FW: [ct-user] -Loop wiring?

Brian K. Short ke7gh@PrimeNet.Com
Sun, 25 Aug 1996 00:19:22 +-100

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RS-232 Cabling for -LOOP Option in CT Version 9

There are two types of RS-232 devices and four types of

DATA TERMINAL EQUIPMENT (DTE) (Most computers and terminals)


The connectors are DB9 and DB25, both male and female.  
Most DTEs have male connectors (at least IBM PC's).


DB9       DB25      Signal    Purpose
          1         PG        Protective Ground:  This is usually
                              the shield in a shielded cable.  It
                              is designed to be connected to the
                              equipment frame and may be
                              connected to external grounds.

3         2         TxD       Transmitted Data:  Sourced by DTE, received
                              by DCE.  Data terminal equipment cannot send
                              unless RTS, CTS, DSR and DTR are

2         3         RxD       Received Data:  Received by the
                              DTE, sourced by DCE.

7         4         RTS       Request to Send:  Sourced by DTE,
                         .    received by DCE.  RTS is asserted by the DTE 
                              when it wants to send data.  The DCE
                              responds by asserting CTS.

8         5         CTS       Clear to Send:  Sourced by DCE, received by DTE. 
                              CTS must be asserted before the DTE
                              can transmit data.

6         6         DSR       Data Set Ready:  Sourced by DCE, received by DTE. 
                              Indicates that the DCE has made a
                              connection of the telephone line
                              and is ready to receive data.  The
                              DTE must see this asserted before
                              it can transmit data.

5         7         SG        Signal Ground:  Ground reference
                              for the signal is separate from pin
                              1, protective ground.

1         8         DCD       Data Carrier Detect:  Sourced by
                              DCE, received by DTE.  Indicates that a DCE has
                              detected the carrier on the
                              telephone line.  Originally it was
                              used in half-duplex systems but can
                              used in full-duplex systems too.

4         20        DTR       Data Terminal Ready:  Sourced by DTE, received
                              by DCE.  Indicates the DTE is ready
                              for sending or receiving.

9         22        RI        Ring Indicator:  Sourced by DCE, recieved by 
                              DTE.  Indicates that a ringing signal is


DTE devices can be connected directly to DCE.  If two DTE devices
(two computers) are to be connected, a NULL modem (cross over
cable) is needed.

Here are some sample cables:
(-->-- Indicates the signal flow)

DB9 DTE Null Modem to DB9 DTE      DB9 DTE  ----- to ----- DB9 DCE 
DTE                      DTE       DTE                      DCE  
RxD  2  --<--------<-- 3 TxD       RxD  2  --<--------<-- 2 RxD  
TxD  3  -->-------->-- 2 RxD       TxD  3  -->-------->-- 3 TxD  
Gnd  5  -------------- 5 Gnd       Gnd  5  -------------- 5 Gnd  
RTS  7  -->-------->-- 8 CTS       RTS  7  -->-------->-- 7 RTS  
CTS  8  --<--------<-- 7 RTS       CTS  8  --<--------<-- 8 CTS  
DB9 DTE Null Modem to DB25 DTE     DB9 DTE ----- to ---- DB25 DCE
DTE                      DTE       DTE                      DCE
RxD  2  --<--------<-- 2 TxD       RxD  2  --<--------<-- 3 RxD
TxD  3  -->-------->-- 3 RxD       TxD  3  -->-------->-- 2 TxD
Gnd  5  -------------- 7 Gnd       Gnd  5  -------------- 7 Gnd
RTS  7  -->-------->-- 5 CTS       RTS  7  -->-------->-- 4 RTS
CTS  8  --<--------<-- 4 RTS       CTS  8  --<--------<-- 5 CTS

-LOOP Connection for CT

Make up identical Y-adapters with DB9 connectors to plug into
each computer.  This scheme lets you use the normal Null Modem
cables you use for CT operation without the -LOOP option.

DB9 Female                    
At Computer                   

RxD  2  --<--------<-- 2 RxD  DB9 Male
Gnd  5  ---|---------- 5 Gnd
           |---------- 5 Gnd  
TxD  3  -->-------->-- 3 TxD  DB9 Male
RTS  7  --|  RTS and CTS shorted
CTS  8  --|

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