Rs485rs422 differential bus transceiver general description. Featuring an integrated encoder link function, the line drivers can be deactivated and analogdigital signals directly accessed through the line driver output pins. Rs422s capabilities make it a good choice for extending the distance and speed of pointtopoint connections. It never disconnects electrically from the receivers. If b is at least 200 mv greater than a, the output is a logic low. The increased load causes the rs485 transmitter to deliver more current into the load, which has the effect of reducing the differential output voltage developed by the driver ic. How to properly configure and wire rs422 and rs485. Figure 1 shows that a differential signaling interface circuit consists of a driver. Care must be taken however, to ensure that the resistance connected in place of the load is able to dissipate sufficient power without damage.
Whereas the rs422 voltage mode driver increases exponentially in most case between 20 mhz50 mhz. Guide to selecting and using rs232, rs422, and rs485. The sp3072een ltr is a differential line transceiver for bidirectional communication on balanced multipoint bus transmission lines and comply with both rs485 and rs422 eia standards. Eia485 devices may be used in 422 circuits, but eia422 may not be used in 485 circuits because of the lack of an enable line. Low power rs485rs422 transceiver stmicroelectronics. The am26ls31cn is a quadruple differential line driver designed to meet the requirements of ansi tiaeia422b and itu formerly ccitt recommendation v. The usbrs422pcb is a usb to rs422 level serial uart converter pcb incorporating ftdis ft232rq. Am26ls31cn quad differential rs422 line driver tri. Driver differential output delay and transition times ds0096307 ds0096308 figure 5. Its quiescent current remains relatively flat versus switching frequency. Vdds 105 tdmax maximum delay from singleended input to rs422 output 30 ns digital outputs x4, x5, x6 201 isclo output short circuit lo channel as rs422 receiver 8 100 ma v vdds oen1 202 ischi output short circuit hi channel as rs422 receiver 100. Rs422, also known as tiaeia422, is a technical standard originated by the electronic industries alliance that specifies electrical characteristics of a digital signaling circuit. Rs485 eiatia485 differential data transmission system.
An example of a pullup and a pulldown resistor on an rs485 bus to ensure the proper voltages on an inactive bus. Ds96176 rs485rs422 differential bus transceiver rev. Thus, a standard rs485 driver is capable of driving 32 unit loads 32 12k. The slave devices cannot communicate with the driving device. In a balanced differential system the voltage produced by the driver. The main difference between rs422 and rs485 is that while rs422 is strictly for pointtopoint communications and the driver is always enabled, rs485 can be used for multidrop.
The driver or generator is labeled d, the receiver is labeled r, and the termination impedance is zt. Rs422 will support pointtopoint, and multidrop circuits, but not multipoint. Il422 isolated rs422rs485 interface product datasheet. The current mode driver provides substantial benefits over voltage mode drivers, such as an rs422 driver. The load presented to an rs485rs422 driver is quantified in terms of the unit load, which is defined as the input impedance for one standard rs485 receiver 12k. For differences less than 200 mv, the output is undefined. Full guide to serial communication protocol and our rs485. The device consists of a differential driver and differential receiver. Rs422 driver output structure with forwardbiased substrate diode.
Rs485 is similar to rs422, but rs422 allows just one driver with multiple receivers, while rs485 supports multiple drivers and receivers. In situations where data must be communicated over long distances and through electrically noisy areas, rs422 provides a higher reliability replacement for standards such as rs232. For differential rs422 line driver operation, six differential complementary drivers are implemented. They both use the same differential drive with identical voltage swings. Ds90c031b lvds quad cmos differential line driver rev. The driver is capable of transmitting data across 4000 feet recommended limit of cable. When the driver is enabled, a logic low on di forces a low and b high, while a logic high on di forces a high and b low. A simple two wire differential communication from one rs485 master to up to 10 dlsflsc devices can be. Sp3072eenltr max linear rs422 rs485 transceiver ic. A high level enables the driver differential outputs, a and b. Y driver output capability g60 ma maximum y thermal shutdown protection. Rs485rs422 circuit implementation guide analog devices. This is due to the overlap current that flows between the rails of.
The texas instruments sn65hvd37 ic used in this example includes a 3state enable input to force the two driver outputs into a highimpedance state. The part contains a differential line driver and a differential line. How to properly configure and wire rs422 and rs485 networks. It was intended to replace the older rs232c standard with a standard that offered much higher speed, better immunity from noise, and longer cable lengths. April 4, 2007 rs485 rs422 and rs232 cable information. Termination is used to match impedance of a node to the impedance of the. While the rs485 specification does not specify cabling, these recommendations should be used for rs485 systems as well.
