Hi Darren, you didn't ask for a separation of 120 RS485 busses in your original mail!
A switch like I proposed it is essentially unidirectional (following the sparse info in your original mail...). What you need is a kind of hub. If your single pair is meant to only talk to one of the 120 clients at a time, you might manage with some kind of multiplexer/demultiplexer - actually you would need to use the analog multiplexers you are so afraid of! Following my original idea you could use one 74x154 to select 1 out of 15 74x4051 8-channel D/MUX or use one 74x138 to select 1 out of 8 74x4067 16-channel D/MUX for each of the two RS485 lines. However, there is no guaranty that the recommended bus impedance is met with this design. In order to build a _reliable_ system you most probably have to do the multiplexing on a digital line with a RS485 line driver/receiver chip on each of the 121 connections of your circuit! In the latter case you would need a set of multiplexers for either direction but since the logic signals on the other side of the transceivers are no longer differential you end up with the same number of chips... Since it appears to me that this circuit should be located at the end points of either RS485 bus you should not forget to terminate the RS485 lines as well. Managing this kind of setup will most probably also require a whole lot of juggling on the software side.... Uwe. > Hi, > It's a calibration system, and as such it requires high reliability. > Networking and addressing leaves the system open to problems such as > if one RS485 chip goes short circuit, for example, it will bring the whole > network down and screw up the (lengthy) calibration process > Therefore IT MUST be designed this way, unless you can think of anything > better > Separating each channel isolates unit specific problems to just that channel > -----Original Message----- > Declan Moriarty > Sent: 23 August 2005 17:49 > To: Multiple recipients of list CHIPDIR-L > Recently, Somebody Somewhere wrote these words >> Hi, >> >> Thanks. >> >> I think I should have been more specific. >> >> We've got an RS485 comms pair, which we want to multiplex between 120 >> RS485 pairs. >> >> I.e. 120 channels to 1 (or 240 to 2 in terms of actual wires). >> >> Ive seen a 32 channel analogue switch from Analogue devices, but im >> unsure as to whether an Analogue switch will be ok? >> > To quote the Irishman who was asked for directions to somewhere: > "You know... If I was you, I wouldn't start from here at all!" > You are never going to stand up 1-->120 in an RS485 link without such > heartache as to justify a sane design. What's your baud rate? You would > also need sample & holds everywhere. Network the things and address > them. Start over, my friend. > Imagine when you get the complaint "Channel 87 goes intermittent after > about 5 minutes" How will you sort that? > BTW, the trick with analogue switches is never bring the input below the > negative rail. > -- > With best Regards, > Declan Moriarty. > -- > Author: Declan Moriarty > INET: [EMAIL PROTECTED] > Fat City Hosting, San Diego, California -- http://www.fatcity.com > --------------------------------------------------------------------- > To REMOVE yourself from this mailing list, send an E-Mail message > to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in > the message BODY, include a line containing: UNSUB CHIPDIR-L > (or the name of mailing list you want to be removed from). You may > also send the HELP command for other information (like subscribing). -- Author: Uwe Zimmermann INET: [EMAIL PROTECTED] Fat City Hosting, San Diego, California -- http://www.fatcity.com --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
