The RS-485 standard defines a popular and widely used serial data interface and network that’s been in use for over 30 years. It’s found in factories, process control plants, building automation, and other industrial applications. RS-485 popularity stems from its differential transmission format that minimizes the noise inherent in most industrial settings.
Such noise tolerance and immunity is critical to the adoption of a data network for industry, as well as the selection of its cabling and transceivers. For example, a key performance characteristic for RS-485 transceivers is electrical-fast-transient (EFT) protection.
Let’s take a closer look at the RS-485 standard, and where ETF protection is achieved.
A Brief Summary of RS-485
RS-485, a standard of the Electronic Industry Alliance (EIA) and the Telecommunications Industry Association (TIA), is designated as EIA/TIA-485-A. The American National Standards Institute (ANSI) also recognizes this standard. Its main features include:
- Unshielded twisted-pair (UTP) transmission line (like CAT5).
- Differential transmission.
- Mulitpoint drops or connections for up to 32 nodes.
- Half-duplex operation (single cable), full-duplex (two cables)
- Maximum range up to 5000 feet.
- Data rate depends on network length, from 100 kb/s to 50 Mb/s.
Though no formal data protocol is specified, the UART format with start and stop bits and up to 8 bits of data is commonly used, as are other proprietary protocols. Specific connectors haven’t been defined.