This interface standard is designed for two communication devices which each use a data source and a data sink. For data transfer, three wires are needed, a send wire (TX), a receive wire (RX) and a common ground.
The signals of RS-232 are bipolar. A logical ‘0’ is represented by a voltage of +12 Volt, a logical ‘1’ by -12 V. The ratio of signals and disturbance is therefore significantly bigger than on a parallel interface. This makes it possible to transfer data over relatively long distances without bigger disturbance.
Cable lengths of more than 20 meters are not recommended.
double-current, also named TTY
This transfer type uses a wire with impressed current (20 mA). Therefore, the voltage loss does not influence the transfer that much so that cable lengths of up to 100 meters can be used.
The data signal is transfered in push-pull operations (like the RS-422), which significantly reduces the disturbance and overcoupling. Therefore, smaller signal amplitudes can be used (e.g. +/- 5V).
The maximal data transfer rate is 115 kBd.
This standard is used for the communication between a sender an a receiver. Four wires are needed, two send wires and two receive wires which are used alternately. On these wires a logical ‘1’ is represented by 5V on the upper wire (TX+ or RX+) and 0V on the lower wire (TX- or RX-),a logical ‘0' the other way round.
The wires on this interface are used in push-pull operations like on the RS-422; however, only two wires are needed which are used in half-duplex mode.
Besides, the RS-485 supports the use of more than one sender or receiver by the use of a protocol. RS-485 supports cable lengths of up to 1.2 km and data transfer rates of up to 1 MBit/s (depending on the serial controller)
Data source, data sink
A data source or data sink is a data end unit, e.g. a PC, which sends and receives data. The data is is always sent from the source to the sink.
Possibility of simultaneous sending and receiving.
Alternating send and receive possibilty on one conduction (multiplexed).
Only sending or receiving.
In push-pull-operation, one wire carries the signal which has to be transmitted, the second one the inverted signal (symmetrical conduction). On the end of the line, the difference of the two signal amplitudes is calculated. That way, common-mode interferences like crosstalk and noise have near to no effect on the transmission.