There are many way to terminate this signal and it depends on the characteristics of the device, in this case an ni 9411 digital input. Selecting and using rs232, rs422, and rs485 serial data. Its a proven, reliable, and economical standard long used in industrial environment. That is to say that the termination of the cable with this impedance gives the. It is a balanced differential interface used mainly for multi drop configuration. Low power rs485rs422 transceiver stmicroelectronics. The rs485 interface supports higher data rate and distance compare to rs232. The termination resistor should match the characteristic impedance of the cable being terminated. Convenient, robust data transmission with rs422 and rs485. To enable the differential driver to be used, the rs422 standard uses a four conductor cable.
A power amplifier may have a rated impedance of 8 ohms, but the actual output impedance will vary depending on circuit conditions. Cable selection for rs422 and rs485 systems making sense of the specifications. Rs485 standard conform drivers provide a differential output of a minimum 1. Differential parallelterminated driver output waveforms. First when setting up such a hardware network is to define if serial data must or will be flowing in both direction at the same time. Rs422 interface ic are available at mouser electronics. A simple 2wire rs422 network a includes only one driver and as many as ten receivers. What is the difference between rs422 communication and rs485. Rs232 pinout standards exist for both db9 and db25 connectors as shown below. The differential receivers can be operated in rs422 or differential highthreshold logic htl modes and are optionally configurable for singleended ttlhtl operation. What is the difference between rs422 communication and. An unbalanced signal is represented by a single signal wire where a voltage level on that one wire is used to transmitreceive binary 1 and 0.
Guide to selecting and using rs232, rs422, and rs485 serial. Rs422 line driver to ni digital input termination ni. When a drivers output impedance is not matched to the transmission line characteristic impedance zo, there are reflections which causes ringing at the receiver as shown by the red waveform in the figure on the left. Numbers on the left indicate the pin assignment of db9 connectors. Need a discrete configurable rs422 driver io board for. The effective impedance of the rmc serials termination resistor and biasing resistors is 114w. Reliable data communications can be difficult in the presence of induced noise, ground level differences, impedance mismatches, failure to effectively bias for idle line conditions, and other hazards associated with installation of a network. Rs485 networks can also operate using 2 pairs of wires, in fullduplex mode, as described for rs422. Some explanation of these concepts is included, but if the reader is unfamiliar with these areas, some additional study may be required. April 4, 2007 rs485 rs422 and rs232 cable information. Scm3402asi halfduplex rs485rs422 transceiver mornsun. Rs422 is a serial communications standard that provides asynchronous communication capabilities, such as hardware flow control, software flow control, and parity check. Because a drivers output depends on the current it must supply into a load, adding transceivers and.
Rs422, rs485 differential line driver texas instruments. The interface has been tested at the baud rate of 19. The max3040max3045 is a family of 5v quad rs 485rs422 transmitters designed. B rs422 ttl rs422 ttl circuits provide a constant 5 v signal level that is not dependent on the supply voltage. This ability to compensate for manufacturing tolerances of some components enables test specifications to be relaxed and saves component and tester costs. The left hand is ttl level while the right hand is rs422 level. Need a discrete configurable rs422 driver io board for pc. The scm3402asi has a 1 8 load that allows 256 scm3402asi transceivers to be connected to the same. When the cable is cut to any length and terminated with this characteristic impedance, measurements of the cable will be identical to values obtained from the. The usbrs422pcb is a usb to rs422 level serial uart converter pcb incorporating ftdis ft232rq.
The effective impedance of the rmc7 x ss termination resistor and biasing resistors is 114 w. The connector k3 provide the rs422 bus signals plus the. Figure 4 shows the crosssection of a fourpair, utp cable typically. Rs422 is a high speed andor long distance data transmission. The output is differential mode driving, so it has two wires for each driver. A receiver that provides failsafe for an open input condition, may provide failsafe for idle drivers output idle and tristate drivers output disabled conditions if the bus is not parallel terminated. An rs232 port can supply only limited power to another device. Max14890e incremental encoder interface for rs422, htl. 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. The cable impedance can be obtained from the cable manufacturer.
