Null Modem Serial Cable Pinout
What is a null modem?
Using the RS232 protocol, null modems let you connect two computers that do not have modems. The RS232 protocol was developed to connect teletype machines and the modems they used for communication. Asymmetric connections employing RS232 needed a modem on one side and a data source or consumer at the other. Many wiring configurations are possible, as there is no standard null modem connection. Reception and transmission lines in null modem connections allow two-way data transmission by being connected crosswise.
Serial; Crossover or 'Null Modem' vs. Straight Through Serial Cable; Crossover or 'Null Modem' vs. Straight Through Serial Cable. The RS-232 specification defines two types of serial devices, DTE and DCE. DTE normally refers to the serial port on a PC or terminal, while DCE refers to a modem or other type of communications device. Pinouts cables serial. Nullmodem 9 to 9 Pin. It appears that you are using AdBlocking software. The cost of running this website is covered by advertisements. Letotech Cross Wired USB serial Cable FTDI ft232r USB RS232 to DB9 Female Adapter Null Modem Cable PC Control TV Cable (Standard Pinout). Null modem cable (DB9-DB9). Pinout and signals for building a serial (RS232) nullmodem cable. Use this cable between two DTE (Data Terminal Equipment) devices (for instance two computers). Depending of the software used, some sort of handshaking may however be necessary. Use the RS232 null modem selection table to find the right null modem cable for each purpose. For a Windows 95/98/ME Direct Cable Connection, the RS232 null modem cable with loop back handshaking is a good choice.
Currently, the primary use of null modems is for data exchange between older laptops and computers that have no network card or USB port. An RS323 null modem is the only way to transfer data between these types of machine.
Null modem pinout and wiring
It makes sense that null modem cables are required to make null modem connections. What exactly is a null modem cable? It is made up of three lines, one of which is the signal ground with the other two lines handling the data transmission. You may need an authenticating handshake depending in the software you use in this connection.
Below are the most common schemes of null modem cables.
Here is the RS232 cable wiring recommended by Microsoft that includes full authentication handshaking.
The industry standard for RS232 employs a cable comprised of seven wires and was developed by Microsoft.
Virtual null modem emulator
As previously mentioned, high-speed data transfer is not possible using a null modem cable. The theoretical top speed of a COM port is the limiting factor, and is capped at 115 kb/s. Cable length impacts transfer speed and the serial port’s maximum capacity is never approached.
You can effectively achieve higher data transfer speed by the use of a null modem emulator, which is an application that uses a virtual null modem cable to connect COM ports.
You are able to create pairs of virtual COM ports using virtual null modem software. Two way communication is facilitated between the COM ports by the software and data transmitted to one COM port is available instantly at the paired port.
Transmitting data trough a virtual cable by employing virtual null modem software is preferable to using a physical cable. Virtual Com Port Driver is a null modem emulator that supports the standard hardware line signals such as DRT/DSR, RTS/CTS, RING, ERROR, etc. This application supports strict baud rate emulation and full HandFlow control. You can also simulate a line break in you serial connection using this virtual null modem emulator.
Null modem is a communication method to directly connect two DTEs (computer, terminal, printer, etc.) using an RS-232serial cable. The name stems from the historical use of RS-232 cables to connect two teleprinter devices or two modems in order to communicate with one another; null modem communication refers to using a crossed-over RS-232 cable to connect the teleprinters directly to one another without the modems. It is also used to serially connect a computer to a printer, since both are DTE, and is known as a Printer Cable.
The RS-232 standard is asymmetric as to the definitions of the two ends of the communications link, assuming that one end is a DTE and the other is a DCE, e.g. a modem. With a null modem connection the transmit and receive lines are crosslinked. Depending on the purpose, sometimes also one or more handshake lines are crosslinked. Several wiring layouts are in use because the null modem connection is not covered by the RS-232 standard.
