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Amazing Launched a Polarity-Free RS485 with a Powerful System-Level ESD Protection Capability2014/03/24


The RS485 has several benefits in terms of cost, noise immunity, simple structure, high reliability, long distance laying and wide common mode range. Therefore RS485 become a basic built-in communication interface of smart electric meter application in smart grid. But its might be damaged or fail to communication due to reverse wiring the polarity differential communication port. As seen in the current situation, there are twenty to thirty percent chance of networking communication failure is reversing wiring the polarized RS485 differential port. According to present China domestic demand, there are about 70 million electric meters per year. It is very important to provide polarity-free RS485 reducing the failure rate by reversing wiring to safe the huge labor debugging costs and time costs and increasing network efficiency.
AMC brings the polarity-free RS485 transceiver which has already success integrated the polarity detector inside. AZRS5485 detects the polarity of differential communication port automatically and then start the self-correction procedure to amend the reverse polarized wire or keep the correct polarized wire. AZRS485 transceiver also includes a powerful ESD protection capability   which against system-level ESD (IEC 61000-4-2, Contact Discharge) to ±23 kV, greatly improve the reliability ofthe smart electric meter.
AMC is the first electrostatic discharge (ESD) protection IC design house in Taiwan that can provides the specializing ESD protection solutions.  
Current Solutions for Reverse Wiring of RS485 Network Communication 
1.    Hardware detection and manual correction
RS485 communication port with the external light-emitting diode (LED) to ground and human judgment polarity is correct or not by monitor the LED. If operator finds the wrong alarm they switch switched to adjust the wrong connection.
Disadvantages: Requires additional LED, switch and professional construction operator.
2.    Software detection and self-correction
Add additional procedure for software to a polar confirmation before the network communication. The master terminal sends a detection signal to the slave terminal. Slave terminal receives the detection signal and send the handshake back to master terminal. In the set time, if master have not received the handshake signal from the slave terminal, the host will be determined from the end of the line connection error, the master sends a control signal to changes the slave connection of the external electromagnetic relay switch connected to the interface circuit to do further testing correction action.
Disadvantages: Requires additional electromagnetic relays and software validation procedures.
3.    Encoding transmission
Transfer the output signal to differential Manchester encoding or Manchester encoding can be realized polarity-free connection.
Disadvantages: Requires additional clock, encoding and decoding chip between the MCU and RS485 transceiver.
Implementation of Polarity-Free RS485 Transceiver (AZRS5485)
AZRS5485 has built-polarity detection and judging circuit, it does not need external hardware to manually polarity judgment nor MCU software polarity detection procedure and extra chips to perform complex signal encoding and decoding.
As long as the power-on the AZRS5485 and wire to communication network, the built-in polarity judgment circuit will detect the polarity and judge the polarity is correct or not automatically. If a connection is correct, then keep the situation. If the connection is error, AZRS5485 can in-house calibration and correction. It eliminates the need for external switches and electromagnetic relay switch polarity switching operation to do, as shown in Figure 1. the construction may not be professionals, even students can be arbitrarily connected the communication without considering the polarity, without any additional extra hardware, software or code encoding decoding. All of the detection, judgment and corrective actions are completed in AZRS5485 themselves within the chip.
Features of Polarity-Free RS485 Transceiver (AZRS5485)
AZRS5485 is design for easy field wiring construction. The construction worker can arbitrarily wiring without considering the polarity of differential signal just like plug and play.
The Polarity-Free network doesn’t need the extra encoder circuit, switching replay or special communication protocol. As long as polarity-free network adopts AZRS5485 transceiver, the polarity detection and polarity correction will be done by itself automatically. 
The AZRS5485 is a Polarity-Free half-duplex RS485 transceiver IC, which contains one transmitter and one receiver inside. This device is fully compliant with the EIA/TIA-485 standard with 5V power supply.
The AZRS5485 has wide common-mode range (-7V~12V) and high ESD protection levels of these devices make them suitable for demanding applications. This transceiver is SOP8 package which matches the industry-standard footprint of SN75176.
The AZRS5485 features a fail-safe receiver, which guarantees the output of the receiver to be logic high when the differential inputs (bus pins, A and B) of the receiver inputs are open, short or idle under abnormal operating conditions.
The AZRS5485 has the thermal shutdown and the current limited function in the transmitter to protect the device from damage by system fault conditions during normal operating condition. The AZRS485 is designed 1/8 unit load with minimum 96kohm of input impedance, which can connect 256 devices on a bus at least.
ESD Protection Capability of Polarity-Free RS485 Transceiver
In Order to meet DL/T 645 standard, the bus pin (A and B) have HBM 15kV ESD protection capability of traditional RS485 transceiver. The HBM protection level is only guarantee a Component-Level ESD that is not enough to meet the System-Level ESD requirement for real application. To solve the System-Level ESD event, the powerful ESD protector TVS (Transient Voltage Suppressor) is integrated in the RS485 transceivers to against System-Level ESD energy. The detail specification is shown below.
1.        IEC 61000-4-2, Contact Discharge ±23 kV
2.        IEC 61000-4-2, Air-Gap Discharge ±23 kV
AZRS5485 greatly enhance the realibilaty of smart electric meter without any extar ESD protector.
Not only the terminal A and terminal B have a complete System-Level ESD protection capability, but they also have embadeed wholechip ESD protection circuit to provide efficient ESD protection, which gurantee every pins to sustain HBM (Human Body Model)8kV and the MM (Machine Model) protection is up to 800V for Industrial Networks application. For stability and realibility consideration, that Charge Device Model (CDM) and Latchup imminity are very important. AZRS5485 provide 1kV CDM protection capability and the latchup immunity is up to 400mA,it is very suitable for noisy environment application.
Table1 Features of AZRS5485

