Comport Connector

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COM (Serial) port Pins layout Connector : male DB9
D	Description	Name	Pin
I	Data Carrier Detect	DCD	1
I	Receive Data	RX	2
O	Transmit Data	TX	3
O	Data Terminal Ready	DTR	4
/	Signal Ground	GND	5
I	Data Set Ready	DSR	6
O	Request To Send	RTS	7
I	Clear To Send	CTS	8
O	Ring Indicator	RI	9
/	Chassis Ground	GND	S
The DB25 Serial port was designed for more signals than the DB9 version, but as most serial devices used only pins available on the DB9 version, manufacturer started to make DB25 with only the DB9 signals (that's why the flat cable used to connect a DB25 to the motherboard is exactly the same as the cable for the DB9) when ATX arrived, they decided to stop DB25 and use only DB9. For devices requiering all the serial signals, complete serial adapters are available as expansion cards.
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Connector : male DB25
D	Description	Name	Pin
/	Signal Ground	GND	1
0	Transmit Data	TX	2
I	Receive Data	RX	3
O	Request To Send	RTS	4
I	Clear To Send	CTS	5
I	Data Set Ready	DSR	6
/	Signal Ground	GND	7
I	Data Carrier Detect	DCD	8
?	Test voltage +	?	9
?	Test voltage -	?	10
?	Select Transmit Freq	?	11
?	Data Carrier Detect 2	DCD 2	12
?	Clear To Send 2	CTS 2	13
?	Transmit Data 2	TX 2	14
?	Transmitter Clock	?	15
?	Receive Data 2	RX 2	16
?	Receiver Clock	?	17
/	Not connected	NC	18
?	Request To Send 2	RTS 2	19
O	Data Terminal Ready	DTR	20
?	Signal Quality Detect	?	21
0	Ring Indicator	RI	22
?	Data Rate Select	?	23
?	Transmitter Clock	?	24
?	BUSY	BUSY	25
/	Chassis Ground	GND	s
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The RJ-45 serial port is used on systems such as MultiPort serial cards (like Comtrol RocketPort), Cisco,... This connector is very specific and should be used only if DB9 is too big or for interfaces with machines using them. If needed, RJ-45 to DB9 adapters are easy to find.
Connector : RJ-45
Pin	Name	Description	D
1	RTS	Request To Send	0
2	DTR	Data Terminal Ready	0
3	GND	Ground	/
4	TX	Transmit Data	0
5	RX	Receive Data	I
6	DCD	Data Carrier Detect	I
7	DSR	Data Set Ready	I
8	CTS	Clear To Send	I
Note: Standard Ethernet cables can be used, crossed Ethernet cables will be just like null-modem serial cables.
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Signals The serial port was first designed for modems, that's why many signals are refering to modem signals. Manufacturers can use these signals as they want, so you'll have to look for how they used them before trying to control their devices. DCD (Data Carrier Detect) active high input Connection established with the other computer. RX (Receive Data) input Data input from the device. TX (Transmit Data) output Data output to the device. DTR (Data Terminal Ready) active high output Tells the other computer that the port is ready to receive data. DSR (Data Set Ready) active high input Tells that the other computer have data ready to be send. RTS (Ready To Send) active high output Tells the other computer that you want to send data. CTS (Clear To Send) active high input Tells that the other computer is ready to send data. RI (Ring Indicator) active high input Tells that another computer want to start a connection. Note: RS232 signals are 0V to 12V.

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Programming Standard Resources :
COM	I/O	IRQ
1	03F8-03FF	4
2	02F8-02FF	3
3	03E8-03EF	4
4	02E8-02EF	3
Note : these IRQ are the ones available for on-board chipset and usually used when you have only two serial ports, when more serial ports are installed using a serial interface card, IrDA,... COM3 and COM4 IRQ are different of COM1 and COM2. For example, to avoid IRQ conflict when installing an internal modem when the two motherboard ports are enabled, set the modem to COM3 IRQ5. Some MultiPort cards can use one IRQ for all ports or no IRQ at all.

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