Disappearing Key Assignments

[Sylvain Cote]

(01-02-2020 07:03 PM)ijabbott Wrote:  The 0s and 1s seem to be transposed, but apart from that it looks OK. Logic 0 is "asserted" (space) in RS-232, and logic 1 is "deasserted" (mark). The quiescent state of RXD and TXD (when no characters are being sent) is logic 1.

RS-232C use inverted logic vs voltage level, a logic zero is declared when the voltage goes over +3V and a logic 1 is declared when the voltage goes under -3V.

(01-02-2020 07:03 PM)ijabbott Wrote:  From the set-up you posted earlier, the Prolific adapter (controlled by clupdate) is acting as the DTE (data terminal equipment), and the FTDI adapter via the Null Modem dongle (controlled by the Python test program) is acting as the DCE (data circuit-terminating equipment). The Null Modem dongle will cross over RXD & TXD, CTS & RTS, DSR & DTR, and wire DCD to DSR at each end. So on the tester:

• DCD shows the state of DTR output by the FTDI before the Null Modem
• RXD shows the state of TXD output by the FTDI before the Null Modem
• TXD shows the state of TXD output by the Prolific
• DTR shows the state of DTR output by the Prolific
• DSR shows the state of DTR output by the FTDI before the Null Modem
• RTS shows the state of RTS output by the Prolific
• CTS shows the state of RTS output by the FTDI before the Null Modem
• RI is not passed through the tester (it would normally be output by a real modem)
  

Yep, you are right. RI line is wired but not driven, that is why no LED is lighten up.

Lots of peoples have a hard time understanding RS-232C DTE & DCE roles and communications direction that goes with it.
It was designed in the 1960's and reflect the reality of the equipments back then.

Data Terminal Equipments (DTE): Teletype, Terminal, Computer, etc
Data Communication Equipments (DCE): Modem, Printer, Plotter, etc.

As an example, when using a DB9 connector ...
a) the transmit data (TXD, pin 3) on the DTE side is an output line but on the DCE side it is an input line
b) the received data (RXD, pin 2) on the DTE side is an input line but on the DCE side it is an output line

It is why when using our USB-to-RS-232C DTE dongles we need a NULL MODEM dongle that switches wires to create a DCE device on the other side.
Direct DTE-to-DTE or DCE-to-DCE communication is not possible without a NULL MODEM adapter, the interface was designed at the hardware level to only works when a DTE device is connected to a DCE device.

Sylvain