application note 1 an207 advantages of enhanced receiver fail safe features in rs-485/rs -422 serial transceivers 1.0 introduction rs-485/rs-422 receiver failsafe keeps the data bus in a known state when there are no active drivers on the bus. the exar rs-485/rs-422 transceivers offer two different types of receiver failsafe: standard and enhanced. this a pplication note will discuss how the enhanced re ceiver failsafe feature can help customers with failsafe problems while terminated. 2.0 standard receiver failsafe rs-485/rs-422 devices such as exar?s sp491 offer a standard receiver failsafe. the standard receiver failsafe feature for this device will prov ide a logic high at the receiver outp ut when the receiver inputs are open circuit or floating. for exam ple, when the data cable is disconnected, the re ceiver output will keep the line in a known high state. stan dard receiver failsafe has an internal weak biasing network that will force the open inputs to >200mv thus resulting in a logic high output . figure 1 shows the receiver configuration for failsafe logic high output. f igure 1. o pen i nput r eceiver f ailsafe some applications operating at high speeds incorpor ate a termination resistor at the ends of the bus to prevent signal reflections. the termination resistor va lue should match the characte ristic impedance of the data cable. in a majority of applications, the termination resist or is typically 120 ohms. when a termination resistor is connected between re ceiver inputs "a" and "b" the standard rece iver failsafe fe ature will not operate properly (see figure 2). f igure 2. t erminated r eceiver i nput
application note 2 an207 this has been a common problem with many standard fa ilsafe receiver devices. th e termination resistor causes a potential difference of 0v because the internal biasing circuit is not strong enough to overcome the termination load. the receiver sensitivity, as per rs-485 specif ications, is +/-200mv (see figure 3). f igure 3. r eceiver s ensitivity a potential difference of 0v is within the indeterminate ra nge of sensitivity. this may result in an unknown state. under this condition, many may find the receiver output state on some devices to be good while others bad. in fact, some devices with enough noise pr esent may produce an oscilla ting clock pulse at ou tput. one solution to this terminated failsafe problem is to include an external biasing network that will pr ovide a >+200mv potential difference across the termination resistor at receiver in put. this external biasing network is a simple pull-up resistor from "a" input to vcc and a pull-down resistor fr om "b" input to gnd (see figure 4). the values of the pull-up/down resistors are selected to provid e at least 200mv across "a" and "b" inputs. f igure 4. t erminated r eceiver i nput w ith b iasing n etwork 3.0 enhanced receiver failsafe exar has developed several new fa milies of rs-485/rs-422 transcei vers, sp1486, sp 3080-3088, sp3495- 3497, xr3170-3178 and xr5486-5488 with an enhanced re ceiver failsafe feature. this enhanced receiver failsafe provides failsafe for three different conditions: o pen input, shorted input and un-driven terminated lines.
application note 3 an207 this feature is very useful for terminated applications in that no external biasing network is required for proper failsafe operation. many of exar?s original rs-485 tran sceivers with standard failsafe may be upgraded to a pin compatible design using sp1486, sp3080-3088, sp3495-3497, xr3170-3178 and xr5486-5488 products. the sp3086e is pin compatible to sp491. the sp3086e receiver offers an enhanced failsafe feature and has a negative input threshold (see figure 5). f igure 5. e nhanced r eceiver f ailsafe s ensitivity when the connected driver is disabled while the receiver input is terminated the potential difference at receiver input will be 0v. since the en hanced receiver failsafe device has a ne gative threshold the output will still be logic high when the differential voltage is 0v. figure s 6, 7 and 8 show the three different conditions for enhanced receiver failsafe logic high output. f igure 6. o pen i nput
f igure 7. u n - driven t erminated i nput f igure 8. s horted i nput application note 4 an207
application note 5 an207 4.0 exar transceivers with enhanced receiver failsafe t able 1: e nhanced r eceiver f ailsafe t ransceivers p art n umber v cc m aximum d ata r ate p ackage sp1486e 5.0v 20mbps 8-pin nsoic sp4082e 5.0v 115kbps 8-pin nsoic sp3495e 3.3v 32mbps 8-pin nsoic sp3496e 3.3v 32mbps 8-pin nsoic sp3497e 3.3v 32mbps 14-pin nsoic sp3070e 3.3v 250kbps 14-pin nsoic sp3071e 3.3v 250kbps 8-pin nsoic sp3072e 3.3v 250kbps 8-pin nsoic sp3073e 3.3v 500kbps 14-pin nsoic sp3074e 3.3v 500kbps 8-pin nsoic sp3075e 3.3v 500kbps 8-pin nsoic sp3076e 3.3v 16mbps 14-pin nsoic sp3077e 3.3v 16mbps 8-pin nsoic sp3078e 3.3v 16mbps 8-pin nsoic sp3080e 5.0v 115kbps 14-pin nsoic sp3081e 5.0v 115kbps 8-pin nsoic sp3082e 5.0v 115kbps 8-pin nsoic sp3083e 5.0v 500kbps 14-pin nsoic sp3084e 5.0v 500kbps 8-pin nsoic sp3085e 5.0v 500kbps 8-pin nsoic sp3086e 5.0v 20mbps 14-pin nsoic sp3087e 5.0v 20mbps 8-pin nsoic sp3088e 5.0v 20mbps 8-pin nsoic xr3170e 3.3v 250kbps 14-pin nsoic xr3171e 3.3v 250kbps 8-pin nsoic xr3172e 3.3v 250kbps 8-pin nsoic xr3173e 3.3v 500kbps 14-pin nsoic xr3174e 3.3v 500kbps 8-pin nsoic XR3175E 3.3v 500kbps 8-pin nsoic xr3176e 3.3v 20mbps 14-pin nsoic xr3177e 3.3v 20mbps 8-pin nsoic xr3178e 3.3v 20mbps 8-pin nsoic xr5486e 5.0v 52mbps 14-pin nsoic xr5487e 5.0v 52mbps 8-pin nsoic xr5488e 5.0v 52mbps 8-pin nsoic
6 notice exar corporation reserves the right to make changes to the products contained in this publicat ion in order to improve design, performanc e or reliability. exar corp oration assumes no responsibility for the use of any circuits described herein, conveys no license under any patent or other right, and makes no representation that the circuits are free of patent in fringement. charts and sc hedules contained here in are only for illustration purposes and may vary depending upon a user?s specific application. while the information in this publication has been carefully ch ecked; no responsibility, however , is assumed for inaccuracies. exar corporation does not re commend the use of any of its products in life suppo rt applications where the failure or malfunction of the product can reasonably be ex pected to cause failure of the life support system or to significantly affect its safety or effectiveness. products ar e not authorized for use in such applications unless exar corporation receives , in writing, assurances to its satisfaction that: (a) the ri sk of injury or damage has been minimized; (b) the us er assumes all such risks; (c) potential liability of exar corporation is adequately protected under the circumstances. copyright 2010 exar corporation datasheet june 2010. reproduction, in part or whole, without the prior written consent of exar co rporation is prohibited. application note an207 5.0 technical support technical questions about this application note should be e-mailed to uarttechsupport@exar.com.
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