source should be connected to an AI channel pin (AI 0, AI 1, ...) and the other lead to AI GND.
Nonreferenced Single-Ended mode
If you have many custom devices to connect and you are not sure whether they are properly grounded to earth, Nonreferenced Single-Ended should be the mode of your choice. In this mode, one lead must be connected to an AI channel pin (AI 0, AI 1, ...) and the other lead to AI SENSE.