RS-485 is a signal transmission standard approved by the Electronic Industries Association (EIA) in 1983. It is only an electrical standard and does not involve upper layer protocol specifications, only physical layer specifications for interfaces.
1, RS-485Characteristics of
1) Using differential signals for transmission, Increased noise resistance capability, Reduce interference caused by noise radiation;
2) Support long-distance transmission, The longest it can reach4000foot(About)1219Rice);
3) High transmission speed, in40Within feet, The speed can reach up to10Mbps;
4) The same bus can connect multiple drivers and receivers;
5) Large common mode voltage range, The maximum allowable common mode voltage can reach -7V - 12V;
2, RS-485Common Circuit Connection Methods
The image used in the above picture485The chip isSP3485, R0To send pins, DITo receive pins, RE/DETo send and receive control pins. 485The chip can only transmit data in one direction at a time, Cannot send and receive simultaneously, And the transmission and reception must control the level status of the transmission and reception pins to achieve the function of transmission and reception.
3, RS-485The level signal
RS-485The transmission is carried out using differential signals, ThroughABThe voltage difference between two wires is used as a level signal, The interference of differential signals on external factors is reflected in the same way on both lines, The voltage difference between the two wires remains unchanged, Signal transmission will not be interfered with, Therefore, it will have a stronger ability to resist noise interference.
RS-485The differential line includes the following two signals:
A: Non reverse (non-inverting) signal
B: reverse (inverting) signal
logic1: The signal voltage of line B is higher than that of line A. general +2V ~ +6V.
logic0: The signal voltage of line A is higher than that of line B. general -2V ~ -6V.
according toRS485Related standards for busesWhenRS485Bus differential voltage greater than+200mVWhen,RS485Transceiver output high levelWhenRS485The differential voltage of the bus is less than-200mVWhen,RS485Transceiver output low levelWhenRS485The voltage on the bus is-200mV~+200mVBetween times, RS485The transceiver may output a high level or a low level. Therefore, to preventRS-485The differential voltage on the bus is at-200mV~+200mVbetween, We need to use pull-up and pull-down resistors to clamp the voltage difference on the bus. The blocking of up and down resistors is generally selected4.7K, What is the actual required resistance, The selection value needs to be calculated based on the actual line load and circuit conditions.
4, RS-485Number of transceivers on the bus
RS-485The maximum number of transmitters and receivers that can be carried on the bus is related to the load characteristics. according to485The instructions in the regulatory document, A maximum of can be mounted on a transmission bus128A transceiver.
5, RS-485The working mode of the bus
RS-485Bus can be designed with two communication methods: Half duplex and full duplex.
1) Half-duplex
Only allow one-way transmission of data on the bus, Only one of them can be sent or received at a time. The connection method of a half duplex bus is as follows:
2) full-duplex
Allow bidirectional transmission of data on the bus, That is, each transmission and reception can be carried out simultaneously. The connection method of a full duplex bus is as follows:
6, RS-485Bus terminal resistance
To avoid signal reflection, When the length of the communication line is very long, the data transmission line must have an endpoint, And the branch length should be as short as possible. The correct communication line terminal requires terminal resistanceRTmatch, RS-485Standard recommendation for cables Z0=120Ω. Communication lines usually have terminal matching120ΩThe resistance, One at the end of each cable. as shown in the figure below:
Leave a Reply