RS-485
| TIA-485-A (Revision of EIA-485) | |
|---|---|
| Standard | ANSI/TIA/EIA-485-A-1998 Approved: March 3, 1998 Reaffirmed: March 28, 2003 |
| Physical media | Balanced Interconnecting Cable |
| Network topology | Point-to-point, Multi-dropped, Multi-point |
| Maximum devices | At least 32 unit loads |
| Maximum distance | Not specified |
| Mode of operation | Different Receiver levels: Binary 1 (OFF) (Voa-Vob < -200 mV) Binary 0 (ON) (Voa-Vob > +200 mV) |
| Available signals | A, B, C |
| Connector types | Not specified |
RS-485, also known as TIA-485(-A), EIA-485, is a standard defining the electrical characteristics of drivers and receivers for use in serial communications systems. Electrical signaling is balanced and multipoint systems are supported. The standard is jointly published by the Telecommunications Industry Association and Electronic Industries Alliance (TIA/EIA). Digital communications networks implementing the standard can be used effectively over long distances and in electrically noisy environments. Multiple receivers may be connected to such a network in a linear, multi-drop configuration. These characteristics make such networks useful in industrial environments and similar applications.
The EIA once labeled all its standards with the prefix "RS" (Recommended Standard), but the EIA-TIA officially replaced "RS" with "EIA/TIA" to help identify the origin of its standards. The EIA has officially disbanded and the standard is now maintained by the TIA. The RS-485 standard is superseded by TIA-485, but often engineers and applications guides continue to use the RS-485 designation.
RS-485 supports inexpensive local networks and multidrop communications links, using the same differential balanced line over twisted pair as RS-422. It is generally accepted that RS-485 can be used with data rates up to 10 Mbit/s and distances up to 1,200 m (4,000 ft), but not at the same time. A rule of thumb is that the speed in bit/s multiplied by the length in meters should not exceed 108. Thus a 50 meter cable should not signal faster than 2 Mbit/s. Under some conditions it can be used up to data transmission speeds of 64 Mbit/s.
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