- 2Cables and adapters
- 4Types of null modem
Origins[edit]
Originally, the RS-232 standard was developed and used for teleprinter machines which could communicate with each other over phone lines. Each teleprinter would be physically connected to its modem via an RS-232 connection and the modems could call each other to establish a remote connection between the teleprinters. If a user wished to connect two teleprinters directly without modems (null modem) then they would crosslink the connections. The term null modem may also refer to the cable or adapter itself as well as the connection method.[1] Null modem cables were a popular method for transferring data between the early personal computers from the 1980s to the early 1990s.
Cables and adapters[edit]
A null modem cable is a RS-232 serial cable where the transmit and receive lines are crosslinked. In some cables there are also handshake lines crosslinked. In many situations a straight-through serial cable is used, together with a null modem adapter. The adapter contains the necessary crosslinks between the signals.[2][3]
Wiring diagrams[edit]
Below is a very common wiring diagram for a null modem cable to interconnect two DTEs (e.g. two PCs) providing full handshaking, which works with software relying on proper assertion of the Data Carrier Detect (DCD) signal:[2]
| One side | Signal direction | Other side | |||||
|---|---|---|---|---|---|---|---|
| Signal and abbreviations | DB-25 pin | DE-9 pin | DE-9 pin | DB-25 pin | Signal | ||
| Frame Ground | FG | 1 | N/A | Common | N/A | 1 | FG |
| Transmitted Data | TxD, TD | 2 | 3 | → | 2 | 3 | RxD |
| Received Data | RxD, RD | 3 | 2 | ← | 3 | 2 | TxD |
| Request To Send | RTS | 4 | 7 | → | 8 | 5 | CTS |
| Clear To Send | CTS | 5 | 8 | ← | 7 | 4 | RTS |
| Signal Ground | SG | 7 | 5 | Common | 5 | 7 | SG |
| Data Set Ready | DSR | 6 | 6 | ← | 4 | 20 | DTR |
| Data Carrier Detect | DCD, CD | 8 | 1 | ||||
| Data Terminal Ready | DTR | 20 | 4 | → | 1 | 8 | DCD |
| 6 | 6 | DSR | |||||
Applications[edit]
The original application of a null modem was to connect two teleprinter terminals directly without using modems. As the RS-232 standard was adopted by other types of equipment, designers needed to decide whether their devices would have DTE-like or DCE-like interfaces. When an application required that two DTEs (or two DCEs) needed to communicate with each other, then a null modem was necessary.[4]
Null modems were commonly used for file transfer between computers, or remote operation. Under the Microsoft Windowsoperating system, the direct cable connection can be used over a null modem connection. The later versions of MS-DOS were shipped with the InterLnk program. Both pieces of software allow the mapping of a hard disk on one computer as a network drive on the other computer. No Ethernet hardware (such as a network interface card or a modem) is required for this.[5] On the Commodore Amiga system, a null modem connection was a common way of playing multiplayer games between two machines.
The popularity and availability of faster information exchange systems such as Ethernet made the use of null modem cables less common. In modern systems, such a cable can still be useful for kernel mode development, since it allows the user to remotely debug a kernel with a minimum of device drivers and code (a serial driver mainly consists of two FIFO buffers and an interrupt service routine). KGDB for Linux, ddb for BSD, and WinDbg or KD for Windows can be used to remotely debug systems, for example. This can also provide a serial console through which the in-kernel debugger can be dropped to in case of kernel panics, in which case the local monitor and keyboard may not be usable anymore (the GUI reserves those resources and dropping to the debugger in the case of a panic won't free them).
Another context where these cables can be useful is when administering 'headless' devices providing a serial administration console (i.e. managed switches, rackmount server units, and various embedded systems). An example of embedded systems that widely use null modems for remote monitoring include RTUs, device controllers, and smart sensing devices. These devices tend to reside in close proximity and lend themselves to short run serial communication through protocols such as DNP3, Modbus, and other IEC variants. The Electric, Oil, Gas, and Water Utilities are slow to respond to newer networking technologies which may be due to large investments in capital equipment that has useful service life measured in decades. Serial ports and null modem cables are still widely used in these industries with Ethernet just slowly becoming a widely available option.
