Null Modem
DCEs and DTE Terminal Equipment Cable Standards
Cable Standards
Originally, the concept of DCEs and DTEs was based on two types of equipment: terminal equipment that generated or received data, and communication equipment that only relayed data. In the development of the RS-232 standard, there were reasons why 25-pin RS-232 connectors on these two types of equipment needed to be wired differently.
Full Post:
DCEs and DTE Terminal Equipment Cable Standards.
DCEs and DTE Terminal Equipment Cable Standards.
These reasons are no longer significant, but we are left with two different types of cables: one for connecting a DTE to a DCE, and another for connecting two DTEs directly to each other.
The DTE/DCE interface for a particular standard defines the following specifications:
The DTE/DCE interface for a particular standard defines the following specifications:
Mechanical/physical – Number of pins and connector type
Electrical – Defines voltage levels for 0 and 1
Functional – Specifies the functions that are performed by assigning meanings to each of the signaling lines in the interface
Procedural – Specifies the sequence of events for transmitting data
The original RS-232 standard only defined the connection of DTEs with DCEs, which were modems. However, if you want to connect two DTEs, such as two computers or two routers in the lab, a special cable called a null modem eliminates the need for a DCE. In other words, the two devices can be connected without a modem. A null modem is a communication method to directly connect two DTEs, such as a computer, terminal, or printer, using a RS-232 serial cable. With a null modem connection, the transmit (Tx) and receive (Rx) lines are crosslinked as shown in the figure.
The cable for the DTE to DCE connection is a shielded serial transition cable. The router end of the shielded serial transition cable may be a DB-60 connector, which connects to the DB-60 port on a serial WAN interface card. The other end of the serial transition cable is available with the connector appropriate for the standard that is to be used. The WAN provider or the CSU/DSU usually dictates this cable type. Cisco devices support the EIA/TIA-232, EIA/TIA-449, V.35, X.21, and EIA/TIA-530 serial standards.
To support higher port densities in a smaller form factor, Cisco has introduced a Smart Serial cable. The router interface end of the Smart Serial cable is a 26-pin connector that is significantly more compact than the DB-60 connector.
When using a null modem, keep in mind that synchronous connections require a clock signal. An external device can generate the signal, or one of the DTEs can generate the clock signal. When a DTE and DCE are connected, the serial port on a router is the DTE end of the connection by default, and the clock signal is typically provided by a CSU/DSU or similar DCE device. However, when using a null modem cable in a router-to-router connection, one of the serial interfaces must be configured as the DCE end to provide the clock signal for the connection.
About the Author
Null Modem & Serial Cables
|
|
15ft CMG-Rated DB9 Low Profile Null Modem F-F $7.77 Provides serial connections where CMG rating is required for in-wall installation… |
|
|
75ft CMG-Rated DB9 Low Profile Null Modem F-F $20.14 Provides serial connections where CMG rating is required for in-wall installation… |
|
|
3ft CMG-Rated DB9 Low Profile Null Modem F-F $4.77 Provides serial connections where CMG rating is required for in-wall installation… |
|
|
Null Modem Cable For Printer – Accessory For Automatic Refractometers, Ar6 Series, Reichert $146.85 These refractometers are easy-to-use tools for concentration measurements, purity determination, and chemical identification. Designed for use in a wide range of applications from routine analysis to advanced research. High-sensitivity 2600 cell diode array shadowline detector can easily handle demanding samples such as inks or grape juice. Automatic temperature compensation corrects readings fo… |
|
|
Braveheart – Rare PC Game $3.00 You’ve Seen the Movie You Know the Legend Now LiveIt!Product Information”They may take our lives but they’ll never take our freedom!”Based on Mel Gibson’s award-winning movie Braveheart the game recreates theatmosphere and excitement of Scotland’s legendary struggle for freedom againstthe English invaders.As a Scottish Lord in medieval England you have to unite the clans and defeatthe British army… |
|
|
Braveheart $2.98 Braveheart fans expecting a game of the same quality as the movie should beware. This game does not do the movie justice. We can almost see what the designers were trying to do here: combine a sweeping strategy game and complex diplomatic model with a real-time combat engine to reenact 13th-century battles. Unfortunately the end result is a muddled mess. We thought we were in for a treat when… |
|
|
Battleground 9: Chickamauga $44.95 From December 30, 1862, to January 1, 1863, the battle at Murfreesboro, fought over a three-day period along the Stones River near Nashville, was one of the costliest engagements of the American Civil War. In the end, William Rosecrans’s Army of the Cubmberland overpowered Braxton Bragg’s Confederates, forcing them to retreat ingloriously toward Chattanooga. During the following summer (September … |
|
|
Plugable USB to RS-232 DB9 Serial Adapter (Prolific PL2303HX Chipset) $12.63 Featuring the latest PL-2303HX Rev D chipset for the best combination of performance and wide driver support. Drivers available for Windows 98/ME/2000/XP, and Microsoft signed drivers for 32/64-bit Vista and Windows 7; Mac OS X 8 and later including Snow Leopard 10.6.x; open source drivers included in Linux kernel 2.4.31 and later. Windows 7 automatically detects and guides driver install without … |
|
|
Keyspan by Tripp Lite USA-19HS Hi-Speed USB Serial Adapter, PC, MAC, supports Cisco Break Sequence $25.00 Ideal for travel and mobile users with PC or Mac Supports 230Kbps and use of multiple adapters on 1 PC with no external power adapter requiredUse adapter with any USB cable up to 5m longMicrosoft WHQL approved… |
|
|
DB9 Female/Female Mini Gender Changer (Coupler) $0.01 DB9 Female/Female Mini Gender Changer (Coupler)… |
November 19, 2006 11:32 am 
Subscribe