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PLC-FPGA Interface

Discussion in 'General Electronics Discussion' started by Kamy_king, May 4, 2011.

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  1. Kamy_king


    May 4, 2011

    I am working on an FPGA-based board that will serve as an interface between an Allen Bradley PLC and remote I/O. The PLC uses 24V I/O (opto-coupled, sinking outputs) and the FPGA uses LVTTL I/O but is 5V tolerant.

    I am looking for a simple and cost-effective method to change the 24V sinking outputs of the PLC to LVTTL or 5V and the FPGA outputs back to 24V. The PLC inputs can be wired as sinking or sourcing depending on the FPGA-board design. There are several I/O lines, so real estate is a concern.

    The FPGA board gets the 24V supply from the PLC side and a DC-DC converter and voltage regulators are used to obtain the supply voltages required by the components on the board.

    I have considered using a diode and a resistor for the PLC outputs and a switching transistor (2N3904 or 2N3906) for the FPGA-to-PLC conversion, but I am unsure about whether these are the best choices for this application. Is it better to use opto-couplers?

    Thanks in advance.
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