Strictly speaking, the RS-232 cable is the name of a standard that describes the interface of connecting from a computer to a data transmission device. RS - recommended standard, translates as "recommended standard", and 232 - type number. It was developed back in the 60s of the last century. Today, the new edition of this standard, which was adopted in 1991 by the telecommunication and electronic industry associations , is called EIA / TIA-232-E. However, most people continue to use the name "RS-232 cable", which tightly "stuck" to the interface.
The above interface provides the connection of the following devices: DTE (Data Terminal Equipment) - OOD (Data Terminal Equipment), and DCE (Data Communications Equipment) - OPD (Data Transmission Equipment). OOD usually means a personal computer, and OPD means a modem. Although the RS-232 cable is also used to connect other peripheral devices (printer, mouse, etc.) to the PEOM, as well as to connect to other computers or controllers. It is important to remember the DCE and DTE designations, as they are used in the names of interface signals and help to understand the description of the required device implementation.
Initially, the RS-232 cable had a 25-pin DB25 type connector. The device type DTE was equipped with a socket-socket ("mother"). Later, they began to use a "stripped down" version of the interface with 9-pin DB9 connectors. This type of cable is common today.
RS-232 cable wiring
The following is the pin assignment of a 9-pin DB9 connector. The list shows the wiring of the connector (“male”) of data processing equipment, such as a personal computer. The socket of the data transmission device is soldered in such a way that both connectors are joined via a cable or directly “contact to contact”.
1. Carrier Detect - the presence of a carrier frequency.
2. Received Data - received data.
3. Transmitted Data - the transmitted data.
4. Data Terminal Ready - OOD readiness.
5. Signal Ground - common.
6. Data Set Read - readiness of the OPD.
7. Request To Send - transfer request.
8. ClearToSend - ready to send.
9. Ring Indicator - the presence of a call signal.
Data is transmitted over the RD and TD circuits. The remaining circuits are designed to display the status of DTR and DSR devices, control the transmission of CTS and RTS, and also indicate the status of RI and CD lines. Only when an external modem is connected to a personal computer, a complete set of circuits is used. When connecting other peripherals, such as controllers or mice, the selective circuits needed for specific equipment are used. They depend on the software and hardware implementation of the device.
Description and technical parameters
The standard clearly defines the maximum possible RS-232 cable length of 15 meters with a data transfer rate of 9600 bps. However, in practice it has been verified that stable operation is achieved with a longer wire length. It is believed that when using unshielded cable, you can increase the length to 30 meters, and when using shielded cable - up to 75 meters. And this is without loss of data transfer rate. If you reduce the speed by about half, the length of the cable also doubles. It is recommended to use a cable based on twisted pair, in which case each signal wire is paired with a common wire. It is not recommended to combine the cable shield with a common signal.
You can often find an RS-232-USB cable. It is a standard interface, at one end of which a USB connector is used .