RS232 Signals

The Host system is DTE (Data Terminal Equipment) and the peripheral system is DCI (Data Circuit–Terminating Equipment)

The most important part of the RS232 spec is makeing sure the electrical caracteristics off the equipment and cables is compatiable. The capacitive load on driver must not exceed 2500pF. The maximum risetime through the transition region is 4% of the bittime. The slewrate must not exceed 30V/us. In other words, don't assert or release signals too fast and don't overload the drivers. The result is low noise.

A low level (–3 to –15 volts) is defined as a logic 1. Asserting a logic 1 or turning on a signal is historically referred to as “marking”.

A high level (+3 to +15 volts) is defined as a logic 0. Releaseing to logic 0 or turning off the signal is referred to as “spacing”.

A stop bit happens to be a mark, a start bit is a space. When the line is idle, the transmitter sends out a continuous "marking" since the start bit is of opposite polarity, the first thing the receiver sees when data is transmitted is a transition from a mark to a space. It waits for the duration of half a bit (as per its baud rate setting) and samples the line again to see if the transition was a start bit or noise. If the line is still high, it continues to sample at the baud rate to pick up each bit untill the stop bit. It then waits for the next start bit.

DB25M Pin	DB9M Pin	Abbr	Name	Driven by	Type
7	5	AB	Signal Common		Common	Should not be Chassis Ground
2	3	BA	Transmitted Data (TD)	Host	Data	At the transmitting end, the device is required to drive +/- 5 volts into a 3k Ohm load.
3	2	BB	Received Data (RD)	Periph	Data	At the recieving end, the official spec requires +/- 3 volts, but almost all recieveing equipment today will respond to positive only signals (0 to +3 volts)
4	7	CA	Request to Send (RTS)	Host	Control	Request to Send (RTS): When the host system (DTE) is ready to transmit data to the peripheral system (DCE), RTS is turned ON. In simplex and duplex systems, this condition maintains the DCE in receive mode. In half-duplex systems, this condition maintains the DCE in receive mode and disables transmit mode. The OFF condition maintains the DCE in transmit mode. After RTS is asserted, the DCE must assert CTS before communicationcan commence. Hard-wire to CTS on connector to bypass this function
5	8	CB	Clear to Send (CTS)	Periph	Control	Clear to Send (CTS): CTS is used along with RTS to provide handshaking between the DTE and the DCE. After the DCE sees an asserted RTS, it turns CTS ON when it is ready to begin communication. Hard-wire to RTS on connector to bypass this function
6	6	CC	DCE Ready (DSR)	Periph	Control	Data Set Ready (DSR): This signal is turned on by the DCE to indicate that it is connected to the telecommunications line. Hard-wire to DTR on connector to bypass this function
20	4	CD	DTE Ready (DTR)	Host	Control	Data Terminal Ready (DTR): DTR indicates the readiness of the DTE. This signal is turned ON by the DTE when it is ready to transmit or receive data from the DCE. DTR must be ON before the DCE can assert DSR. Hard-wire to DSR on connector to bypass this function
22	9	CE	Ring Indicator (RI)	Periph	Control	Ring Indicator (RI): RI, when asserted, indicates that a ringing signal is being received on the communications channel.
8	1	CF	Received Line Signal Detector** (DCD)	Periph	Control	Data Carrier Detect (DCD): This signal is turned ON when the DCE is receiving a signal from a remote DCE which meets its suitable signal criteria. This signal remains ON as long as the a suitable carrier signal can be detected.
		CG	Signal Quality Detector	Periph	Control
		CH	Data Signal Rate Detector from DTE	Host	Control
		CI	Data Signal Rate Detector from DCE	Periph	Control
		CJ	Ready for Receiving	Host	Control
21		RL	Remote Loopback	Host	Control
18		LL	Local Loopback	Host	Control
25		TM	Test Mode	Periph	Control
		DA	Transmitter Signal Element Timing	Host	Timing
15		DB	Transmitter Signal Element Timing from DCE	Periph	Timing
17		DD	Receiver Signal Element Timing From DCE	Periph	Timing
14		SBA	Secondary Transmitted Data	Host	Data
16		SBB	Secondary Received Data	Periph	Data
19		SCA	Secondary Request to Send	Host	Control
13		SCB	Secondary Clear to Send	Periph	Control
12		SCF	Secondary Received Line Signal Detector	Periph	Control

Note: This is for the DB9 Male pinout. Female DB9s often have the pins swapped to include the "null modem" function.

Comments:

Cool, thanks.+

Questions:

SPAMnadinlarajeshspam at KILLspamyahoo.com asks: " /techref/io/serial/rs232sigs.htm what is mnem there
" James Newton replies: A very poor abbreviation for mnemomic.+
+
Sholdn't "Data Circuit–Terminating Equipment" be abbreviated DCE instead of DCI (in the first line of this page)?+

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