Part Number
Supply Voltage
Data Rate
IEC 61000-4-2 Contact on A,B
Special Function
Package Type
± 23kV
Polarity Free

Introduction of Polarity-Free RS485 Network Communication
The major benefits of constitution the polarity-free RS485 network communication are easy on-site construction and convenient maintenance. Therefore; greatly reduces the RS485 network communication failure probability and safe the debugging time and labor costs. It is impossible to install one meter to test one meter immediately when construct the large-scale intelligent meter networking. It must be completed fully assembled and then back for RS485 network communication testing and debugging. There are twenty to thirty percent error causes due to reverse polarity wiring. So long as the use of polarity-free RS485 communication network can avoid from these troubles, it can be used in a completely polarity-free communications network, or a hybrid of polarize and polarity-free communications network. It is very convenient to replace the meter directly without considering the polarity of differential communications network bus, as shown in Fig. 4.
Polarity-free Meter Hardware Introduction:
1.    Polarity-free meter has NO pull_up and pull_down resistor as shown in Fig.3.
2.    Polarity-free transceiver can be adopted in either three potocouplers or two potocouplers system as shown in Fig.2.
3.    The polarity-free communication network only needs one pair of pull_up and pull_down resistor. (Recommend place at collector).
AMC RS-485 Products
AZRS485: 5V Transceiver embedded System-Level ESD protection TVS
AZRS485 is a ±15 kV IEC 61000-4-2 Contact Discharge protected, half-duplex transceiver designed for RS-485 data bus network, which contains one transmitter and one receiver with 5V power supply. This device is fully compliant with the EIA/TIA-485 standard.
AZRS3078: 3.3V Transceiver embedded System-Level ESD protection TVS
AZRS3078 is a 3.3V supply voltage high speed RS-485 transceiver, and likewise which also integrates the external TVS into transceiver and meets the EIA/TIA-485 electrical standard. This Transceiver is very suitable for IP CAM application.
AZRS3082AZRS3088: Transceiver embedded Surge protection TVS
AMC base on their own technology develop the Surge TVS on traditional CMOS process to enhance the System-Level ESD protection capability (IEC 61000-4-2, Contact Discharge ±30 kV) and lightning surge protection capability (IEC 61000-4-5, Surge ±30 A). The AZRS3082 and AZRS3088 have ultra low standby current and low quiescent current to meet longer standby time. Furthermore, AZRS3082 and AZRS3088 transmit larger differential output voltage for cable dissipation those provide longer cable transmission capability at same data baud rate.
Table2 AMC RS-485 Products

T/R (#)
Data Rate (kbps)
Vcc (V)
Surge (A)

                                                                                   Fig.1  AZRS5485 Function Logic Diagram
                                        Fig.2 AZRS5485 is Suitable for Either Three or Two photocouplers Application in Smart Electric Meter

                                                                       Fig.3 Compare the External Circuit of Polarity-Free Transceiver and Polarized Transceiver.

                                                   Fig.4 RS485Communication of Polarity-Free Network, Polarized Network and Hybrid Network

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