Post by thread:RS485 & RS422

I'm looking at designing a multi drop comm's system for control purposes. I have been investigating RS485, and RS422 keeps on coming up along with it, so I know there can't be much difference between them, but what is the difference ? Thanks in advance for any responses, Wayne. ------------------------------------ Wayne G Boyd (Digital Sys. Eng.) E-mail: spam_OUTwayneTakeThisOuTjce.wintermute.co.uk ------------------------------------

pic<.....picKILLspam@spam@jce.wintermute.co.uk> wrote: > I'm looking at designing a multi drop comm's system for control purposes. > I have been investigating RS485, and RS422 keeps on coming up along with it, > so I know there can't be much difference between them, but what is the > difference ? They are both 5V differential (twisted pair) but RS422 is unidirectional - the drivers are always enabled (but you can have more than one receiver) whereas RS485 is bi-directional - the transmitters are tri-stated when not talking. RS485 also has better common-mode isolation specs. -- Clyde Smith-Stubbs | HI-TECH Software, | Voice: +61 7 3300 5011 clydeKILLspamhitech.com.au | P.O. Box 103, Alderley, | Fax: +61 7 3300 5246 http://www.hitech.com.au | QLD, 4051, AUSTRALIA. | BBS: +61 7 3300 5235 ---------------------------------------------------------------------------- For info on the World's best C cross compilers for embedded systems, point your WWW browser at http://www.hitech.com.au, or email .....infoKILLspam.....hitech.com.au

pic wrote: > > I'm looking at designing a multi drop comm's system for control purposes. > I have been investigating RS485, and RS422 keeps on coming up along with it, > so I know there can't be much difference between them, but what is the > difference ? > > Thanks in advance for any responses, > > Wayne. > ------------------------------------ > Wayne G Boyd > (Digital Sys. Eng.) > E-mail: EraseMEwaynespam_OUTTakeThisOuTjce.wintermute.co.uk > ------------------------------------ RS485 is a multidrop version of RS422.

> > I'm looking at designing a multi drop comm's system for control purposes. > I have been investigating RS485, and RS422 keeps on coming up along with it, > so I know there can't be much difference between them, but what is the > difference ? > > Thanks in advance for any responses, > > Wayne. This will probably be beaten to death but..... 422 and 485 are very similar, in many cases 485 transceivers are used in 422 applications. The primary difference is that an RS-485 driver can be tri-stated, allowing multiple drivers to share a line, which allows 2-wire mode to be used. 2-wire mode ties each nodes driver and receiver together on the same twisted pair and communicates half duplex with up to 32 other nodes (more with special hardware). 422 can have multiple receivers sharing a driver, but since it doesn't tri-state the drivers no two drivers may share a line. Distances and baud rates are very similar if not identical to 485. The input impedance of a 422 receiver is a bit lower than 485, which allows less nodes to be hung on one driver (10 instead of 32). RS-422 can be used in point to point systems or as the master of a master/slave 4-wire 485 network. We have a pretty decent app note on 422 and 485 on our web site at http://www.bb-elec.com Other good info can be found in the app notes in the back of NSC's latest Interface Data Book. Hope this helps. -mike -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- Mike Fahrion mfahrionspam_OUTbb-elec.com http://www.bb-elec.com/ B&B Electronics Mfg Co ph.(815) 433-5100 ext.215 fax (815) 434-7094 707 Dayton Road PO Box 1040 Ottawa IL 61350 -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

On Thu, 13 Jun 1996, pic wrote: > I'm looking at designing a multi drop comm's system for control purposes. > I have been investigating RS485, and RS422 keeps on coming up along with it, > so I know there can't be much difference between them, but what is the > difference ? RS485 is a true multi-drop setup - I believe the spec is 32 drivers/receivers on a twisted pair bus (about a mile long?)? RS422 is meant for single send/receive pairs - no multidrop. To meet RS485 automatically qualifies the chip for use as RS422. So, manufactures - being frugal - make one chip in high quantities (and at cheaper cost) than two different chips. Personally, I am not aware of a commercially 422 only chip you can buy (I believe you can buy 422 pre-assembled boards though - even though you can't use them as 485 - they use the 485 chips). My source on info comes from the National Semiconductor Communications Data Book - I like the SN75176 chip in particular - cheap from DigiKey and well designed. If you order from DigiKey - you can pay an extra buck and get the data sheet for the chip. \\\|/// \\ ~ ~ // ( @ @ ) --|---------------------------oOOo-(_)-oOOo---------------------------------- --| Troy D.F. Nelson | EMAIL @spam@nelsonKILLspamcerc.wes.army.mil --| Coastal Engineering Research Center | Voice 601-634-3568 --| USAE Waterways Experiment Station | FAX 601-634-3151 --|------------------------------------Oooo.--------------------------------- .oooO ( ) ( ) ) / \ ( (_/ \_)