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Level Voltage in NI-6537

I have a system that sends data (5V) directly to 32 lines of acquisition of a NI-6534. Now, I'm interested in using my system with a NI-6537. Can the NI-6537 receive 5V trough its lines of Data?, because I've read the NI-6537 Specifications Guide and there says: "5V TTL compatible", but in brackets.

 

Thank You.

 

Marcos Inonan

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Hey Marcos,

 

In the NI PXIe/PCIe-6535/6536/6537 Specifications document, it talks about the Acquisition Voltage Levels. The High Voltage Threshold Minimum is 2.3 V, so anything over that will read as a high value. It says 5V TTL Compatible because you can input a 5V TTL signal and the board will be fine. Also, it specs the Input Protection to be -1V to +6V, and there are even Diode clamps in the design that may provide additional protection outside this range. So all in all, yes the NI 6537 can receive 5V into its data lines and PFI lines.

 

As a side note, another reason it mentions "5V TTL compatible" is because on the Generation side, the board can not actually output a full 5V, but instead it outputs 3.3V (typical) when set to the 5V Level, which most of the time works in 5V TTL applications.

 

Regards,

DJ L.

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Actually there is a difference between "5V TTL levels" and "5V TTL compatible". A true 5V TTL input will recognize anything below 0.8V as "low level" (0) and anything above 2.3V as "high level" (1). Voltage levels between 0.8 and 2.3V should be avoided since it is not specified how the receiving device will react in this region.

 

Anyhow, a threshold of 2.3V makes sense and will work with virtually all devices having a TTL compatible output. Usually the minimum high level output voltage is specified with 2.8V, but I do not remember having seen any TTL circuit with a high output voltage of less than 3.1V.

 

Similar for the outputs of the board. A 3.3V level will be recognized as "low level" (0) by all TTL compatible peripheral devices.

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Marcos,

 

As DJ mentioned in his post the 6537 will read the logic levels according to page 4 of the 6537 Specifications. It appears like anything below 1V will be seen as a low, and anything above 2.3V will be seen as a logic high. Hope this clears the issue up.

Aaron W.
National Instruments
CLA, CTA and CPI
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