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RS-485 interface chip introduction and application

Published on Oct 28 2010 // Micrcontroller circuits


RS-485 interface chip introduction and application

Abstract This paper describes the practical application of RS-485 interface chip types and some common solutions to problems.
RS-485 interface chip has been widely used in industrial control, instrumentation, instrumentation, multimedia network, mechatronic products and other areas. Can be used for RS-485 interface chip types is also increasing. How in the wide variety of interface chips to find the most suitable chip is placed in front of each user of a problem. RS-485 interface in the use of different occasions, the requirements of the chip and use are different. User selection of the chip and circuit design of what factors should be considered, due to the inherent characteristics of certain chips, some of communication failures even need to be adjusted in the software, and so on. Hope this solve some of the RS-485 interface, frequently asked questions helpful.
Nodes
Model
32
SN75176,SN75276, SN75179, SN75180, MAX485, MAX488, MAX490
64
SN75LBC184
128
MAX487,MAX1487
256
MAX1482,MAX1483, MAX3080 MAX3089 ~
2RS-485 interface standard
Transmission: Differential
Transmission media: twisted pair
Standard nodes: 32
Longest communication distance: 1200m common-mode voltage maximum and minimum values: +12 V;-7V
Differential input range:-7V ~ +12 V
Receiver Input Sensitivity: ± 200mV
Receiver input impedance: = 12kO
Three points and the half-duplex and full duplex communication
3.1 Points
The so-called nodes, that each RS-485 interface chip driver can drive the number of standard RS-485 load. According to regulations, standard RS-485 interface, the input impedance of = 12kO, the appropriate standards-driven nodes is 32. More communication nodes to adapt to situations, some of the input impedance of the chip design into a 1 / 2 load (= 24kO), 1 / 4 load (= 48kO) or 1 / 8 load (= 96kO), the corresponding number of nodes can be increased to 64,128 and 256. Table 1 shows some common chip nodes. Table 1
3.2 half-duplex and full duplex
RS-485 interface can be connected in two half-duplex and full duplex communication, shown in Figure 1. Half-duplex communication chips are SN75176, SN75276, SN75LBC184, MAX485, MAX1487, MAX3082, MAX1483, etc.; full-duplex communication chips are SN75179, SN75180, MAX488 ~ MAX491, MAX1482 and so on.

(A) half-duplex communication circuit

(B) full-duplex communication circuits
Figure 1
4 Application FAQs
4.1 The impact of anti-lightning and static
RS-485 interface chip in the use of welding equipment in transit or are likely to impact damage by static electricity. Erection of transmission lines used in outdoor situations, the interface chip and the whole system may also be stricken lightning attacks. Use anti-static or anti-lightning chip can effectively prevent such losses, the common chip has MAX485E, MAX487E, MAX1487E and so on. It is particularly worth mentioning is SN75LBC184, it not only the impact of lightning resistant and can withstand shocks of up to 8kV of electrostatic discharge is rare on the market a product.
4.2 The slew-rate drivers
Because the signal transmission process will produce electromagnetic interference and terminal reflection, so that the effective signal and invalid signal transmission line between superposition, and in severe traffic will not work correctly. To solve this problem, some of the chips designed to limit the slope of the drive way, so not too steep edge of the output signal to the transmission line Buzhi Yu excessive high frequency components to effectively curb the interference generated. Eg MAX487, SN75LBC184 and so has this feature.
4.3 Fault Protection
Fault protection techniques in the last two years generated some of the new RS-485 chips have adopted the technology, such as the SN75276, MAX3080 ~ MAX3089. What is the fault protection, why should there fault protection, fault protection if not what would be the consequences?
As we all know, RS-485 Interface uses a differential transmission mode, the communication between nodes through a pair of (half-duplex) or two (duplex) pair as a transmission medium. According to the provisions of RS-485 standard, the receiver sensitivity of the receiver ± 200mV, the receiving end of the differential voltage greater than, equal to +200 mV, the receiver output is high; less than, equal to-200mV, the receiver output low; range between ± 200mV, the receiver output uncertainty. Transmission line, the bus is idle all the nodes that are to receive state and open or short circuit in the transmission line failure, failure to take special measures, the high output of the receiver may also be output low. Once a receiver node will generate low the receiver to the serial (UART) can not find the start bit, causing abnormal communication to solve such problems in two ways:
(1) the use of the chip with fault protection, it will open in the bus, under short-circuit and idle, so that the output of the receiver is high. Ensure that the bus is free, high receiver output when the circuit is to change the receiver input threshold to achieve. For example, MAX3080 ~ MAX3089 input sensitivity -50mV/-200mV, the differential receiver input voltage UA-B =-50mV, the receiver output logic high; If the UA-B =-200mV, the output logic low . When the receiver circuit input bus or bus is prohibited on all of the transmitter, the receiver differential input is 0V, so that the receiver output high. Similarly, SN75276 sensitivity 0mV/-300mV, thus achieving the purpose of fault protection.
(2) If the use of non-fault protection of the chip, such as the SN75176, MAX1487, etc., can make some deal in the software, thus avoiding communication exception. That is before entering the normal data communication, the bus driver in advance by the host is greater than +200 mV, and for some time, so that all nodes have high receiver output. Thus, the issue of valid data, all receivers correctly receive start bit, and then received the full data.
4.4 optical isolation
In some areas of industrial control, because the scene is very complex, very high between the various nodes common-mode voltage. Although the RS-485 interface uses the differential transmission, has a certain anti-interference ability of the common mode, but when the common mode voltage exceeds the RS-485 receiver to receive the voltage limit, that is greater than or less than-7V +12 V when the receiver could no longer work properly, even when serious burn chips and equipment.
To solve such problems is through the system DC-DC power supply and RS-485 transceiver power isolation; the signal through the opto isolated, the total elimination of common mode voltage. Ways to implement this program can be divided into:
(1) with a coupler, isolated DC-DC, RS-485 chip to build the circuit;
(2) the use of the second integrated chip, such as PS1480, MAX1480 and so on.
Focuses on different occasions over how to choose the appropriate RS-485 interface chip, and may encounter the problem solution, in order to avoid communication exception. As for the other, such as termination, transmission line selection and screening, communication speed selection, etc., in some relevant information can be found in the answer, there is no longer introduced.