The Host system is DTE (Data Terminal Equipment) and the peripheral system is DCI (Data CircuitTerminating 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|
|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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<A HREF="http://www.piclist.com/techref/io/serial/rs232sigs.htm"> RS232 Signals </A>
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Ashley Roll has put together a really nice little unit here. Leave off the MAX232 and keep these handy for the few times you need true RS232!
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