RS485相關學習

 

 

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.

該段大體講解了RS-485的由來。它定義了一套用於串行通訊系統的驅動器和及接收器的電氣特性的標準。電気信號是均衡的，和多點支持的系統。該標準由電氣行業協會和電子行業聯盟（TIA/EIA）聯合發佈。實現該標準的數字通訊網絡可有效的用於長距離和電噪環境中。多個接收器能夠經過線性，分支總線的網絡鏈接。這些特性使得RS485普遍用於工業控制系統及相似應用中。

 

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支持廉價的本地網絡和多路傳輸鏈路，使用與RS-422相同的在雙絞線上的差分信號。人們廣泛認爲RS-485能夠用於數據速率高達10 Mbit/s[a]，或者在低速時能夠使用到1200米(4000英尺)。根據經驗，比特/秒的速率乘以米的長度不該超過108。所以，50米的電纜信號不該超過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.

直接上機翻了。不得不說，這裏太多的專有名詞，啃起來很吃力。

與RS-422具備沒法關閉的驅動電路相比，RS-485驅動程序使用三態邏輯，容許單個發射機被停用。這容許RS-485只使用兩條線實現線性總線拓撲。位於RS-485線上的設備可互換地稱爲節點、站或設備。推薦的導線排列方式是做爲一系列的點對點(多點)節點，即一條線或總線，而不是星形、環形或多重鏈接網絡。因爲信號反射或太低或太高的終端阻抗，不推薦星形和環形拓撲結構。若是沒法避免星型配置，能夠使用特殊的RS-485中繼器，在每一個跨上雙向偵聽數據，而後將數據從新傳輸到全部其餘跨

 

Typical bias network together with termination. Biasing and termination values are not specified in the RS-485 standard.

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.

典型的偏置網絡和終端。RS-485標準中沒有指定偏置和終止值。

理想狀況下，電纜的兩端將有一個終端電阻鏈接在兩根電線上。若是沒有終端電阻，快速驅動邊緣的反射會致使數據損壞。終端電阻也下降了因爲低阻抗的電噪聲靈敏度。每一個終端電阻器的值應該等於電纜特性阻抗(一般爲雙絞線的120歐姆)。在一組電線的某處，當線路沒有被任何設備驅動時，拉起或拉下電阻器是創建在故障安全誤差的基礎上的。這樣，線路就會偏向已知電壓，節點就不會把無驅動線路的噪聲解釋爲實際數據;在沒有偏置電阻的狀況下，當全部的設備站都是靜音或無電源時，數據線就會以這樣一種方式浮動:當全部的設備站都是靜音或無電源時，電噪聲靈敏度最高。

 

Standard標準

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曾經把全部標準都標上「RS」(推薦標準)，但EIA-TIA正式將「RS」替換爲「EIA/TIA」，以幫助肯定其標準的來源。EIA已正式解散，TIA如今將該標準維持爲TIA-485，但工程師和應用指南仍在使用RS-485。

RS-485只指定發電機和接收機的電氣特性。它不指定或推薦任何通訊協議，只指定物理層。其餘標準定義了RS-485鏈路上的通訊協議。標準的前言推薦電信系統公告TSB-89，其中包含應用指南，包括數據信令率與電纜長度、存根長度和配置。

第4節定義了發電機(發射機或驅動器)、接收機、收發器和系統的電氣特性。這些特性包括:單位負載的定義、電壓範圍、開路電壓、閾值和暫態公差。它還定義了三個發電機接口點(信號線);「A」、「B」和「C」。數據在「A」和「B」上傳輸。「C」是地面參考。本節還經過A和B終端之間的極性定義邏輯狀態1 (off)和0 (on)。若是A對B是負的，狀態是二進制1。反極性(A + B−)是二進制0。標準沒有爲這兩種狀態分配任何邏輯函數

 

 

 

 

 

 

 

 

 

 

參考資料：https://en.wikipedia.org/wiki/RS-485