Editing Null modem (section)

== Cables and adapters ==
[[File:Null modem cable 1.jpg|thumb|A null modem cable]]

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 (computing)|adapter]]. The adapter contains the necessary crosslinks between the signals.<ref name="null-modem">{{cite web |url=http://www.nullmodem.com/NullModem.htm |title=Null Modem |publisher=nullmodem.com |date=2008-11-07 |access-date=2013-12-26}}</ref><ref>{{cite web |url=http://www.hardwarebook.info/Nullmodem_(9-9) |title=Nullmodem (9-9) - HwB |publisher=hardwarebook.info |date=2006-12-27 |access-date=2013-12-26}}</ref>

=== Wiring diagrams ===
[[File:D25 Null Modem Wiring.png|thumb|DB-25 null modem wiring diagram]]
[[File:D9 Null Modem Wiring.png|thumb|DE-9 null modem wiring diagram]]
{{See also|Serial port#Pinouts|l1=Serial port: Pinouts}}

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:<ref name="null-modem" />
{| class="wikitable"
! colspan="4" | One side
! rowspan="2" | Signal<br />direction
! colspan="3" | Other side
|-
! colspan="2" | Signal and abbreviations
!               [[DB-25]] pin
!               [[D-subminiature|DE-9]] pin
!               DE-9 pin
!               DB-25 pin
!               Signal
|-
| [[Ground (electricity)|Frame Ground]]
| FG      || 1 || {{n/a}}
| style="text-align:center;"   {{n/a|Common}}
| {{n/a}} || 1 || FG
|-
| [[Transmitted Data]]
| TxD, TD || 2 || 3
| style="text-align:center;" | →
| 2       || 3 || RxD
|-
| [[Received Data]]
| RxD, RD || 3 || 2
| style="text-align:center;" | ←
| 3       || 2 || TxD
|-
| [[RS-232 RTS/CTS|Request To Send]]
| RTS     || 4 || 7
| style="text-align:center;" | →
| 8       || 5 || CTS
|-
| [[RS-232 RTS/CTS|Clear To Send]]
| CTS     || 5 || 8
| style="text-align:center;" | ←
| 7       || 4 || RTS
|-
| [[Single-ended signalling|Signal Ground]]
| SG      || 7 || 5
| style="text-align:center;" | <small>Common</small>
| 5       || 7 || SG
|-
| [[Data Set Ready]]
| DSR     || 6 || 6
| style="text-align:center;" rowspan="2" | ←
| rowspan="2" | 4   || rowspan="2" | 20 || rowspan="2" | DTR
|-
| [[Data Carrier Detect]]
| DCD, CD || 8 || 1
|-
| rowspan="2" | [[Data Terminal Ready]]
| rowspan="2" | DTR || rowspan="2" | 20 || rowspan="2" | 4
| style="text-align:center;" rowspan="2" | →
| 1       || 8 || DCD
|-
| 6       || 6 || DSR
|}