628:
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752:
703:
661:
168:
139:
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588:, 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.
31:
596:
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
124:
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
575:
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
489:
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
684:
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.
567:
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
775:
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.
688:
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.
146:
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
646:
has to be implemented in the software. The use of this cable is restricted to data-traffic only on its cross-connected Rx and Tx lines. This cable can also be used in devices that do not need or make use of modem control signals.
129:
may also refer to the cable or adapter itself as well as the connection method. Null modem cables were a popular method for transferring data between the early personal computers from the 1980s to the early 1990s.
738:(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.
80:
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
642:
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
181:
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
796:
solution which emulates a hardware null modem within the computer. All features of a hardware null modem are available in a virtual null modem as well. There are some advantages to this:
839:
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.
580:, 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
734:(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
543:
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
1036:
597:
simulated by connecting DSR and DCD internally in the connector, thus obtaining CD from the remote DTR signal. One feature of the
112:
lines are crosslinked. Several wiring layouts are in use because the null modem connection is not covered by the RS-232 standard.
726:
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
92:
The RS-232 standard is asymmetric as to the definitions of the two ends of the communications link, assuming that one end is a
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105:
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made the use of null modem cables less common. In modern systems, such a cable can still be useful for
17:
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that two DTEs (or two DCEs) needed to communicate with each other, then a null modem was necessary.
858:
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370:
93:
82:
42:
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Higher transmission speed of serial data, limited only by computer performance and network speed
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reserves those resources and dropping to the debugger in the case of a panic won't free them).
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on one computer as a network drive on the other computer. No
Ethernet hardware (such as a
8:
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444:
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54:
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Virtual connections over local network or
Internet, mitigating cable length restrictions
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and receive lines are crosslinked. Depending on the purpose, sometimes also one or more
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50:
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605:" that just reverses pins 1 through 8 on one end to 8 through 1 on the other end.
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without modems (null modem) then they would crosslink the connections. The term
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The popularity and availability of faster information exchange systems such as
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1016:"Null-modem emulator | Download Null-modem emulator software for free at"
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1037:"BerliOS Developer: Project Summary - N8VB_vCOM Virtual Null Modem Cable"
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151:. The adapter contains the necessary crosslinks between the signals.
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can be used over a null modem connection. The later versions of
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A virtual null modem is a communication method to connect two
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101:
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30:
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Wiring pinouts for DB-25 (left) and DE-9 (right) connectors
720:
Wiring pinouts for DB-25 (left) and DE-9 (right) connectors
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Wiring pinouts for DB-25 (left) and DE-9 (right) connectors
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Wiring pinouts for DB-25 (left) and DE-9 (right) connectors
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Originally, the RS-232 standard was developed and used for
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program. Both pieces of software allow the mapping of a
792:. Unlike a null modem cable, a virtual null modem is a
528:
computer, a null modem connection was a common way of
905:"RS232 serial null modem cable wiring and tutorial"
497:between computers, or remote operation. Under the
41:is a communication method to directly connect two
806:Virtually unlimited number of virtual connections
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64:. The name stems from the historical use of
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601:standard is that a null modem cable is a "
76:in order to communicate with one another;
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524:or a modem) is required for this. On the
1039:. berlios.de. 2005-07-15. Archived from
902:
831:Another common example consists of Unix
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14:
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992:"MS-DOS External commands - INTERLNK"
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27:Serial cable connecting two computers
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493:Null modems were commonly used for
104:. With a null modem connection the
24:
828:games to use virtual null modems.
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25:
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971:"ADTPro - ADTPro Serial Cabling"
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952:. hardwarebook.info. 2006-12-27
835:(pty) which present a standard
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163:DB-25 null modem wiring diagram
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1008:
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963:
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171:DE-9 null modem wiring diagram
13:
1:
973:. sourceforge.net. 2011-01-25
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874:Serial Line Internet Protocol
1091:Multiplayer null modem games
7:
931:. nullmodem.com. 2008-11-07
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10:
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809:No need for a serial cable
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549:interrupt service routine
530:playing multiplayer games
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204:Signal and abbreviations
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812:The computer's physical
950:"Nullmodem (9-9) - HwB"
859:Direct cable connection
609:No hardware handshaking
506:direct cable connection
1096:Out-of-band management
532:between two machines.
