توضیحات

آي سي MAX485 يکي از آي سي هاي ساخت شرکت Maxim است که يک انتقال دهنده (Transceivers) براي استاندارهاي RS485 و RS422 مي باشد. لايه فيزيکي rs485 به عنوان TIA-485(-A), EIA-485 نيز شناخته ميشود که يک استاندارد ارسال و دريافت اطلاعات از طريق پورت سريال ميباشد و مهمترين مزيت ان قابليت شبکه کردن چند دستگاه روي يک باس اطلاعات ميباشد. چرا بايد از اين لايه فيزيکي استفاده کنم؟ اين لايه فيزيکي يکي از لايه فيزيکي هاي پرکاربرد در صنعت و همچنين الکترونيک ميباشد. -امکان شبکه کردن چند دستگاه روي يک باس اطلاعاتي -امکان انتقال ديتا به مسافت هاي دور تا 1 کيلومتر(از اين فاصله بيشتر نيز پشتيباني ميشود که بايد از Reapeter يا تکرار کننده ها استفاده نمود) -تاثير نويز بسيار کم روي اين پورت زيرا اين لايه فيزيکي سيگنال هاي اطلاعات TTL را تبديل به سيگنال هاي ديفرانسيلي مينمايد که در اين صورت در مکان هايي که نويز زياد است روي هردو خط اطلاعات به يک مقدار نويز تاثير ميگذاريد بنابراين تغييري در صحت اطلاعات به وجود نميايد. و يکي از معايبي که ميتوان ذکر نمود امکان انتقال اطلاعات به صورت half duplex ميباشد به اين معني که باس اطلاعاتي به صورت نيمه دوطرفه اطلاعات را انتقال ميدهد يعني در يک لحظه نميتوان هم ديتا دريافت نمود و هم ديتا ارسال کرد. (البته در اين مورد اگر نياز به ارسال و دريافت همزمان اطلاعات باشد از لايه فيزيکي rs422 استفاده ميکنند که خود از دو چيپ تبديل کننده TTL به RS485 استفاده ميشود)

Description

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. 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] 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.