RS232 Multiple Rx on Single Tx Line - Page 1
Forum section: electronics / beginners

author: jmdejoanelli
author_rank: Contributor
timestamp: April 11, 2013, 10:31:26 pm
content: Hi all,Can someone, or everyone explain why multiple RX lines connected to a single TX line might be a bad idea (we're talking RS232 here)?I'm peer reviewing a colleagues drawing () and I thought strictly only a single RX and TX line could be connected to one another.This is a high reliability application, so if your answer is "it's a bad idea... blah blah blah... decreases reliability... blah blah blah...", can you explain why?Thanks,Josef

author: c4757p
author_rank: Super Contributor
timestamp: April 11, 2013, 10:54:23 pm
content: If these are just general RS-232 lines, then the problem is impedance and loading. Multiple receivers will present a higher load to the transmitter, which may pull the output voltage low enough (through the transmitter's own output impedance) to corrupt the data. Obviously, if these aren'tgeneralRS-232 lines and you actually know what they are and what input and output impedances they have, it might not be a problem at all. At high speeds you could have problems with signal integrity related to impedance mismatches, but RS-232 is rarely "high speed".

author: MasterOfNone
author_rank: Regular Contributor
timestamp: April 11, 2013, 11:13:29 pm
content: Also from a functionality point of view. Does the transmitter need confirmation that the message has been received? I.e. does it matter if a receiver misses a message?If yes:- You could have a different port on the transmitting device for each receiver and implement hardware and/or software handshaking. If that isn’t feasible you could look at a half-duplex Multidrop (485) type interface and implement a polling scheme where you can poll each receiver response in turn with it’s status.If No:- Then for this design may not be a bad idea for this system.

author: Rufus
author_rank: Super Contributor
timestamp: April 11, 2013, 11:14:32 pm
content: Quote from: jmdejoanelli on April 11, 2013, 10:31:26 pmMultiple loads will reduce noise margins to the point where it won't work at all with enough loads. There is no reason that can't be overcome with a more powerful driver.Quote from: jmdejoanelli on April 11, 2013, 10:31:26 pmIf you are talking about communication reliability broadcasting isn't. Jumping though hoops (like heartbeat broadcasts and missing message sequence detection) you can detect some failure at the receiving end. On hardware reliability more failures will take down all communication, on the other hand there is less to fail.

author: digsys
author_rank: Unknown
timestamp: April 11, 2013, 11:24:08 pm
content: I do this in many ways, even have "network" RS232 setups - Multiple Rxd / Txd on 2 wires. IF you DON'T want to do any mods toyour existing RS232 ports (whether PC etc) then you MAY get away with 2-4 Receivers at below 115Kb. Totally depends on whatdriver / receiver chips they use. I have seen 6+ Receivers work OK on 19K2 and below, but you need to check it.Otherwise it's not very difficult to make up your own driver / receiver network - TTL/OPTO/OC/485 etc etc

author: AlfBaz
author_rank: Super Contributor
timestamp: April 11, 2013, 11:37:48 pm
content: RS232 is meant to be point to point. If this is a high reliability application with one transmitter and multiple receivers then at a minimum he should of gone with 422

author: 4to20Milliamps
author_rank: Regular Contributor
timestamp: April 12, 2013, 12:01:12 am
content: We use serial communication for multiple radios but always use a port splitter, sending two signals at the same time and at the same frequency will most likely cause problems with data collision, it works but you will also get garbled data quite a bit.

author: jmdejoanelli
author_rank: Contributor
timestamp: April 12, 2013, 02:59:15 am
content: My colleagues are "sure" that "it will be fine". Despite the RS232 devices being black boxes (don't know what kind of drivers are in use). You would think we would opt on the side of caution, especially since these are aircraft systems we are working with.But, I'm not the boss...Thanks anyway guys, very helpful answers none the less.-Josef

author: c4757p
author_rank: Super Contributor
timestamp: April 12, 2013, 03:02:35 am
content: Quote from: jmdejoanelli on April 12, 2013, 02:59:15 amThe problem with black boxes is that you don't even know it's really (standard-compliant) RS-232. They could have cut corners just like we're discussing doing, assuming your colleague would not. This is not OK.Quote

author: AlfBaz
author_rank: Super Contributor
timestamp: April 12, 2013, 06:13:16 am
content: Quote from: jmdejoanelli on April 12, 2013, 02:59:15 amThere must be something else we don't know about. I find impossible to believe that you will be allowed to use a spec that is not designed for multidrop in an industry with such tight regulations. If you have no control over the black boxes design and they stipulate 232 then you should have a 232 port for each and every box

author: jmdejoanelli
author_rank: Contributor
timestamp: April 12, 2013, 11:04:56 am
content: Unfortunately that is everything. The justification I received was that there are only two receivers attached to the transmitter, it is a short distance, and the baud rate is low(ish) at 9600-19200 baud.I would say if it reduces reliability byanyamount then it shouldn't be done.And I absolutely agree with you.

