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RS232 - V.24/V.28 - IS2110 - X.20 bis (for Async) - X.21 bis (for Sync)

General

In this document the term RS232 will be used when refered to this serial interface. The description of RS232 is an EIA/TIA norm and is identical to CCITT V.24/V.28, X.20bis/X.21bis and ISO IS2110. The only difference is that CCITT has split the interface into its electrical description (V.28) and a mechanical part (V.24) or Asynchronous (X.20 bis) and Synchronous (X.21 bis) where the EIA/TIA describes everything under RS232.

As said before RS232 is a serial interface. It can be found in many different applications where the most common ones are modems and Personal Computers. All pinning specifications are writen for the DTE side.

All DTE-DCE cables are straight through meaning the pins are connected one on one. DTE-DTE and DCE-DCE cables are cross cables. To make a destiction between all different types of cables we have to use a naming convention.
DTE - DCE: Straight Cable
DTE - DTE: Null-Modem Cable
DCE - DCE: Tail Circuit Cable

Interface Mechanical

RS232 can be found on different connectors. There are special specifications for this. The CCITT only defines a Sub-D 25 pins version where the EIA/TIA has two versions RS232C and RS232D which are resp. on a Sub-D25 and a RJ45. Next to this IBM has added a Sub-D 9 version which is found an almost all Personal Computers and is described in TIA 457.




Male	Female

Pinning

RS232-C	Description	Circuit EIA	Circuit CCITT	RJ45	TIA 457
1	Shield Ground	AA
7	Signal Ground	AB	102	4	5
2	Transmitted Data	BA	103	6	3
3	Received Data	BB	104	5	2
4	Request To Send	CA	105	8	7
5	Clear To Send	CB	106	7	8
6	DCE Ready	CC	107	1	6
20	DTE Ready	CD	108.2	3	4
22	Ring Indicator	CE	125	1	9
8	Received Line Signal Detector	CF	109	2	1
23	Data Signal Rate Select (DTE/DCE Source>	CH/CI	111/112
24	Transmit Signal Element Timing (DTE Source)	DA	113
15	Transmitter Signal Element Timing (DCE Source)	DB	114
17	Receiver Signal Element Timing (DCE Source)	DD	115
18	Local Loopback / Quality Detector	LL	141
21	Remote Loopback	RL/CG	140/110
14	Secondary Transmitted Data	SBA	118
16	Secondary Received Data	SBB	119
19	Secondary Request To Send	SCA	120
13	Secondary Clear To Send	SCB	121
12	Secondary Received Line Signal Detector/ Data signal Rate Select (DCE Source)	SCF/CI	122/112
25	Test Mode	TM	142
9	Reserved for Testing
10	Reserved for Testing
11	Unassigned

Interface Electrical

All signals are measured in reference to a common ground, which is called the signal ground (AB). A positive voltage between 3 and 15 Vdc represents a logical 0 and a negative voltage between 3 and 15 Vdc represents a logical 1.
This switching between positive and negative is called bipolar. The zero state is not defined in RS232 and is considered a fault condition (this happens when a device is turned off).
According to the above a maximum distance of 50 ft or 15 m. can be reached at a maximum speed of 20k bps. This is according to the official specifications, the distance can be exceeded with the use of Line Drivers.

Functional description

Description	Circuit	Function
Shield Ground	AA	Also known as protective ground. This is the chassis ground connection between DTE and DCE.
Signal Ground	AB	The reference ground between a DTE and a DCE. Has the value 0 Vdc.
Transmitted Data	BA	Data send by the DTE.
Received Data	BB	Data received by the DTE.
Request To Send	CA	Originated by the DTE to initiate transmission by the DCE.
Clear To Send	CB	Send by the DCE as a reply on the RTS after a delay in ms, which gives the DCEs enough time to energize their circuits and synchronize on basic modulation patterns.
DCE Ready	CC	Known as DSR. Originated by the DCE indicating that it is basically operating (power on, and in functional mode).
DTE Ready	CD	Known as DTR. Originated by the DTE to instruct the DCE to setup a connection. Actually it means that the DTE is up and running and ready to communicate.
Ring Indicator	CE	A signal from the DCE to the DTE that there is an incomming call (telephone is ringing). Only used on switched circuit connections.
Received Line Signal Detector	CF	Known as DCD. A signal send from DCE to its DTE to indicate that it has received a basic carrier signal from a (remote) DCE.
Data Signal Rate Select (DTE/DCE Source>	CH/CI	A control signal that can be used to change the transmission speed.
Transmit Signal Element Timing (DTE Source)	DA	Timing signals used by the DTE for transmission, where the clock is originated by the DTE and the DCE is the slave.
Transmitter Signal Element Timing (DCE Source)	DB	Timing signals used by the DTE for transmission.
Receiver Signal Element Timing (DCE Source)	DD	Timing signals used by the DTE when receiving data.
Local Loopback / Quality Detector	LL
Remote Loopback	RL/CG	Originated by the DCE that changes state when the analog signal received from the (remote) DCE becomes marginal.
Test Mode	TM
Reserved for Testing

The secondary signals are used on some DCE's. Those units have the possibility to transmit and/or receive on a secondary channel. Those secondary channels are mostly of a lower speed than the normal ones and are mainly used for administrative functions.

Cable pinnings
Here are some cable pinnings that might be useful. Not all applications are covered, it is just a help:

Straight DB25 Cable

DB25 Null- modem cable (Async)

DB25 Tail- circuit cable (Sync)

DB25 to DB9 DTE - DCE cable

DB25 to DB9 DTE - DTE cable

Pin	Pin
1	1
2	2
3	3
4	4
5	5
6	6
7	7
8	8
9	9
10	10
11	11
12	12
13	13
14	14
15	15
16	16
17	17
18	18
19	19
20	20
21	21
22	22
23	23
24	24
25	25

Pin	Pin
1	1
2	3
3	2
4	5
5	4
6, 8	20
7	7
20	6, 8

DB9 Null- modem cable

1,6	4
2	3
3	2
4	1,6
5	5
7	8
8	7

Pin	Pin
1	1
2	3
3	2
4	8
6	20
7	7
8	4
17	24
20	6
24	17

Pin	Pin
2	3
3	2
4	7
5	8
6	6
7	5
8	1
20	4
22	9

Pin	Pin
2	2
3	3
4	8
5	7
6	4
7	5
20	1, 6

Resources:

http://www.rs485.com/rs485spec.html: RS485, RS232, RS422, RS423, Quick Reference Guide