PCIe-7842R (R series FPGA) digital output not work properly

[iron_curtain]

Greetings,

I'm having some problem outputing correct TTL voltage with my PCIe-7842R FPGA board.
 

The block diagram of my orignal LV FPGA code is shown in "analog to digital.png". The idea was to convert an analog input (decimal value) to a 16-bit binary code and output it through 16 DIO ports. I'm using SCB-68A connector block as the terminal and connected it to the FPGA RDIO Connector 1 with SHC68-68-RDIO shielded cable.
 

The code compiled ok. But during the test, I noticed some ports didn't output TTL levels correctly. For example, for input 1000 decimal,  I would expect binary code 0000001111101000. However, some ports (DIO #6, #7, #9, etc), which are supposed to output ~3.3V TTL high (i.e., digital 1), actually output 0.8V. I attached the measured result in "digital output test.png".

To make sure the issue was not due to the LV code, I did some further test on DIO #6 with a basic example (simple digital output.png). The output was ~1V this time at digital 1.

This is really confusing as the digital output is supposed to be straightforward. I used the same FPGA board to control shutters with TTL signals before and it worked fine.

Did anyone have similar issues? Any suggestions are greatly appreciated.

[crossrulz]

What are you using to measure these voltages?  What are the DIO connecting to (past the terminal block)?

[iron_curtain]

Hi crossrulz,

I used both multimeter and oscilloscope to measure the voltages and got same readouts. 

DIOs are connected to a Cambridge Tech digital galvo controller. But I measured the voltages at the connector block terminals.

[crossrulz · Accepted solution]

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@iron_curtain wrote:

DIOs are connected to a Cambridge Tech digital galvo controller. But I measured the voltages at the connector block terminals.

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If you disconnect the DIOs from the galvo controller, do they look good (have the right voltages).  Do you know how much current those inputs to the controller need?  I'm thinking you are hitting the limit on the total current available for the DIO in the board.

[iron_curtain]

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@crossrulz wrote:

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@iron_curtain wrote:

DIOs are connected to a Cambridge Tech digital galvo controller. But I measured the voltages at the connector block terminals.

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If you disconnect the DIOs from the galvo controller, do they look good (have the right voltages).  Do you know how much current those inputs to the controller need?  I'm thinking you are hitting the limit on the total current available for the DIO in the board.

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crossrulz, you are right. I disconnected the DIOs from the controller and now the voltages are correct. This is interesting, because the controller itself was powered off during all the previous test.

If I do need to drive the controller digitally with the FPGA, is there possible solution? Thanks very much.

[crossrulz · Accepted solution]

Tyically in these situations, you use an extra chip that acts as an isolator to allow for more current.  Go look on Digikey for some buffers and you will find plenty of parts that should do the job for you.  You will likely need another power supply for thiese chips.

[iron_curtain]

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@crossrulz wrote:

Tyically in these situations, you use an extra chip that acts as an isolator to allow for more current.  Go look on Digikey for some buffers and you will find plenty of parts that should do the job for you.  You will likely need another power supply for thiese chips.

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I will try that. Thanks for much for your help!