522:network interface card
512:were shipped with the
172:
164:
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68:cables to connect two
35:
786:computer applications
170:
162:
141:
33:
1072:at Wikimedia Commons
651:Loopback handshaking
177:Serial port: Pinouts
85:, and is known as a
34:A null modem adapter
790:virtual serial port
693:Partial handshaking
592:Types of null modem
445:Data Terminal Ready
429:Data Carrier Detect
183:Data Carrier Detect
134:Cables and adapters
96:and the other is a
824:has allowed older
780:Virtual null modem
173:
165:
144:
142:A null modem cable
36:
1068:Media related to
1018:. sourceforge.net
788:directly using a
499:Microsoft Windows
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57:, etc.) using an
16:(Redirected from
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907:. lammertbies.nl
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742:Full handshaking
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502:operating system
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259:Transmitted Data
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833:pseudoterminals
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155:Wiring diagrams
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1043:on 2013-12-26
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864:LapLink cable
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732:Clear To Send
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87:Printer Cable
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44:
40:
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19:
1045:. Retrieved
1041:the original
1031:
1020:. Retrieved
1010:
999:. Retrieved
986:
975:. Retrieved
965:
954:. Retrieved
944:
933:. Retrieved
929:"Null Modem"
909:. Retrieved
830:
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814:serial ports
783:
774:
735:
731:
727:
725:
687:
683:
644:flow control
641:
595:
574:
565:WinDbg or KD
534:
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485:Applications
231:Frame Ground
182:
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145:
126:
119:
91:
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62:serial cable
38:
37:
1070:Null modems
816:remain free
199:Other side
122:teleprinter
70:teleprinter
1080:Categories
1047:2013-12-26
1022:2013-12-26
1001:2013-12-26
977:2013-12-26
956:2013-12-26
935:2013-12-26
911:2013-12-26
880:References
730:(RTS) and
559:, ddb for
222:DB-25 pin
196:direction
175:See also:
127:null modem
78:null modem
39:Null modem
18:Null-modem
996:Angelfire
854:Debugging
518:hard disk
219:DE-9 pin
191:One side
110:handshake
100:, e.g. a
843:See also
794:software
537:Ethernet
514:InterLnk
106:transmit
51:terminal
47:computer
433:DCD, CD
291:RxD, RD
263:TxD, TD
244:Common
225:Signal
149:adapter
116:Origins
55:printer
1086:Modems
822:DOSBox
586:Modbus
563:, and
541:kernel
510:MS-DOS
504:, the
385:Common
194:Signal
74:modems
66:RS-232
59:RS-232
557:Linux
526:Amiga
208:DB-25
102:modem
599:Yost
582:DNP3
578:RTUs
555:for
553:KGDB
545:FIFO
478:DSR
467:DCD
423:DTR
365:RTS
337:CTS
309:TxD
281:RxD
216:pin
214:DE-9
210:pin
43:DTEs
837:tty
826:DOS
570:GUI
561:BSD
551:).
449:DTR
405:DSR
395:SG
347:CTS
319:RTS
253:FG
98:DCE
94:DTE
83:DTE
1082::
994:.
920:^
887:^
584:,
458:→
455:4
452:20
439:1
420:20
414:←
411:6
381:5
375:SG
356:←
353:8
328:→
325:7
300:←
297:2
272:→
269:3
241:—
235:FG
89:.
53:,
49:,
1050:.
1025:.
1004:.
980:.
959:.
938:.
914:.
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278:3
275:2
266:2
250:1
247:—
238:1
45:(
20:)
Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
