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?