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RS232 Wiring

RS232 DB9 Connector
DB-9M Function Abbreviation
Pin #1 Data Carrier Detect CD
Pin #2 Receive Data RD or RX or RXD
Pin #3 Transmit Data TD or TX or TXD
Pin #4 Data Terminal Ready DTR
Pin #5 Signal Ground GND
Pin #6 Data Set Ready DSR
Pin #7 Request To Send RTS
Pin #8 Clear To Send CTS
Pin #9 Ring Indicator RI
RS232 9 Pin Connector

RS232 DB25 Connector
DB-25M Function Abbreviation
Pin #1 Chassis/Frame Ground GND
Pin #2 Transmit Data TD
Pin #3 Receive Data RD
Pin #4 Request To Send RTS
Pin #5 Clear To Send CTS
Pin #6 Data Set Ready DSR
Pin #7 Signal Ground GND
Pin #8 Data Carrier Detect DCD or CD
Pin #9 Transmit + (Current Loop) TD+
Pin #11 Transmit - (Current Loop) TD-
Pin #18 Receive + (Current Loop) RD+
Pin #20 Data Terminal Ready DTR
Pin #22 Ring Indicator RI
Pin #25 Receive - (Current Loop) RD-
RS232 25 Pin Connector

DTE vs DCE

This is one of the most misunderstood areas of RS-232. DTE stands for Data Terminal Equipment, and DCE stands for Data Communications Equipment. DTE is typically either a dumb terminal or the serial port on a computer/workstation. DCE is typically a modem, DSU/CSU, or other piece of data communications equipment... hence the names. This is all most people really need to know about these terms.

Where it gets confusing is when you start to talk about signal definitions and direction. For example, it's easy for someone to understand that when you transmit data, you send it out. However, when you talk in terms of the DCE, it becomes an input. This is because the specification was written from the perspective of the DTE end of the link. Another example is that the Receive Data signal is an input to DTE, but an output from DCE.

Therefore, a straight through "one to one" cable is all that is necessary between a modem and a standard DTE serial port. However, if you want to connect two DTE ports together, you have to simulate the existence of the pair of DCE devices, typically modems, that would normally be between the two DTE devices. This is where the null-modem device or cable comes in.


RS-232 signal information

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".


RS-232 Flow Control Primer

Hardware flow control uses pins >RTS and CTS to gate flow back and forth between two connected serial devices. The DTE device uses RTS to start and stop flow from the DCE device, and the DCE device uses CTS to gate flow from the DTE device.

This method is popular for higher speed connections where flow control reaction time is more critical. It's also popular where the data stream is such that embedded flow control characters can not be tolerated by the protocol running on the link. The cost is that you have to run two more wires in the cable.

Software flow control uses special START (XON) and STOP (XOFF) characters embedded in the data stream to gate flow. In other words, the receiving device would send a STOP character (typically a control-s) to the sending device to halt flow. It would then later send a START character (typically a control-q) to resume flow.

This method of flow control is more popular for slower links whose protocol can support embedded flow control characters. It's popular because it only requires that you run 3 wires for the link to function. Receive (RxD), Transmit (TxD), and Ground (GND).


Null Modem Wirings

Null Modem Wiring