Here is a decent reference for pinouts and meanings
[Link: interfacebus.com]The provided explanation of RTS must be read in the context of a modem (a Data Set). The device raises it to the modem (it is an output from the device (Data Terminal) to the modem (Data Set). The description in the reference is poor in that it does not consistently talk about a single entiry e.g. the connector on the device (Data terminal).
In Null Modem work CTS may also be used to control transmission. If held asserted, then the device may transmit freely. If controlled the device may transmit only when CTS is asserted. This then becomes the crux of the matter when there is no modem. How does one control CTS at the distant End from the near End? RTS, a controllable output signal on a good implementation may be used to act as CTS at the other end. In this case a 5 wire cable is needed with RTS crossed over to CTS just as Tx is crossed over to Rx. The real secret of dealing with RS232 cables and modems/null modems is understanding what is supposed (according to the specs) to be going on.
In RS232 work (but of course Marantz or Phgilips may not be obeying the RS232 protocols) RTS and CTS are the pin pair that are used to control transmission, sometimes called "flow control". RTS is an output signal (on either side of the cable). No device should ever look at RTS to make a decision. It asserts RTS when it wants to send data (RTS=Request To Send). CTS is the pin that is always an input. Its meaning when asserted is that the device is being told that it is okay to transmit. That is why tying RTS and CTS together on one side of the cable should satisfy the RTS/CTS signaling protocol. Doing this does eliminate flow control and the receiving device must be able to receive data at any time no matter how busy they are.
A terminal device may not always be ready to receive data. If that is the case some sort of flow control is needed. Flow control is performed by controlling the transmission of data, not its reception. Flow control is initiated by the receiving side and obeyed by the transmitting side.
The two other pins of interest are DCD (DCD-Data Carrer Detected) and DSR (DSR=Data Set Ready). They are inputs to a device. DSR says the data set, the modem, is ready for operation, and the DCD signal says that a carrier has been detected by the data set (modem) so data may be forthcoming. In three wire work a device should assume DTR and DSR are always asserted as there is no modem.
IMHO a well engineered serial device will supply on one of the pins (normally DTR) a signal that is always asserted as long as the unit is powered up and the interface is ready to receive data. This signal can then be used at the other end to satisfy DSR and DCD.
What you are describing is interesting. My understanding is that the Philips RFX devices only use three pins of the DB9 connector (2,3,5) and no other pins contain voltage or signal. Bear in mind that I do not own or use any RFX devices, so I am talking only theory. In theory, theory and practice are pretty close, In practice they are not.
Hope that helps explain it.
Last edited by Barry Gordon
on December 2, 2008 11:07.