--- chapter.sgml.orig Sat Sep 3 14:52:36 2005
+++ chapter.sgml.2 Tue Sep 6 22:36:41 2005
@@ -119,15 +119,20 @@
A null-modem cable passes some signals, such as signal
ground
, straight through, but switches other signals. For
- example, the send data
pin on one end goes to the
- receive data
pin on the other end.
+ example, the transmitted data
pin on one end goes to the
+ received data
pin on the other end.
- If you like making your own cables, you can construct
- a null-modem cable for use with
- terminals. This table shows the RS-232C signal names and the pin
- numbers on a DB-25 connector.
+ You can also construct your own null-modem cable for use with
+ terminals (e.g., for quality purposes). This table shows the RS-232C
+ signal names and the pin numbers on a DB-25 connector. Note that the
+ standard also calls for a straight-through pin 1 to pin 1
+ protective ground line, but it is often omitted.
+ Some terminals can get by using only pins 2, 3 and 7, while others
+ require different configurations than the examples shown below.
-
+
+ Null-modem DB-25 to DB-25 cable
+
@@ -189,11 +194,11 @@
- DCD
+ DTR
+ 20
+ connects to
8
-
- 6
- DSR
+ DCD
@@ -203,16 +208,269 @@
20
DTR
+
+
+ DCD
+ 8
+ connects to
+ 20
+ DTR
+
-
+
+ Here are two other schemes more common nowadays.
+
+
+ Null-modem DB-9 to DB-9 cable
+
+
+
+
+ Signal
+ Pin #
+
+ Pin #
+ Signal
+
+
+
+
+
+ RD
+ 2
+ connects to
+ 3
+ TD
+
+
+
+ TD
+ 3
+ connects to
+ 2
+ RD
+
+
+
+ DTR
+ 4
+ connects to
+ 6
+ DSR
+
+
+
+ DTR
+ 4
+ connects to
+ 1
+ DCD
+
+
+
+ SG
+ 5
+ connects to
+ 5
+ SG
+
+
+
+ DSR
+ 6
+ connects to
+ 4
+ DTR
+
+
+
+ DCD
+ 1
+ connects to
+ 4
+ DTR
+
+
+
+ RTS
+ 7
+ connects to
+ 8
+ CTS
+
+
+
+ CTS
+ 8
+ connects to
+ 7
+ RTS
+
+
+
+
+
+
+ Null-modem DB-9 to DB-25 cable
+
+
+
+
+ Signal
+ Pin #
+
+ Pin #
+ Signal
+
+
+
+
+
+ RD
+ 2
+ connects to
+ 2
+ TD
+
+
+
+ TD
+ 3
+ connects to
+ 3
+ RD
+
+
+
+ DTR
+ 4
+ connects to
+ 6
+ DSR
+
+
+
+ DTR
+ 4
+ connects to
+ 8
+ DCD
+
+
+
+ SG
+ 5
+ connects to
+ 7
+ SG
+
+
+
+ DSR
+ 6
+ connects to
+ 20
+ DTR
+
+
+
+ DCD
+ 1
+ connects to
+ 20
+ DTR
+
+
+
+ RTS
+ 7
+ connects to
+ 5
+ CTS
+
+
+
+ CTS
+ 8
+ connects to
+ 4
+ RTS
+
+
+
+
+
- Connect Data Set Ready
(DSR) and
- Data Carrier Detect
(DCD) internally in the
- connector hood, and then to Data Terminal
- Ready
(DTR) in the remote hood.
+ Usually, when one pin at one end connects to a pair of pins
+ at the other end, it is implemented with one short wire between
+ the pair of pins in their connector and a long wire to the other
+ single pin.
+
+ The above designs seems to be the most popular. In another
+ variation (explained in the book RS-232 Made
+ Easy, ISBN: 0137498543) SG connects to SG, TD connects
+ to RD, RTS and CTS connect to DCD, DTR connects to DSR, and
+ vice-versa.
+
+ The correspondance between acronyms used and signal names
+ follow.
+
+
+ Signal acronyms
+
+
+
+
+ Acronyms
+ Signal names
+
+
+
+
+
+ RD
+ Received Data
+
+
+
+ TD
+ Transmitted Data
+
+
+
+ DTR
+ Data Terminal Ready
+
+
+
+ DSR
+ Data Set Ready
+
+
+
+ DCD
+ Data Carrier Detect
+
+
+
+ SG
+ System Ground
+
+
+
+ RTS
+ Request To Send
+
+
+
+ CTS
+ Clear To Send
+
+
+
@@ -220,8 +478,8 @@
RS-232C cables
A standard serial cable passes all the RS-232C signals
- straight-through. That is, the send data
pin on one
- end of the cable goes to the send data
pin on the
+ straight-through. That is, the transmitted data
pin on one
+ end of the cable goes to the transmitted data
pin on the
other end. This is the type of cable to use to connect a modem to your
FreeBSD system, and is also appropriate for some
terminals.