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RS232 connection as separate transmit and splitter

RS232 connection as separate transmit and splitter

 The RS232 connection has separate transmit and receive lines, which means that the hardware can send data and receive data at the same time ("full duplex"). However, the majority of the protocols are half duplex: also called "master-slave" or "query-reply". What the splitter cable does is to re-route the transmit lines of both devices to the read line for the monitoring PC. This will only work correctly for half duplex protocols; when both devices that are involved in the serial connection send at the same, the signals will collide on the split-off receive line.

RS232 connection as separate transmit and splitter.

Here is how it works: when a device (on either side of the communication link) is not sending, it pulls its transmit line low (where low is typically a voltage between -12V and -5V). If neither device is sending, both transmit lines are therefore low and they pull the receive line of the split-off cable low as well. If either device is sending, the "high" pulses (between +5V and +12V) pass through a diode and pull the receive line of the split-off cable up.

The two 4k7 resistors cause some cross-talk between the transmit lines of the two communicating devices, but this should not cause any problems.

On the PC that must monitor the data stream, you can run any common RS232 communication program, such as the HyperTerminal that comes with Windows. Note that the monitoring PC cannot send anything: the transmit line is left unconnected.

added 12.09.09 16:35:10 | looked over 3777 times