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RS-232 Pin, Signal, and Cabling Information


Definitions

TxD, or Transmit Data:
It is an output for DTE devices and an input for DCE devices. This is the data channel from the DTE device to the DCE device.

RxD, or Receive Data:
It is an input for DTE devices and an output for DCE devices. This is the data channel from the DCE device to the DTE device.

RTS, or Request To Send:
It is an output for DTE devices and an input for DCE devices. This signal is typically used to gate flow from the DCE device to the DTE device. In other words, the workstation serial port would drop this signal to halt flow from the modem, and then later raise it to resume flow.

CTS, or Clear To Send:
It is an input for DTE devices and an output for DCE devices. This signal typically is used to gate flow from the DTE device to the DCE device. In other words, a modem may drop this signal to halt flow from the workstation, and then later raise it to resume flow.

DSR, or Data Set Ready:
It is an input for DTE devices and an output for DCE devices. This signal is not widely used in UNIX, except on some DEC machines, which will block on open if it is not true in some cases.

GND, or Signal Ground:
Signal return for all signal lines.

DCD, or Data Carrier Detect:
It is an input for DTE devices and an output for DCE devices. This signal is used to show that there is a valid connection between the DTE and DCE devices. It is typically used to block opens on a port before connections, and to generate UNIX "hang up" signals upon loss of a connection.

DTR, or Data Terminal Ready:
It is an output for DTE devices and an input for DCE devices. This signal is typically used in UNIX to show that the port has been activated or "opened".


Cabling Information

Sample DTE to DTE "null-modem" wirings

Null-Modem Diagrams


Notes:

All the silly ASCII art below:

I wanted to make sure this information was not locked away from those unfortunate enough to only have access to ASCII-based browsers such as lynx.

Our DB-25 connector:

We use the standard DTE pinout for the DB-25 connector, but we use female connectors to avoid possible pin shorts via pens or paper clips and such.

This connector is used on our ST-1016, ST-1616, ST-1600, ST-1002, and ST-1008 products.

                              +--------------------------+
                              | pin | signal | direction |
      Female DB-25 DTE        |--------------------------|
                              |  2  |  TxD   |    out    |
        ___________           |  3  |  RxD   |    in     |
       ( 13......1 )          |  4  |  RTS   |    out    |
        \ 25...14 /           |  5  |  CTS   |    in     |
         `-------'            |  6  |  DSR   |    in     |
                              |  7  |  GND   |    n/a    |
                              |  8  |  DCD   |    in     |
                              | 20  |  DTR   |    out    |
                              +--------------------------+

Our RJ-45 modular connector:

We use our own RJ-45 pin configuration, which is noted where appropriate below. To avoid confusion, an ASCII representation of an RJ-45 receptacle (the female connector like the ones used on our units) is shown below with pin numbering.

This connector is used on our SC-1016, SC-1600, ST-2008, ST-1800, ST-1800P, ST-1400, ST-1610, and ST-8008 products.

         .---.                +--------------------------+
        _|   |_               | pin | signal | direction |
       |       |              |--------------------------|
 .-----~       ~-----.        |  1  |  RTS   |    out    |
 |                   |        |  2  |  DSR   |    in     |
 |                   |        |  3  |  DCD   |    in     |
 |                   |        |  4  |  RxD   |    in     |
 |                   |        |  5  |  TxD   |    out    |
 |                   |        |  6  |  GND   |    n/a    |
 |                   |        |  7  |  DTR   |    out    |
 +__|_|_|_|_|_|_|_|__+        |  8  |  CTS   |    in     |
    8 7 6 5 4 3 2 1           +--------------------------+

The DB-9 PC-AT style connector:

Many people ask about configuring our ports with DB-9 (PC-AT style) connectors. For convenience, I've redocumented that pinout here, with an ASCII representation of the connector and its pin numbering.