Differential lvds, rs422, lvpecl, necl, pecl or ttl to rs422 logic level translation differential line driverreceiver essential lab tools for interfacing with high speed data communications equipment features. The purpose of this signal is to connect the driver to its output terminals, a and. It supports distance of 10 meters for 3035 mbps and 1200 meters for the data rate of about 100 kbps. Bus drivers in slave devices must have the capability to switch their outputs into a high. Il422 isolated rs422rs485 interface product datasheet. February 2009 rev 16 119 19 st485b st485c low power rs485rs422 transceiver features low quiescent current. We plan to use this card for serial io anyway and my understanding of this device is that there is a uart behind the rs422 interface and the outputs will either be tristated or if biased be in a highlow state when idle. Line drivers and receivers are commonly used to exchange data between two or more points nodes on a network. Drivers output impedance from ibis bert simonovichs. The circuit for ttltors422 converter is very simple. Termination should be placed at the end of the network for each wire pair.
If the enable signal is off, one can consider the driver as disconnected from the transmission line. Now, imagine the driver sends out voc 2v the differential voltage. Universal differential receiver, rs422 outputs pulse. These monolithic quadruple differential line drivers with 3state outputs are designed to meet the requirements of ansi standards eiatia422b and rs485 and itu recommendation v. The slave devices cannot communicate with the driving device. An rs422 driver may have this signal, but it is not always required. This section summarises the key features of the ft232rq which apply to the usbrs422pcb. Industrial rs485rs422 ttl 5v converter part number.
Rs422485 is a four or twowire, full or halfduplex, differential, moderatespeed serial communication interface that supports a multidrop rs422 or multipoint rs485 bus architecture. View more information about our rs485rs422 transceivers by category. A single transmitting master device to one or more receiving slave devices on a single pair of wires. 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. Ltm2885 6500vrms isolated rs485rs422 module transceiver.
When the cable is cut to any length and terminated with this characteristic impedance, measurements of the cable will be identical to values obtained from the infinite length cable. Output driver calibration also allows board designers to compensate for variations in trace impedance and termination voltage caused by manufacturing and assembly processes. The main reason for the existence of rs422485 is to accommodate multi drop communications over rather long lengths and or high baud rates. When installing an rs422 device in a rs485 bus network should also be noted that a rs422 device rs485 bus network so heavily loaded as 3 rs485 devices, ie the maximum number of devices per station. The max3040max3045 is a family of 5v quad rs 485rs422 transmitters designed for digital data transmission over twistedpair balanced lines. Each device turns on its line driver only when transmitting data, and keeps it off in high impedance state for the remaining time to allow other devices to transmit. You will see that one of the major differences between rs232 and rs422rs485 is the signaling mode. Highlevel output current driver ioh 60 ma highlevel digital output current receiver. Change in magnitude of common mode output voltage7. Rs422 will support pointtopoint, and multidrop circuits, but not multipoint. 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, rs485, rs232, rs422, telemetry, process control, translators, extenders. At the driver, the voltage difference must be at least 1. Rs485rs422 circuit implementation guide analog devices. The output impedance is a measure of the sources propensity to drop in voltage when the load draws current, the source network being the portion of the network that transmits and.
Eiatia422 defines a balanced differential interface. Some explanation of these concepts is included, but if the reader is unfamiliar with these. You will see that one of the major differences between rs232 and rs422 rs485 is the signaling mode. Using differential signals the output fully complies to the rs422 standard.
