The digital I/O of USB-2408

[Roger Lo]

Hi everyone,

Recently we've tested the USB-2408 and found some issues.

1. For the analog input channels, we set up a power supply to generate +5VDC as an input. The voltage measurement is working fine. But we found the other channels those have no input also show the same voltage as the channel with input.

For example, as the attachment analog_input_0.jpg we set up +5VDC to channel 5. There is no load on channel 6. But the software shows the channel 6 with +5VDC input. It is the same as other channels those are without any load.

BUT when the channel 6 are set up with a load, like +5VDC power input, it works normal.

Is it normal?

 

2. We know the default setting of digital I/O is pull-high. But when the power supply is connected to the USB-2408 and isn't turned on. The digital input indicator on the DAQami  shows "low". And then we turned the power supply on, the indicator shows high.

Is it correct?

 

3. We tried to set up a loop back test. We connect one of the digital output channel (DIO3) to one of the digital input channel (DIO4).

We found when we set the DIO3 as high status, the DIO4 represents low status. The DIO3 is low, the DIO4 shows high.

Is it normal?

 

Thank you

 

[JRys]

1. It's normal to see stray voltages on unused channels. Attach them to a ground pin or turn them off. On many other devices, you can attach +5V to the first channel and if you look at all the channels, you can see the +5V decay exponentially from the first to the last. This is because there's a tiny charge passed from one channel to the next, and if the channel is open, it has nowhere to go but to the next channel until it peters out. 

2. Check with the power supply manufacturer about their output impedance when the unit is turned off. I'm betting there's a low impedance path to ground that drags the input low. 

3. The digital output uses an open-drain configuration, which acts like a switch or relay. When you turn it on, the output has a low impedance path to the ground, so it's low. When off, it's high impedance with a pull-up to +5v. Think in terms of negative logic.

[Roger Lo]

On 9/26/2023 at 8:26 PM, JRys said:

1. It's normal to see stray voltages on unused channels. Attach them to a ground pin or turn them off. On many other devices, you can attach +5V to the first channel and if you look at all the channels, you can see the +5V decay exponentially from the first to the last. This is because there's a tiny charge passed from one channel to the next, and if the channel is open, it has nowhere to go but to the next channel until it peters out. 

2. Check with the power supply manufacturer about their output impedance when the unit is turned off. I'm betting there's a low impedance path to ground that drags the input low. 

3. The digital output uses an open-drain configuration, which acts like a switch or relay. When you turn it on, the output has a low impedance path to the ground, so it's low. When off, it's high impedance with a pull-up to +5v. Think in terms of negative logic.

Dear JRys,

Understood.

Thank you so much for your reply