Resistor choose in PCI-6527 card

[labviewer]

I think I need more help for PCI-6527 card.

Problem 1: In PCI-6527 user manual,page 3-11, last paragraph about the source current. The instrument require digital I/O card can source current 7mA (better10mA) on each pin for activating switch. The switch is TTL(0-1V/3.5-5V)(Which is all information the instrument provided). If I choose the resistor 5K which is recommended in the manual, the source current will be very small(0.3mA) and can not achieved the current for activating switch(10mA). Is my understanding correct or not? If not, how should I choose right resistor value for my current application?

Problem 2 : for current application, I can use the 5V source board since total current on all channels below 1A. Also I can use gro
und line of board to connect to DIG- pin for reference line.Is it right?

Problem 3: Three pins of instrument cable(25 pin sub-D cable) are required to connect to ground line, could I connect these three pins directly to ground line on board or I have to use additional ground lines.

Thanks
zhou

[nestor-sg]

Zhou,

Answer 1: The formulas for choosing your resistor are there in page 3-11 and 3-12. Your instrument switch is going to be connected to VOUT (DIG+ and DIG-), you have to check your instrument to see if it requires only the 10 mA current for activating or if it requires 5V or if it requires both. In case it requires only 10mA if you perform the formula V=RI, you are going to find that you need a total resistance of 500 ohms, your board already has a 35ohm resistance so you need to connect a 465 ohm resistance in serial but not as RL, in that case RL is just jumper wire (0 ohm) your 465 resistance is in serial connection with the 35R and all of them make VOUT. Just take a note that your switch may be acting as a relay and it may have a coil on it which adds mo
re resistance, so measure the resistance also of your switch and the sum of 35R + your switch coil resistance + your configurable resistance = 500 ohms, this is in case you need 5V and 10mA in the input of your switch. Check this out because switches may need only to be applied a certain current or a certain voltage but not usually both.

Answer 2: If you are using the 5V source of the board for DIG+ you have to use the ground reference of the board also for DIG-... if you are using an external 5V source, then you need DIG- to connect to the external ground.

Answer 3: We don't know what are this three pins for. Your switch must have a + and - inputs, in which you directly connect DIG+ and DIG-, are these GND pins this - inputs ?... if so then yes. If not, then you should what are this GND pins for in your instrument, if they have other functionality most probably you don't have to connect to the board ground, because you are not getting any source from the board, you are only act
ivating the switches, but I cannot guarantee this because I don't know your instrument.

Hope this is helpful for you Zhou, good luck!...

Nestor Sanchez
Applications Engineer
National Instruments