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RS-232 (Recommended Standard 232)

RS-232 standard is used to transfer data at a rate of several thousands of bits per second. Bipolar levels from ± 5 to ± 15V are defined by this standard. Besides that this standard describes interface control signals, data transfer and connectors types. There are asynchronous and synchronous modes of exchange provided by the standard but COM-ports support only asynchronous mode. An interface does not provide galvanic isolation of devices and requires a safety ground for connected devices if both of them are supplied by AC network and have network filters. This standard application allows using multi-core cables without any screening as the maximum voltage change rate of formers to minimize crosstalk is intentionally limited to 30 V/ms.

In the standard modem connection cables with DB25P, DB9P plugs and DB25S, DB9S sockets are connected in accordance with the picture below:

modem connection cables with DB25P, DB9P plugs and DB25S, DB9S sockets

If devices are the same (2 computers) there are null-modem cables used because their sockets are the same:

DB9/DB9
Receive Data	2	3	Transmit Data
Transmit Data	3	2	Receive Data
Data Terminal Ready	4	6+1	Data Set Ready + Carrier Detect
System Ground	5	5	System Ground
Data Set Ready + Carrier Detect	6+1	4	Data Terminal Ready
Request to Send	7	8	Clear to Send
Clear to Send	8	7	Request to Send

DB25/DB25
Receive Data	3	2	Transmit Data
Transmit Data	2	3	Receive Data
Data Terminal Ready	20	6+8	Data Set Ready + Carrier Detect
System Ground	7	7	System Ground
Data Set Ready + Carrier Detect	6+8	20	Data Terminal Ready
Request to Send	4	5	Clear to Send
Clear to Send	5	4	Request to Send

DB9/DB25
Receive Data	2	2	Transmit Data
Transmit Data	3	3	Receive Data
Data Terminal Ready	4	6+8	Data Set Ready + Carrier Detect
System Ground	5	7	System Ground
Data Set Ready + Carrier Detect	6+1	20	Data Terminal Ready
Request to Send	7	5	Clear to Send
Clear to Send	8	4	Request to Send

The animated picture below demonstrates a signal waveform in the data transfer loop sending 16 bytes of data with 00h, 11h, 22h,…FFh symbols without P parity bit

RS-232