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?

  • 0
    •  Analog Employees 
    on Mar 18, 2015 2:06 PM over 5 years ago

    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.



  • Hi Tracey,

    Thanks for the reply.  I think you're right, especially as the only other HART modem I know of (A5191HRT) doesn't have a CTS signal either, but I'm still not sure I understand.

    The /RTS is an input to the modem and is described in a different SAP - one from the Link Layer to the Physical layer.  I would have thought that the reciprocal SAP would need to be an output from the modem (please see the attached diagram).

    I thought that I might be able to implement this by entering full duplex mode and using the Carrier Detect pin, although this wouldn't actually achieve much, since the state of CD would be checked before the RTS is sent.  This would have just been a bodge to try and implement (my understanding of) the requirements.

    Also, this spec seems to suggest that the AD5700's /RESET pin is not optional, as alluded to in this question:

    since there needs to be a SAP that can be passed to the Physcial Layer in order to request it to undergo a physical reset.  Is this correct?  Thanks.

  • 0
    •  Analog Employees 
    on Mar 19, 2015 8:58 AM over 5 years ago

    Hi there,

    Upon conferring with my colleague, we agree that the CTS signal is not required in the case of a HART modem which does not have any memory. RTS/CTS signaling was used in older modems to help with flow control.

    When the master wishes to send data, it takes the Request to Send line active. If the modem has room for this data, then the modem will reply by taking the Clear to Send line active and the master starts sending data. If the modem does not have the room, then it will not send an active Clear to Send signal.

    For a transparent modem, such as the AD5700/AD5700-1, the CTS signal is not applicable.

    If you would like to discuss the DLL spec itself and a potential update to it, I would suggest contacting the HART Communication Foundation.

    Hope this helps,


  • Many thanks for your help.  Contacting the HCF is never a bad idea; I think I'll get their take on it too.