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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.

DB25 Pin DB9 Pin mnem 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

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