AD5700/AD5700-1 Clear To Send (CTS)


The HART Protocol Token-Passing Data Link Layer Specification (HCF_SPEC-81 Revision 8.2) talks about Service Access Points (SAPs) that must be supported by a field device.

One of the Physical Layer SAPs is described as follows:


This SAP is passed back by the Physical Layer to confirm its ability to accept data for transmission, or to indicate it will accept no further data.  In a modem implementation this SAP is usually mapped into the clear to send (CTS) signal from a modem."

Since the AD5700/AD5700-1 devices don't have a CTS signal, how can this requirement be implemented?

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  • Hi there,

    As implemented in the AD5700/AD5700-1 devices, the /RTS (Request To Send) digital input covers the two cases of:

    1. enabling the demodulator and disabling the modulator - i.e. modem is waiting for CD to assert and thus begin receiving data and passing this data back to the controller via RXD.
    2. enabling the modulator and disabling the demodulator - i.e. modem is transmitting data received from the controller via TXD onto HART_OUT.

    From the SPEC-81 description, the role of the CTS (Clear To Send) signal is to confirm ability to accept data for transmission (which is what /RTS low does) or to indicate that it will accept no further transmission data (which is what /RTS high does).

    Please let me know if you have any further questions.



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