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7318 Model S20 Guide and Reference

Appendix B: Wiring Serial Devices

For many installations, the most troublesome part of installing communications equipment is making the physical connection work. The following information describes the terminology and issues involved in making a successful connection:


Two types of devices can connect to a serial port: Data Terminal Equipment (DTE) and Data Communications Equipment (DCE). As you might guess from the name, a terminal is a DTE. A modem is the most common example of a DCE. These terms derive from a phone company perspective as seen in the Phone Network figure:

A DTE is any device that drives the transmit data signal and listens to the receive data signal. A DCE is any device that listens to the transmit data signal and drives the receive data signal.

Logically, the 7318 can be a DCE or a DTE, depending on the application, since it can communicate with both terminals and modems. Physically, the 7318 ports are wired as DTEs. To connect another DTE device, such as a terminal, to the 7318, you must use a null modem.

A null modem is a connection device that eliminates the need for the modems and phone networks between like devices (two DTE devices). For more information on null modems, refer to "Null Modems".

Communications Standards

For two devices to communicate, they must speak a common language (use common electrical signals). To provide a common language for all manufacturers, the Electronic Industries Association (EIA), a U.S. standards body, devised the RS series of standards. The three most common standards in use for serial devices (terminals, printers, and modems) are:

In Europe, CCITT, an international organization, has published equivalent standards as shown in the following table:

Communications Standards
RS-232 V.28
RS-423 V.10
RS-422 V.11

The 7318 uses the RS-423 standard on its ports. As used in the 7318, this standard is compatible with both RS-232 devices and RS-422 devices and offers performance benefits over RS-232. Refer to "General Specifications" for information on RS-232 and RS-422 interoperability.


Common communications standards, such as RS-232, define many different circuits as part of the interface between the data communication equipment (DCE) and the data terminal equipment (DTE). Terminals do not use many of these circuits. Furthermore, not all terminal manufacturers interpret and use these circuits in the same way. Terminals commonly use the following circuits:

Commonly Used Circuits
Symbol Name Direction Function
TD, TxD, XMT Transmit Data From DTE The terminal sends information on this circuit.
RD, RxD, RCV Receive Data To DTE The terminal receives information on this circuit.
DTR Data Terminal Ready From DTE Indicates that the terminal is operational. Some terminals use this circuit for flow control.
DSR Data Set Ready To DTE Input to the terminal indicating that the DCE is ready. Some terminals will not receive if this circuit is not asserted.
RTS Request to Send From DTE On most terminals, this circuit is always asserted.
CTS Clear to Send To DTE Indicates that the DCE is ready to receive. Most terminals will not transmit if this circuit is not asserted.
DCD Data Carrier Detect To DTE Indicates that the DCE has a carrier, a fact that is of no significance to ASCII terminals. Some terminals will not receive if this circuit is not asserted.
GND, SGND Signal Ground
Common reference point for all data and control circuits.
TD REF, TXD Transmitter Reference From DTE Reference circuit for transmitter circuits in RS-423 and RS-422 implementations.
RD REF RXD Receive Reference To DTE Reference circuit for receive circuits in RS-423 and RS-422 implementations.
PGND Protective Ground
Used for the shield of a shielded cable.

Usually, you can get a terminal talking if you connect transmit, receive and ground, leaving the other signals unconnected. Adding other control signals can sometimes increase the performance or functionality of the terminal.

For the RS-232 standard, there is a single reference point for all circuits called signal ground. Conceptually, its voltage relative to the earth ground is zero. However, in a realistic setting (a building with many electrical distribution paths), the voltage of signal ground relative to earth ground may not be zero and may even be different between the DCE and the DTE. This voltage difference between the DCE and the DTE, referred to as ground shift, can make all other circuits, especially data circuits, less reliable and prone to errors.

Newer standards (such as RS-423) separate the reference points of the transmit and receive circuits to reduce ground shift. When using separate transmit and receive ground references, the receive reference of the DCE connects to the transmit reference of the DTE and vice versa.

On the 7318, the TD REF circuit is the reference point for TD, DTR, and RTS. The RD REF circuit is the reference point for RD, CTS, and DCD. When communicating with an RS-232 terminal, these two circuits are tied together at the terminal to the signal ground circuit of the terminal.

When communicating with an RS-423 terminal with separate receive and transmit reference points, the 7318 GND circuit should be connected to the terminal's receive reference circuit and the 7318 RD REF circuit should be connected to the terminal's transmit reference circuit.

The RS-232 standard contains another circuit, called protective ground, that has nothing to do with any of the other circuits. Generally connected to the chassis of the device, it may also connect to the shield in a shielded cable. The 7318 does not support protective ground.

Null Modems

When connecting a DTE-wired terminal to another DTE-wired device, such as the 7318, you must make a circuit that replaces the modems and phone network. A null modem performs this function. To make a null modem, reverse the transmit and receive signals.

A null modem permits data flow between devices but does not pass control information. The control signals provide additional functionality useful with some applications. Refer to "Connector Pinouts" for the pinouts of a null modem cable that connects a terminal to the 7318.


Two types of connectors are commonly used for serial connections on terminals:

D-type Connectors

D-type connectors (named because of their shape) come in 25-pin (DB-25) and 9-pin (DB-9) configurations. Most terminals use the DB-25 connector. The RS-232 standard says that the DTE should have a male connector, but many terminal vendors use a female connector instead.

Some terminals have two DB-25 connectors that are often labeled modem and aux . To connect to the 7318, you should usually select the port labeled modem . In most terminals, this port is DTE-wired regardless of what the connector looks like. A terminal adapter (Feature Code 7904) is available for use with terminals with female DB-25 connectors such as the 3151 or Wyse. This adapter is wired as a null modem so that you can use a straight-through cable to attach the terminal to the 7318.

Most PCs use a male DB-9 connector wired as a DTE for their COM1 port. Some terminals also use DB-9 connectors. The DB-9 uses the same circuits as the DB-25 but connects them to different pin numbers.

MMJ Connectors

Recent Digital Equipment Corporation (DEC) terminals (and clones) use a connector that DEC calls an MMJ (modified modular jack) connector. To attach a terminal with this type of connector, you need a special adapter. Refer to "Connector Pinouts" for the pinouts of this adapter.

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