What is basic purpose of RS232 to RS485 Converter?

Answer 1

RS232, a single-ended data communication connection was introduced in 1962, and despite rumors has remained widely used throughout the industry. The specification allows for data transmission from one transmitter to one receiver at relatively slow data rates (up to 20K bits/second) and short distances (up to 50 feet at the maximum data rate).

Independent channels are established for two-way (full-duplex) communications. RS232 has numerous handshaking lines (primarily used with modems), and also specifies a communications protocol. In general if you are not connected to a modem the handshaking lines can present a lot of problems if not disabled in software or accounted for in the hardware (loop-back or pulled-up or pulled-down). The RTS (Request to send) handshaking signal does have some utility in certain applications. RS423 is another single ended specification with enhanced operation over RS232; however, it has not been widely used in the industry.

When communicating at high data rates or over long distances in real world environments, single-ended methods are often inadequate. Differential data transmission (balanced differential signal) offers superior performance in most applications, as it can help nullify the effects of ground shifts and induced noise signals that can appear as common mode voltages on a network bus in commonly noisy environments.

RS485 meets the requirements for a truly multi-point communications network, and the standard specifies up to 32 drivers and 32 receivers on a single two wire bus. With the introduction of repeaters and high-impedance drivers & receivers this 'limitation' can be extended to hundreds of nodes on a network or larger. RS485 extends the common mode range for both drivers and receivers in the 'tri-state' mode and with power off. Also, RS485 drivers are able to withstand data collisions, or bus contention problems, and bus fault conditions.

To solve the 'data collision' problem sometimes present in multi-drop networks, hardware units (converters, repeaters, micro-processor controls) can be constructed to remain in a receive mode until they are ready to transmit data. Single master systems (there are many other communication designs) offer straightforward and simple means to avoid data collisions in a typical two wire, half-duplex, multi-drop system. The master initiates a communications request to a slave node by addressing that unit. The hardware detects the start-bit of the transmission and automatically enables (on the fly) the RS485 transmitter. Once a character is sent the hardware reverts back into a receive mode in about 1-2 microseconds typically.

Any number of characters can be sent, and the transmitter will automatically re-trigger with each new character (or in many cases a bit-oriented timing scheme is used in conjunction with network biasing for fully automatic operation, including any Baud rate or any communications spec, for instance, 9600,N,8,1). Once a slave unit is addressed it is able to respond immediately because of the fast transmitter turn-off time of the automatic device; it is NOT necessary to introduce long delays in a network to avoid data collisions. And, because delays are not required, networks can be constructed that will utilize the data communications bandwidth with up to 100% throughput.

In summary, each RS232 to RS485 converter could be used to connect a single simple device, that has no differential interface, to an efficiently networked communications bus with many other nodes, or over much longer distances.