author: hlavac
author_rank: Frequent Contributor
timestamp: April 12, 2013, 02:27:15 pm
content: That will be fine, calm downThis is quite common way of doing cheap simple unidirectional broadcast.RS232 has huge margins for noise and uses voltage levels for signaling,receivers have quite high input impedance, easily 10x higher than the drivers.If you dont use any handshake for flow control, Rx and Tx are two completely independent channels.You can use just one, and they dont have to go into the same device.For example you can connect many devices into a ring bus,by having Rx go one way and Tx the other along a circle.Devices then pass messages in one direction along the circle and the reply will come all around the circle and arrives from the other side. All the devices need just one rs232 port.

author: deephaven
author_rank: Frequent Contributor
timestamp: April 12, 2013, 03:13:46 pm
content: Quote from: jmdejoanelli on April 12, 2013, 11:04:56 amAs these are 'unknown' boxes I would set it all up and put a scope on the signal, you will soon see if it is well within the RS232 spec or getting marginal. "Flying blind" is a bad idea.

author: AlfBaz
author_rank: Super Contributor
timestamp: April 12, 2013, 03:48:25 pm
content: Quote from: hlavac on April 12, 2013, 02:27:15 pmI'm sure it probably would be fine, the problem is the design has a compromise, then there's the manufacture, installation, use, maintenance. If each one of those groups starts with the "It'll be fine, I'm sure the designers of this aircraft product wouldn't have taken any shortcuts, besides this is the only cable we have..."Engineering in a heavily regulated field is not just about "will it work", its about covering all the bases including your arse

author: hlavac
author_rank: Frequent Contributor
timestamp: April 13, 2013, 03:56:00 pm
content: Okay so if you don't want your drone to crash when someone nearby gets a call on a mobile phone, you need proper differential bus like CAN or RS 485. RS232's single ended nature is vulnerable to interference and proper shielding is heavy...

author: jmdejoanelli
author_rank: Contributor
timestamp: April 14, 2013, 06:09:57 am
content: "Good enough is enemy of the best"I'm sure everything would be fine, and in practice it has been. It just annoys me that some engineers can blindly say, "it'll be fine" with no measurable evidence to support the claim. I don't believe in half arsing anything in any case, but especially not in aviation.I've had a few of these issues at work and because I'm a noobie engineer there, I have no authority to have any say in anything. I guess I'm just venting a little at the same time.Anyway, good comments guys, thanks.

author: notsob
author_rank: Frequent Contributor
timestamp: April 14, 2013, 06:27:18 am
content: Not sure if it's still available, but see if you can track down a description of "Burroughs' POL/SELECT protocol. Basically it was a 2 wire serial, with something like a 8,1,1 protocol around 9600bps with voltage between 0 and -12V, supporting up to 10 devices on each line, 9 was better as there was less load on the com processor driver and less failures. Each device had a unique address, and a group address (2 bits) allowing for individual comms or group broadcasts.Gives you a good understanding of all the setup procedures just to communicate with a single device on a line with multiple devices attached.

author: Bored@Work
author_rank: Super Contributor
timestamp: April 14, 2013, 08:48:13 am
content: Quote from: jmdejoanelli on April 14, 2013, 06:09:57 amIt is called experience. When you grow up you will have it, too.

author: ptricks
author_rank: Frequent Contributor
timestamp: April 14, 2013, 11:21:30 am
content: If you want an easy to use replacement for RS232 check out LIN. Low cost to use and feature rich. Created for the auto market but because of the inherent noise that comes with that industry it also works well for other things.http://en.wikipedia.org/wiki/Local_Interconnect_Network

author: MasterOfNone
author_rank: Regular Contributor
timestamp: April 14, 2013, 11:27:30 am
content: Quote from: jmdejoanelli on April 14, 2013, 06:09:57 amIn real life the best solution isn’t always the best engineering.We really can’t say whether the idea is good or not because we don’t know anything know anything about the system, apart from the baud rates, it has two (or more) serial receivers serial and one transmitter .We don’t know anything about the importance of the data on this system.We don’t know don’t know if there is multiple redundancy built into the system. So if this single wire breaks there are different paths for this info.We don’t know the weight/space limitation for this system, aircraft fuel isn’t free so weight cost money.We don’t know what type of testing will be performed on the system to ensure it meets the requirements.And even if it turns out that this system doesn’t meet the requirements and has to be redesigned, we still won’t know if wasn’t just better to implement the system with the limitations and then get the managers to allocate extra resources for implementing a better solution, rather than spending extra time/cost now on something no-one else at your company believes is needed. Think of if this way if you turn out to be right at least you can say I told you so, rather than being blamed for causing an over spend.There are minimum requirements for safety critical systems. I don’t believe airplanes will be falling out Sky, because I don’t believe an RS232 single wire multi-drop system, with no form of feedback would be used on a safety critical system (not even is a recession).