RS232 cables for all applications

RS232 cables can be an area of confusion. Even manufacturers manuals don't always specify the correct
configuration. Any two RS232 devices should be able to communicate using one of the following configurations, figures
A to D. Modems will need the Ring Indicator- RI signal connected. The pins of an RS232 port should not
be left unconnected as floating voltages can cause unreliable communications. Shielded, low capacitance
cables are preferable and should have Protective Ground (PG) connected to the shield. Cable length should
not exceed 15m / 50 ft. You will need to know whether the device on each end is a DTE or DCE.
DTE= Data Terminal Equipment e.g. a PC
DCE= Data Circuit-terminating Equipment e.g. a modem
If you need a null modem / crossover / DTE-DTE cable, this is the type defined in figures C and D.
If you see a cable specification which includes only TD, RD and SG (3 wire connection), you can also loopback
the handshaking lines (figs B and D). This will not affect communications and will ensure no floating voltages
are present on these lines.
RS232 voltages are:  Signal > +3v = 0,  Signal < -3v = 1
The output signal level usually swings between +12v and -12v. The area between +3v and -3v is to provide line noise immunity.





The cable in Fig E can be used to monitor communications between two devices.


Communication sequence

DTE Transmitting:
1. Assert DTR and RTS
2. Wait for DSR
3. Wait for CTS
4. Transmit data

DTE Receiving:
1. Assert DTR
2. Wait for DSR
3. Receive data

RS232 signal functions

 
RJ45 
8 pin *		 3 wire cable * D-type 
9 pin		 D-type 25 pin Meaning Mnemonic re:DTE
* varies by manufacturer
 
      1 Protective ground (shield) PG  
X X 3  2 Transmit data TD OUTPUT
X X 2 3 Receive data RD INPUT
X   7 4 Request to send RTS OUTPUT
X   8 5 Clear to send CTS INPUT
X   6 6 Data set ready DSR INPUT
X X 5 7 Signal ground SG  
X   1 8 Data carrier detect DCD INPUT
      9 Reserved for testing +V    
      10 Reserved for testing -V    
      11 Unassigned    
      12 Secondary Data carrier detect S-DCD INPUT
      13 Secondary Clear to send S-CTS INPUT
      14 Secondary Transmit data S-TD OUTPUT
      15 Secondary Transmit clock S-TC INPUT
      16 Secondary Receive data S-RD INPUT
      17 Receive clock RC INPUT
      18 Divided clock receiver DCR OUTPUT
      19 Secondary Request to send S-RTS OUTPUT
X   4 20 Data terminal ready DTR OUTPUT
      21 Signal quality detect SQ OUTPUT
    9 22 Ring indicator RI INPUT
      23 Data rate selector   OUTPUT
      24 External transmitter clock   OUTPUT
      25 Busy   INPUT