In the market male connector is called DRXX and female connector is called DBXX, for example the serial port on our computer is called DR9 but not DB9 in the market, many people mistakenly call it DB9, in fact, DB9 is two connectors to connect two DR9 together. Generally in textbooks or articles, we often refer to all serial device interfaces as RS232 interfaces.

RS232 interface is a standard for serial communications which jointly formulated by Electronic Industries Association (EIA), Bell System, modem manufacturers and computer terminal manufacturers in 1970, full name is “Technical Standard for Serial Binary Data Exchange Interface between Data Terminal Equipment (DTE) and Data Communications Equipment (DCE)”. This standard specifies the use of a 25-pin DB25 connector, stipulates the signal content of each pin of the connector, and also stipulates the levels of various signals. The serial port of DB25 generally only uses three pins: 2 (RXD), 3 (TXD), and 7 (GND). With continuous improvement of equipment, DB25 pin is rarely seen now, it is replaced by DB9, the pins used by DB9 are slightly different from those of DB25, they are 2 (RXD), 3 (TXD), and 5 (GND). Therefore, RS232 interface is now called DB9.

Since RS232 interface standard appeared earlier so it inevitably has 4 shortcomings as below:

1.The signal level of the interface is relatively high which can damage the chip of the interface circuit easily, a level conversion circuit is required as it is incompatible with the TTL level.

2. The transmission rate is low, the baud rate is 20Kbps while asynchronous transmission.  

3. The interface uses a signal line and a signal return line to form a common ground transmission form, which is easy to cause common-mode interference so the immunity to noise interference is weak.

4. Transmission distance is limited, maximum theoretical transmission distance is 50 feet, but it can actually reachable 50 meters.

Aiming at the shortcomings of RS232 interface, some new interface standards emerged continuously, RS-485 is one of them and it has the following characteristics:

1.Electrical characteristics of RS-485: Logic “1” is represented by the voltage difference between the two lines being +(2-6) V; logic “0” is represented by the voltage difference between the two lines being -(2-6) V. The interface signal level is lower than RS-232 which makes it not damage the chip of the interface circuit easy, meanwhile the level is compatible with the TTL level so it can connect with the TTL circuit conveniently.  

2. The maximum transmission rate is 10Mbps.

3. RS-485 interface uses a combination of a balanced driver and a differential receiver, which has enhanced resistance to common mode dryness and good immunity to noise interference.

4. Maximum theoretical transmission distance is 4000 feet and actually reachable 3000 meters.

5. In addition, RS-232 interface allows only one transceiver to connect with the bus which is single-station capability, but RS-485 interface allows up to 128 transceivers to connect with the bus, that is it has multi-station capability so users can establish a device network with a single RS-485 interface easily.

Because the half-duplex network composed of the RS485 interface generally requires only two lines (we generally call it AB lines), so RS485 interface uses shielded twisted pairs for transmission.

Since some devices have RS232 interfaces and some have RS485 interfaces, if there is a RS232 interface device communicating with a RS485 interface device, then an RS232/RS485 converter is needed to convert the RS232 signal of the RS232 interface device into RS485 signal, then communicate with the device on RS485 interface. If two RS232 interface devices need to communicate over long distances, just add two RS232/RS485 conversion circuits is ok.

The so-called “passive RS232/RS485 converter” in the market uses the technology of stealing power from the computer serial port, while the circuit schematic diagram of the “active RS232/RS485 converter” is similar to the former, just little changes on the power supply part.