RS-232 Interface

The types of signals carried by the RS-232 interface are listed below:

Signal	Description
Signal Ground (SG)	Common electrical ground path for all interface circuits.
Transmit Data (TD)	Command codes or data sent from DTE to DCE; won't be sent unless RTS, CTS, DSR and DTR are set to on.
Receive Data (RD)	DCE responses to DTE commands or data received from a remote DCE.
Request To Send (RTS)	Set to on, the DTE tells the DCE to remain in transmit mode; set to off, the DTE tells the DCE to receive.
Clear To Send (CTS)	Set to on, the DCE reports it is ready to receive; set to off, the DCE tells the DTE not to transmit.
Data Set Ready (DSR)	Set to on, the DCE reports it is ready to operate.
Data Terminal Ready (DTR)	Set to on, the DTE tells the DCE to connect to a communications channel (required to be on for automatic answering by DCE). Set to off, the DTE tells the DCE to break the connection when it has finished the current transmission.
Ring Indicator (RI)	Set to on, the DCE reports it detects the presence of a ringing signal on the communications channel. Set to off, the DCE indicates the absence of a ringing signal.
Carrier Detect (CD)	Set to on, the DCE reports receiving a signal from the communications channel that a connection can be established.

The RS-232 standard utilizes nine pins for signaling, although many serial interfaces provide 25. The other 16 pins can be used for testing or secondary signaling. Smaller 9-pin connectors support standard signals and save space.

DB-25

25 pin connectors, or DB-25, use pins 2 though 8 and pins 20 and 22 as shown:

Pin	Signal	Signal Direction
1	Protective Ground	both
2	Transmit Data (TD)	from DTE
3	Receive Data (RD)	from DCE
4	Request To Send (RTS)	from DTE
5	Clear To Send (CTS)	from DCE
6	Data Set Ready (DSR)	from DCE
7	Signal Ground	both
8	Carrier Detect (CD)	from DCE
20	Data Terminal Ready (DTR)	from DTE
22	Ring Indicator (RI)	from DCE

DB-9

Standard 9 pin connectors, called DB-9, use the following pin setup:

Pin	Signal	Signal Direction
1	Carrier Detect (CD)	from DCE
2	Receive Data (RD)	from DCE
3	Transmit Data (TD)	from DTE
4	Data Terminal Ready (DTR)	from DTE
5	Signal Ground	both
6	Data Set Ready (DSR)	from DCE
7	Request To Send (RTS)	from DTE
8	Clear To Send (CTS)	from DCE
9	Ring Indicator (RI)	from DCE

DB-9 to DB-25 Conversion

This wiring diagram shows the standard conversion from DB-9 to DB-25:

Pin	DB-9 Pin	DB-25 Pin
Carrier Detect (CD)	1	8
Receive Data (RD)	2	3
Transmit Data (TD)	3	2
Data Terminal Ready (DTR)	4	20
Ground	5	7
Data Set Ready (DSR)	6	6
Request To Send (RTS)	7	4
Clear To Send (CTS)	8	5
Ring Indicator (RI)	9	22

HP Modem Cables

Most HP systems do not use straight through modem cables to connect the modem to the system. Make sure you are using the correct cable that is proper for your system.

HP Modem Cable (Series 700)

The HP 9000/700 series workstation provides a DB-9 port for connection to aysnchronous serial devices. Hewlett-Packard recommends that you use cable part number 24542M to connect a 9000/700 system to a DB-25 asynchronous modem. A similar part number (24542G) describes a printer cable. Do not confuse the two part numbers if you order the Hewlett-Packard cable. If you cannot get the HP part, use a cable like the one described in the table below.

Signal	Direction	DB-9 Pin	DB-25 Pin
Carrier Detect (CD)	from Modem	1	8
Receive Data (RD)	from Modem	2	3
Transmit Data (TD)	from DTE	3	2
Data Terminal Ready (DTR)	from DTE	4	20
Ground	both	5	7
Data Set Ready (DSR)	from Modem	6	6
Request To Send (RTS)	from DTE	7	4
Clear To Send (CTS)	from Modem	8	5
Ring Indicator (RI)	from Modem	9	22

HP Modem Cable (Series 800)

For an 800 series system with a model 5062-3054 MDP "Full Modem" connector panel use the HP part #40233A modem cable. If you cannot get the HP part, use a cable like the one described in the table below.

CPU		DIRECTION	MODEM
Gnd	1	both	1	Gnd
TD	2	from Modem	3	RD
RD	3	from DTE	2	TD
RTS	4	from Modem	8	DCD
DSR	6	from DTE	20	DTR
GND	7	BOTH	7	GND
DCD	8	from DTE	4	RTS
	9	from Modem	22	RI
DTR	20	from Modem	6	DSR
RI	22	from Modem	5	CTS

Null-Modem Cables

You can directly connect the serial ports of two systems with a null-modem cable. Null-modem cables connect pins of one machine to their symmetric counterparts on another machine. For example, pin 2, the Transmit Data (TD) pin of machine A, is paired with the pin 3, the Receive Data (RD) of machine B. If this is not done, both machines would try to use pin 2 to transmit data and neither could receive the others transmission. Sometimes only pins 2, 3, and 7 are connected in a null-modem cable, but hardware handshaking may require optional connections of other pins. Data Set Ready (DSR) and Carrier Detect (CD), pins 6 and 8, may be joined together and connect to the other machine's Data Terminal Ready pin, number 20.

The table below shows typical null-modem pin connections between two DB-25 ports.

DB-25 to DB-25 Null-Modem Connections

DTE Signal	DTE Pin	DCE Pin	DCE Signal
Protective Ground	1	1	Protective Ground
Ground	7	7	Ground
Transmit Data (TD)	2	3	Receive Data (RD)
Receive Data (RD)	3	2	Transmit Data (TD)
Data Set Ready (DSR) & Carrier Detect (CD)	6+8	20	Data Terminal Ready (DTR)
Data Terminal Ready (DTR	20	6+8	Data Set Ready (DSR) & Carrier Detect (CD)
Request To Send (RTS)	4	5	Clear To Send (CTS)
Clear To Send (CTS)	5	4	Request To Send (RTS)

RS-232 Interface

Technical documentation

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