is that any limit on the input current for the digital IO?

[PKIM]

Hi all,

  I wonder if there is any limit on input current for the digital IO port. I have a device outputing the voltage about 0.2volt and the manual said the maximum sink current is 25mA. I am going to connect that signal to a DI input in my 6711 card. I wonder if the current is too much for the DI input or PFI?

[DECONN]

Hi PKIM,

Here is the link the spec sheet for that card:

http://www.ni.com/pdf/manuals/371011c.pdf

Keep in mind the DIO lines operate at TTL/CMOS levels. To be safe, you could consider protection circuitry prior to wiring the device. 

[Buechsenschuetz]

As you have cited correctly, your peripheral device has a SINK current specification. This means that it will accept the specified current without damage and the voltage will not rise any higher than specified. 

A true TTL input sources (delivers) a certain current in low level mode, or, in other words, a certain current has to be drawn from this input in low level mode. The peripheral device must be able to sink this current. However, the current a peripheral device has to sink to drive a single TTL input in low level mode is just 400 micro-amps. 

You can consider the peripheral device as a passive switch when its output is in low mode, and the current spec just is to limit the current across this switch. Most device outputs can sink much more than it would be required to drive a TTL input since in some cases such an output is used to drive a load (a LED indicator, for example) directly. With a sinking capability of 25mA you can easiy switch on a LED (connected to +5VDC and a current limiting resistor to protect the LED from overcurrent). 

BTW, most, if not all, digital outputs of NI products also have a current sink spec much higher than required to drive a TTL level input to low level, usually in the range of 4 to 25mA. See individual data sheets / hardware manuals for detailed specs.

[crossrulz]

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PKIM wrote: the manual said the maximum sink current is 25mA.

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That is just a maximum current.  It can certainly sink less.  The spec for the 6711 is showing that it will require a maximum of 320uA to drive it low.  This is the current you can expect your device to put out.

That maximum current your device can source/sink is important when you are trying to drive relays, LEDs, and other non-TTL/CMOS devices with the digital signals.

[PKIM]

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@crossrulz wrote:

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PKIM wrote: the manual said the maximum sink current is 25mA.

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That is just a maximum current.  It can certainly sink less.  The spec for the 6711 is showing that it will require a maximum of 320uA to drive it low.  This is the current you can expect your device to put out.

That maximum current your device can source/sink is important when you are trying to drive relays, LEDs, and other non-TTL/CMOS devices with the digital signals.

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Thanks for the reply. Sorry but I am not good at electronic, so I just wonder if 320uA is the maximum to drive it low, does it mean 25mA is too much and will burn the line?

[Buechsenschuetz · Accepted solution]

As already mentioned, there is no current flowing out of the peripheral device in low level mode.

The current specs mentioned refer to the maximum current that can be handled by the peripheral device in low level mode.

I hope not to confuse you more than necessary. On a TTL level data line, there is current flowing from the peripheral device to the main circuit in high level mode. In low level mode, there is current flowing out of the main circuit into the peripheral device. This current is inherently limited by the circuitry.

So if you have a spec of 25mA max. of the peripheral device in low level mode, there will never flow too much current out of the main circuit. You can connect both devices without any risk. The maximum current is just specified in case you need to handle more current with the peripheral device, an LED indicator or a relay for example. In this case, the current of the load must be limitied so that you do not damage the peripheral device. But - as mentioned - this is NOT the case in your application.