Now, if i understand it correctly line a if more positive than line b, then the reciever outputs a logic 1 and vice. The receiver also includes an enable signal equipment manufacturers could use to turn off the receiver. The driver or generator is labeled d, the receiver is labeled r, and the termination impedance is zt. I am purchasing an incremental encoder that has an rs422 line driver ouput. This is a differential signal and requires transmission line termination. Twoway communication requires two rs422 buses in parallel. The rs422 standard defines a serial communications standard. We plan to use this card for serial io anyway and my understanding of this device is that there is a uart behind the rs422 interface and the outputs will either be tristated or if. The receiver input has a failsafe feature that guarantees a logic high output. Rs422, also known as tiaeia422, is a technical standard originated by the electronic industries alliance that specifies electrical characteristics of a digital signaling circuit. The circuit for ttlto rs422 converter is very simple. How to properly configure and wire rs422 and rs485.
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. The biggest difference between rs422 and rs485 is how they communicate with devices on a single pair of differential wires. The rated output impedance is the impedance into which the amplifier can deliver its maximum amount of power without failing. Over distances up to 40 feet the maximum data rate is 100 kilobytes per second. Oct 22, 2008 what software commands set this cards output to a static output or reads the input high or low. Only one device can transmit at a time, which is called a halfduplex operation. Each signal is carried by a pair of wires and is thus a differential data transmission system.
Rs485 summary of key specifications parameter conditions min max units driver output voltage, open circuit 1. The number of output lines, the type of interface driver ic, and the state of the output lines are important considerations. Figure1 depicts pin configurations of rs485 interface in a 9 pin db9 connector. Rs422, rs485 line driver maxim integrated rs422, rs485. What software commands set this cards output to a static output or reads the input high or low. If b is at least 200 mv greater than a, the output is a logic low. An example of a pullup and a pulldown resistor on an rs485 bus to ensure the proper voltages on an inactive bus. The cable impedance may also be measured using tdr, time domain reflectometry, techniques. So an rs485 device can be used in an rs422 network, but rs422 devices are not necessarily compatible with an existing rs485 network. The texas instruments sn65hvd37 ic used in this example includes a 3state enable input to force the two driver outputs into a high impedance state. The output impedance of an electrical network is the measure of the opposition to current flow impedance, both static and dynamic, into the load network being connected that is internal to the electrical source. I was looking at the differential output driver of a rs422 transceiver with my scope, i had one output connect to the scope probe and the other connected to the scope probe ground. Therefore, cabling with impedance of 100w to 120w is recommended.
Maxim offers over 160 rs485rs422 transceivers for applications such as automotive, hvac, metering, and point of sale pos. But since its dc levels i am reckoning it will be in the range of 6v and 0v. Dec 22, 2010 when a drivers output impedance is not matched to the transmission line characteristic impedance zo, there are reflections which causes ringing at the receiver as shown by the red waveform in the figure on the left. Therefore, cabling with impedance of 100 w to 120 w is recommended. The actual output impedance for most devices is not the same as the rated output impedance. A value based on the inherent conductance, resistance, capacitance and inductance of a cable that represents the impedance of an infinitely long cable.
When you must have responses from slave devices, use a 4wire enhanced multidrop bus b. Unlike rs232, rs422 is capable of data transmissions over long distances up to 4000 feet. The txd and rxd lines status are provided by data indicating leds. For rs485 2wire, pointtopoint or multidrop, terminate the wire pair at each end of the network. Now, consider a driver rs422 connected to a reciever. The types of driver ics used in serial ports can be divided into three general categories. For differences less than 200 mv, the output is undefined. Rs422 systems can transmit data at rates as high as 10.
Figure 1 shows that a differential signaling interface circuit consists of a driver. Ftdi usb driver, the d2xx driver, can also be used with application software to directly access the. Additionally the rs422 standard allows for line impedances down to 50 ohms while supporting the high data rates. The use of rs422 devices on a rs485 bus network is only possible if they are used as recipients have as rs485 and rs422 bus different signal levels. Universal differential inputs accept lvds, lvpecl, necl, pecl, rs422 or ttl inputs differential 124. However, i can not find much information on the electrical characteristics of the 9411. Termination is used to match impedance of a node to the impedance of the. To illustrate their basic differences, a condensed description of each standard is presented in the following subsections. 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. 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.
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