Types of null modem[edit]
Connecting two DTE devices together requires a null modem that acts as a DCE between the devices by swapping the corresponding signals (TD-RD, DTR-DSR, and RTS-CTS). This can be done with a separate device and two cables, or using a cable wired to do this. If devices require Carrier Detect, it can be simulated by connecting DSR and DCD internally in the connector, thus obtaining CD from the remote DTR signal. One feature of the Yost standard is that a null modem cable is a 'rollover cable' that just reverses pins 1 through 8 on one end to 8 through 1 on the other end.[1]
No hardware handshaking[edit]
The simplest type of serial cable has no hardware handshaking. This cable has only the data and signal ground wires connected. All of the other pins have no connection. With this type of cable flow control has to be implemented in the software. The use of this cable is restricted to>
Because of the compatibility issues and potential problems with a simple null modem cable, a solution was developed to trick the software into thinking there was handshaking available. However, the cable pin out merely loops back, and does not physically support the hardware flow control.[1]
This cable could be used with more software but it had no actual enhancements over its predecessor. The software would work thinking it had hardware flow control but could suddenly stop when higher speeds were reached and with no identifiable reason.
Partial handshaking[edit]
In this cable the flow control lines are still looped back to the device. However, they are done so in a way that still permits Request To Send (RTS) and Clear To Send (CTS) flow control but has no actual functionality. The only way the flow control signal would reach the other device is if the opposite device checked for a Carrier Detect (CD) signal (at pin 1 on a DE-9 cable and pin 8 on a DB-25 cable). As a result, only specially designed software could make use of this partial handshaking. Software flow control still worked with this cable.[1]
Full handshaking[edit]
Null Modem Wiring
This cable is incompatible with the previous types of cables' hardware flow control, due to a crossing of its RTS/CTS pins. With suitable software, the cable is capable of much higher speeds than its predecessors. It also supports software flow control.[1]
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Virtual null modem[edit]
A virtual null modem is a communication method to connect two computer applications directly using a virtual serial port. Unlike a null modem cable, a virtual null modem is a software solution which emulates a hardware null modem within the computer.[6][7] All features of a hardware null modem are available in a virtual null modem as well. There are some advantages to this:
- Higher transmission speed of serial data, limited only by computer performance and network speed
- Virtual connections over local network or Internet, mitigating cable length restrictions
- Virtually unlimited number of virtual connections
- No need for a serial cable
- The computer's physical serial ports remain free
For instance, DOSBox has allowed older DOS games to use virtual null modems.
Another common example consists of Unix pseudoterminals (pty) which present a standard tty interface to user applications, including virtual serial controls. Two such ptys may easily be linked together by an application to form a virtual null modem communication path.
See also[edit]
References[edit]
- ^ abcdefLammert Bies. 'RS232 serial null modem cable wiring and tutorial'. lammertbies.nl. Retrieved 2013-12-26.
- ^ ab'Null Modem'. nullmodem.com. 2008-11-07. Retrieved 2013-12-26.
- ^'Nullmodem (9-9) - HwB'. hardwarebook.info. 2006-12-27. Retrieved 2013-12-26.
- ^'ADTPro - ADTPro Serial Cabling'. sourceforge.net. 2011-01-25. Retrieved 2013-12-26.
- ^'MS-DOS External commands - INTERLNK'. angelfire.com. Retrieved 2013-12-26.
- ^'Null-modem emulator Download Null-modem emulator software for free at'. sourceforge.net. Retrieved 2013-12-26.
- ^'BerliOS Developer: Project Summary - N8VB_vCOM Virtual Null Modem Cable'. berlios.de. 2005-07-15. Archived from the original on 2013-12-26. Retrieved 2013-12-26.