| 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, multidrop bus. These characteristics make RS-485 useful in industrial control systems and similar applications.
该段大致讲解了RS-485的由来。它定义了一套用于串行通信系统的驱动器和及接收器的电气特性的标准。电気信号是均衡的，和多点支持的系统。该标准由电气行业协会和电子行业联盟（TIA/EIA）联合发布。实现该标准的数字通信网络可有效的用于长距离和电噪环境中。多个接收器可以通过线性，分支总线的网络连接。这些特性使得RS485广泛用于工业控制系统及类似应用中。
概述部分
RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. It is generally accepted that RS-485 can be used with data rates up to 10 Mbit/s[a] or, at lower speeds, distances up to 1,200 m (4,000 ft).[2] As a rule of thumb, the speed in bit/s multiplied by the length in metres should not exceed 108. Thus a 50 meter cable should not signal faster than 2 Mbit/s.[3]
RS-485支持廉价的本地网络和多路传输链路，使用与RS-422相同的在双绞线上的差分信号。人们普遍认为RS-485可以用于数据速率高达10 Mbit/s[a]，或者在低速时可以使用到1200米(4000英尺)。根据经验，比特/秒的速率乘以米的长度不应超过108。因此，50米的电缆信号不应超过2mbit /s
In contrast to RS-422, which has a driver circuit which cannot be switched off, RS-485 drivers use three-state logic allowing individual transmitters to be deactivated. This allows RS-485 to implement linear bus topologies using only two wires. The equipment located along a set of RS-485 wires are interchangeably called nodes, stations or devices.[4] The recommended arrangement of the wires is as a connected series of point-to-point (multidropped) nodes, i.e. a line or bus, not a star, ring, or multiply connected network. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. If a star configuration is unavoidable, special RS-485 repeaters are available which bidirectionally listen for data on each span and then retransmit the data onto all other spans.
直接上机翻了。不得不说，这里太多的专有名词，啃起来很吃力。
与RS-422具有无法关闭的驱动电路相比，RS-485驱动程序使用三态逻辑，允许单个发射机被停用。这允许RS-485只使用两条线实现线性总线拓扑。位于RS-485线上的设备可互换地称为节点、站或设备。推荐的导线排列方式是作为一系列的点对点(多点)节点，即一条线或总线，而不是星形、环形或多重连接网络。由于信号反射或过低或过高的终端阻抗，不推荐星形和环形拓扑结构。如果无法避免星型配置，可以使用特殊的RS-485中继器，在每个跨上双向侦听数据，然后将数据重新传输到所有其他跨
Ideally, the two ends of the cable will have a termination resistor connected across the two wires. Without termination resistors, reflections of fast driver edges can cause data corruption. Termination resistors also reduce electrical noise sensitivity due to the lower impedance. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). Somewhere along the set of wires, pull up or pull down resistors are established to fail-safe bias each data wire when the lines are not being driven by any device. This way, the lines will be biased to known voltages and nodes will not interpret the noise from undriven lines as actual data; without biasing resistors, the data lines float in such a way that electrical noise sensitivity is greatest when all device stations are silent or unpowered.
典型的偏置网络和终端。RS-485标准中没有指定偏置和终止值。
理想情况下，电缆的两端将有一个终端电阻连接在两根电线上。如果没有终端电阻，快速驱动边缘的反射会导致数据损坏。终端电阻也降低了由于低阻抗的电噪声灵敏度。每个终端电阻器的值应该等于电缆特性阻抗(通常为双绞线的120欧姆)。在一组电线的某处，当线路没有被任何设备驱动时，拉起或拉下电阻器是建立在故障安全偏差的基础上的。这样，线路就会偏向已知电压，节点就不会把无驱动线路的噪声解释为实际数据;在没有偏置电阻的情况下，当所有的设备站都是静音或无电源时，数据线就会以这样一种方式浮动:当所有的设备站都是静音或无电源时，电噪声灵敏度最高。
Standard标准
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.[6] The EIA has officially disbanded and the standard is now maintained by the TIA as TIA-485, but engineers and applications guides continue to use the RS-485 designation.
RS-485 only specifies electrical characteristics of the generator and the receiver. It does not specify or recommend any communications protocol, only the physical layer. Other standards define the protocols for communication over an RS-485 link. The foreword to the standard recommends The Telecommunications Systems Bulletin TSB-89 which contains application guidelines, including data signaling rate vs. cable length, stub length, and configurations.
Section 4 defines the electrical characteristics of the generator (transmitter or driver), receiver, transceiver, and system. These characteristics include: definition of a unit load, voltage ranges, open circuit voltages, thresholds, and transient tolerance. It also defines three generator interface points (signal lines); "A", "B" and "C". The data is transmitted on "A" and "B". "C" is a ground reference. This section also defines the logic states 1 (off) and 0 (on), by the polarity between A and B terminals. If A is negative with respect to B, the state is binary 1. The reversed polarity (A +, B −) is binary 0. The standard does not assign any logic function to the two states.
EIA曾经把所有标准都标上“RS”(推荐标准)，但EIA-TIA正式将“RS”替换为“EIA/TIA”，以帮助确定其标准的来源。EIA已正式解散，TIA现在将该标准维持为TIA-485，但工程师和应用指南仍在使用RS-485。
RS-485只指定发电机和接收机的电气特性。它不指定或推荐任何通信协议，只指定物理层。其他标准定义了RS-485链路上的通信协议。标准的前言推荐电信系统公告TSB-89，其中包含应用指南，包括数据信令率与电缆长度、存根长度和配置。
第4节定义了发电机(发射机或驱动器)、接收机、收发器和系统的电气特性。这些特性包括:单位负载的定义、电压范围、开路电压、阈值和暂态公差。它还定义了三个发电机接口点(信号线);“A”、“B”和“C”。数据在“A”和“B”上传输。“C”是地面参考。本节还通过A和B终端之间的极性定义逻辑状态1 (off)和0 (on)。如果A对B是负的，状态是二进制1。反极性(A + B−)是二进制0。标准没有为这两种状态分配任何逻辑函数