The measurement of output impedance uses the same method as for input impedance but with different connections. Example of rs485 communication cable beyond the network cabling, it is mandatory that the layout of printedcircuit boards and the connector pin assignments of rs485 equipment maintain the electrical characteristics of the network by keeping both signal lines close and equidistant to another. Removing that termination causes issues since youre driving a low output impedance driver into a high input impedance receiver and this will generate signal reflections and ringing. How do i lay out pcb traces for a given differential. Rs422rs485 interface ic are available at mouser electronics. In an rs 422 network, the lone driver always produces a logic1 or a logic0 output. The rs422 driver has the ability to drive ten rs422 receivers connected in parallel. Driver voltage measurement test circuit 2 a b r ldiff c l1 c l2 figure 3. Standard rs422 drivers are guaranteed to source and sink a minimum 20 ma across a 100. The integrated sensor bus interface in silicon based on. If the cable must be longer than one meter, copper strap or braided.
A simple 2wire rs422 network a includes only one driver and as many as ten receivers. Protective ground pin 1 and signal ground pin 7 are. Rs422 systems can transmit data at rates as high as 10. The il422 was the first isolated rs422 interface in a standard 16pin soic package to meet the ansi standards eiatia422b. In an rs422 network, the lone driver always produces a logic1 or a logic0 output. When the driver output voltage is measured differentially, the voltage on the a.
In addition, by having the input pins tied together via a resistor when the driver is not powered the pins will develop the same voltage and the receiver output. Rs485 summary of key specifications parameter conditions min max units driver output voltage, open circuit 1. The purpose of this signal is to connect the driver to its output terminals, a and b. Using the hs26ct31rh radiation hardened rs422 line. Bus drivers in slave devices must have the capability to switch their outputs into a high. Typically, twisted pair wire has an impedance of about 120 ohms.
Since only one driver should be enabled at any time, the output of a disabled or powereddown driver is threestated to avoid overloading the bus. High receiver input impedance allows a large number of transceivers to share a. An214transmission line drivers and receivers for tiaeia. The receiver also includes an enable signal equipment manufacturers could use to turn off the receiver. Driver propagation delay test circuit v out c l a b v cc s2 r l. A system of multiple receivers may require some experimentation with location and size of termination resistors, line lengths, grounding, etc. Output current low iol driver 60 ma receiver 16 ma operating temperature ta 0 25 70. Smith serial communication products is your rs485 rs232 and rs422 authority for converters, repeaters, microprocessors, fiber optics, ethernet serial port server, ethernet serial products, mei, nodes and networks, usb, serial and digital io solutions, serial and analog io solutions including rs485, rs232, rs422, rs 485, rs 232, rs 422, telemetry, process control, translators, extenders. Selecting and using rs232, rs422, and rs485 serial data. When the driver is enabled, a logic low on di forces a low and b high while a logic high on di forces a high and b low. Differential parallelterminated driver output waveforms. The original ansieiatia485a1998 standard was approved in march 1998 to address the shortcomings of rs232 and rs422. The rs422 specification recommends 24awg twisted pair cable with a shunt capacitance of 16 pf per foot and 100 ohm characteristic impedance.
Rs422 connection to rs485 laser distance measurement. The driver provides a low internal impedance that terminates the line at that end. You must measure all the lines in your network to determine the longest path. The rs422 standard recommends that protective ground pin 1 be connected to a good green wire ground. At the driver, the voltage difference must be at least 1. Rs485 driver, in addition to the output states of 1 and 0. The receiver commonmode input resistance, measured between. The bicmos output st age consists of a large nmos sink driver and a pmosnpn emitter follower source driver.
Throughout this application note the generic terms of rs422 and rs485 will be used to represent the. Eiatia422 defines a balanced differential interface. February 2009 rev 16 119 19 st485b st485c low power rs485rs422 transceiver features low quiescent current. But since its dc levels i am reckoning it will be in the range of 6v and 0v. Though rs422 doesnt explicitly mention a maximum commonmode load, the standard specifies that a driver must be able to drive 10 receivers with a minimum input resistance of 4 k. Guide to selecting and using rs232, rs422, and rs485 serial. As rs485 meets all the rs422 specifications table 3, rs485 drivers can be used in rs422 applications. To illustrate their basic differences, a condensed description of each standard is presented in the following subsections. So bus receivers always see a valid logic level, even when no bus activity occurs. The only difference in hardware of the rs 485 and rs 422 circuits is the ability to set the output to high impedance state. The same analysis as above would yield a load of 34ohms if the cable is terminated on both ends with its characteristic impedance of 68ohms. A system of multiple receivers may require some experimentation with location and size of termination. When you must have responses from slave devices, use a 4wire enhanced multidrop bus b.
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