ABUSAR - Associação Brasileira dos Usuários de Acesso Rápido

A finalidade dos cabos seriais é interligar um dispositivo DTE RS 232 com um DCE

RS232 is a voltage loop interface for two-way (full-duplex) communication represented by voltage levels with respect to system ground (common). A common ground between the PC and the associated device is necessary. Maximum serial cable length is defined: 75 feet at 9,600 bps, but today cables up to 1,000 feet are used sucessfully.

The interface is single ended (connecting only two devices with each other), the data rate is less than 20 kbps.

Data:
- logical "1" (Mark) is represented by a negative voltage of -3V to -25V
- logical "0" (Space) is represented by a positive voltage of +3V to +25V

Devices that communicate over a serial interface are divided into two classes: DTE and DCE. The most important difference between these types of devices is that the DCE device supplies the clock signal that paces the communications on the bus. All DTE-DCE cables are straight through, the pins are connected one to one. DTE-DTE and DCE-DCE cables are cross cables.

DTE - DCE is called a 'Straight Cable'
DTE - DTE is called a 'Null-Modem Cable'
DCE - DCE is called a 'Tail Circuit Cable'

DTE (Data Terminal Equipment) devices are: computers, terminals, printers, CSU/DSU, Multiplexers. DCE (Data Communications Equipment) devices are: modems and other equipment.
Hubs and Routers could be either DTE or DCE.

This difference determines, which signal a device expects on which pin. Any device is configured as either a DTE or a DCE device. Cables between DTE and DCE are different from cables between DCE and DCE devices. The connector is a 25-pin D-Sub connector (for possible synchronous communications), or a 9-pin D-Sub connector (the subset for asynchronous communications). With asynchronous communications the serial data bits are not locked to a specific clock at the receiving end. The bits are synchronized by the transmit clock at the sending end.

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.

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

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

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

Cables & Communication Information for Developers

http://www.fentonups.com/Support___Service/Cables___Comm_Info/cables___comm_info.html

Fenton UPS products (except PowerOffice) come with a DB9 female interface port and a custom DB9 male-female cable. Please note: a standard serial cable will not work with Fenton UPS products.

The interface port supports both RS232 and contact closure signals. The contact closure signals are relay signals and are "normal open". The RS232 signals are photo-transistor signals. The pin assignments on the DB9 female interface port on the UPS are as follows:

1 - N/A
2 - AC Failure (Contact Closure)
3 - N/A
4 - Common
5 - Battery Low (Contact Closure)
6 - Rx (RS232) and Turn Off UPS (Contact Closure)
7 - Common
8 - N/A
9 - Tx (RS232)

The pin assignments of the Fenton DB9 male-female cable are as follows:

DB9 female (to computer) DB9 male (to UPS)
2----------------------------------9
3----------------------------------6
4 and 8 shorted-------------------1
5----------------------------------7

In order to interface with a DB25 male serial port on a computer, either a standard serial DB9/25 adapter can be used with the regular Fenton cable, or a custom cable can be made. The pin assignments for a custom DB25 female - DB9 male cable would be as follows:

DB25 female (to computer) DB9 male (to UPS)
2---------------------------------6
3---------------------------------9
5 and 20 shorted------------------1
4, 6, and 8 shorted----------------NC
7---------------------------------7

"Contact closure" cables for applications such as Windows NT and 2000 built-in UPS feature or custom programs can be purchased from Fenton Technologies.


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