I will be interfacing my processor (as a DCE) to a workstation/computer(DTE).
The processor only has RX, TX, CTS, RTS signals. I am wondering what I do with DCD, DTR, RI signals. Does the workstation/computer care what these levels are at?

I saw one design just tie them all together(DCD,DTR,RI). Not sure that makes sense to me.

 

hi Matt.
DTR is an output pin, DCD and RI Input pins, so tying the two inputs to an output is a common connection.

E

 

You tie DTR (out from terminal) to DCD (in to terminal) so the terminal will see Data Carrier Detect (DCE ready) whenever it raises Data Terminal Ready. Ring Indicator would be left unconnected, or tied 'inactive'.

 

You tie DTR (out from terminal) to DCD (in to terminal) so the terminal will see Data Carrier Detect (DCE ready) whenever it raises Data Terminal Ready. Ring Indicator would be left unconnected, or tied 'inactive'.

So are you saying the DTE(computer) will be ignoring the RI signal? Could I just tie them all (DCD, DTR and RI) together?

 

I always thought generally speaking with a normal serial port driver those signals are ignored. The driver only recognizes RTS and CTS and they can be enabled / disabled depending on your actual needs. You can set or read DCD, DTR, RI signals with the driver, but it doesn't actually depend on them for anything.

 

You can do whatever you want with those signals. In the past I used them as cheap Digital IO between your processor and workstation PC. They have nothing to do with UART communication anymore for the most part. If you don't need them just leave them disconnected. They used to be used when we had the old fashioned modems of the past that used a telephone line.

 

With RS232, any line can be shorted to any other without consequences. The old spec used +-25V signals.

RTS/CTS and DTR/DSR have been really messed up from their intended use and usually implement hardware flow control. Hardware flow control is SUPPOSED to be RTS/CTS.
e.g. Request to Send, Clear to Send. DTR and DSR effectively checked for power.

It's been a number of parts/pin issue.

the secondary part of the RS232 spec is "Modem control".
RI - the phone is ringing
*** - Answer the phone (I think that's RTS or request to send)
DCD - You got mail or (Carrier was Detected)
CTS - Clear to Send

You had full "null modems" which would make a serial line which had modem control, to behave as if a modem was attached.

Then there is the simple:
9600 baud, 8 data, 1 stop bit, No parity and no flow control
It might be used for quick and dirty configuring. The lines would be short and the stuff typed would be short.

I had voltage issues too with one device and a Macintosh computer. You can massage RS422 to work as Rs232 at short distances, but it isn't RS232.

RS232 has evolved into a real mess.
The DE-9 connectors took over. They are erroneously called DB-9.
Then there are 8 conductor and 10 conductor modular pinouts, where nothing is standard.