Below, I show a few sample wiring configurations and give examples for DB25 to DB25, and then also DB25 to our flavor of RJ45. Rather than clog up the page with some redundancy, simply use the pin definitions here and translate the DB25 to DB25 cable examples below accordingly. The signal names are the same, and serve the same purpose. Only the pin numbers change in the wiring diagram.

                              +--------------------------+
                              | pin | signal | direction |
  Male AT-style DB-9 DTE      |--------------------------|
                              |  1  |  DCD   |    in     |
        ___________           |  2  |  RxD   |    in     |
       ( 1.......5 )          |  3  |  TxD   |    out    |
        \ 6.....9 /           |  4  |  DTR   |    out    |
         `-------'            |  5  |  GND   |    n/a    |
                              |  6  |  DSR   |    in     |
                              |  7  |  RTS   |    out    |
                              |  8  |  CTS   |    in     |
                              |  9  |  RI    |    in     |
                              +--------------------------+


The most popular FULL handshake null-modem adapter:

(DTE)           (DTE)
-----           -----
 SG  ----------- SG
 TxD ----------- RxD
 RxD ----------- TxD
 RTS ----------- CTS
 CTS ----------- RTS
 DSR --+
 DCD --+-------- DTR
 GND ----------- GND
 DTR --------+-- DSR
             +-- DCD

Example pin connections:
------------------------

DB-25 -> DB-25                CD RJ-45 -> DB-25
--------------                -----------------
    2 -> 3       TxD - RxD          5 -> 3
    3 -> 2       RxD - TxD          4 -> 2
    4 -> 5       RTS - CTS          1 -> 5
    5 -> 4       CTS - RTS          8 -> 4
    7 -> 7       GND - GND          6 -> 7
  6+8 -> 20     DSR+DCD - DTR       3 -> 20  (DCD - DTR)
   20 -> 6+8    DTR - DSR+DCD       7 -> 8   (DTR - DCD)
                                    2 -> n/c or pin 6 (see note below)
Notes:


The most popular NO handshake null-modem adapter:

(DTE)           (DTE)
-----           -----
 SG) ----------- SG
 TxD ----------- RxD
 RxD ----------- TxD
 DCD --+     +-- DCD |
 DSR --+     +-- DSR |- modem control loopback
 DTR --+     +-- DTR |
 GND ----------- GND
 RTS --+     +-- CTS |
 CTS --+     +-- RTS |- hardware flow control loopback

Example pin connections:
------------------------

DB-25 -> DB-25           CD RJ-45 -> DB-25
--------------           -----------------
    2 -> 3     TxD - RxD       5 -> 3
    3 -> 2     RxD - TxD       4 -> 2
    6+8+20    DSR+DCD+DTR      2, 3, and 7 not connected. (see note below)
    7 -> 7     GND - GND       6 -> 7
    4+5         RTS+CTS        1 and 8 not connected. (see note below)
                   
                  
Notes:


Possible Printer null-modem adapter

Some terminals are designed to use DTR and CTS for hardware flow control instead of the more common RTS/CTS pairing. Some serial PRINTERS are also designed this way. In these cases, the following wiring makes the most sense:

(DTE)      (Terminal/Printer)
-----      ------------------
 SG  ------------- SG
 TxD ------------- RxD
 RxD ------------- TxD
 RTS ------------- CTS
 CTS ------------- DTR
 GND ------------- GND
 DSR --+
 DCD --+
 DTR --+

Example pin connections:
------------------------

DB-25 -> DB-25              CD RJ-45 -> DB-25
--------------              -----------------
     2 -> 3      TxD - RxD        5 -> 3
     3 -> 2      RxD - TxD        4 -> 2
     4 -> 5      RTS - CTS        1 -> 5
     5 -> 20     CTS - DTR        8 -> 20
     7 -> 7      GND - GND        6 -> 7
6+8+20 -> n/c   DSR+DCD+DTR       2 -> not connected. +
                                  3 -> not connected. +-(see note below)
                                  7 -> not connected. +
Notes:


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