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Electronics Forum / Basics / May 2006How to switch TX, RX,RTS & CTS from an RS232 port to a tranceiver?
Hi All, My current project involves adding bluetooth to a pda that lacks it. For this I've settled in a BT module with uart interface that will be connected throught a transceiver to the pda's current rs232 port. Instead of anulating the rs232 external port I would like to add an external switch that (probably by means of a relay) would switch TX, RX,RTS & CTS lines from the port to the transceiver when and back. Given that I'm not an expert, the voltages involved and switch current bounce oddities, I'm not even sure this is a good idea or even feasible.. Any pointers? Thaks a lot, David > Hi All, > [quoted text clipped - 9 lines] > bounce oddities, I'm not even sure this is a good idea or even > feasible.. I don't quite understand where the switch comes into play. As long as you're not interrupting data transmissions I can't see any problem. Graham > > Instead of anulating the rs232 external port I would like to add an > > external switch that (probably by means of a relay) would switch TX, > > RX,RTS & CTS lines from the port to the transceiver when and back. > > I don't quite understand where the switch comes into play. Ok, my fault, I'll try to explain better :-) Basically there'd be one rs232 port shared between two transceivers; one being the machine's own one, the second being added by me. My idea about the switch/relay was to be able to attach the port to one transceiver or the other at any one time while maintaining the other one isolated from the channel. I'm concerned that when both transceivers are simultaneously connected to the port one of them would be driving high the other one's output lines (TX,RTS). Given rs232 voltage range maybe this could damage one or both tranceivers.. Here comes the idea of switching these four lines (the others not being used) Alternatively I could perhaps insert some diodes to achieve the same effect? > As long as you're not interrupting data transmissions I can't see any problem. You seem to imply I could just fork the lines in a "Y" fashion and everithing would be well as long as only one of the transceivers is communication at any one time. Did I understand right? Thanks again, David > > > Instead of anulating the rs232 external port I would like to add an > > > external switch that (probably by means of a relay) would switch TX, [quoted text clipped - 14 lines] > lines (TX,RTS). Given rs232 voltage range maybe this could damage one > or both tranceivers.. They're supposed to be quite robust so probably not a problem. > Here comes the idea of switching these four lines (the others not being > used) > Alternatively I could perhaps insert some diodes to achieve the same > effect? Diodes won't work. > > As long as you're not interrupting data transmissions I can't see any problem. > > You seem to imply I could just fork the lines in a "Y" fashion and > everithing would be well as long as only one of the transceivers is > communication at any one time. Did I understand right? No. You can't have them both connected at the same time. You ideally need what's called a 'break before make' changeover switch ( 4 pole ). Graham Now I'm getting confused > > I'm concerned that when both transceivers are simultaneously connected > > to the port one of them would be driving high the other one's output > > lines (TX,RTS). Given rs232 voltage range maybe this could damage one > > or both tranceivers.. > > They're supposed to be quite robust so probably not a problem. hmm... so there is no need to electrically disconnect the transceivers? > > Alternatively I could perhaps insert some diodes to achieve the same > > effect? > > Diodes won't work. Ok, my homework to research why they wont. Main point of this project is to learn :-) > > You seem to imply I could just fork the lines in a "Y" fashion and > > everithing would be well as long as only one of the transceivers is > > communication at any one time. Did I understand right? > > No. You can't have them both connected at the same time. but here it seems one transceiver must be electrically disconnected indeed.. do I miss something? if driving outputs high from the line instead of from the transceiver is not a problem, then what? Please, bear with me if I ask dumb questions. I know very little on electronics. I've some knowledge on digital circuits but thats all. > You ideally need what's called a 'break before make' changeover switch ( 4 pole ). aha, that's it! Original question was about what would I need for performing the switch :-) Do this component exist in the form of an IC? Do exist in some form of relay that could be activated by a line drived from a microswitch at the port's cover lid? Thanks for your patience, David Just to clarify. I got really confused in my explanation.. > > > You seem to imply I could just fork the lines in a "Y" fashion and > > > everithing would be well as long as only one of the transceivers is > > > communication at any one time. Did I understand right? > > > > No. You can't have them both connected at the same time. The setup is that machine's original transceiver will remain allways connected to the channel and the uart of the pda. Sometimes, it's going to "talk" to the added transceiver wich in turn is connected to the BT module.The rest of the time original transceiver should remain connected to the existing rs232 port in order to communicate with a modem or whatever device connected to the port. The switching (if needed) would be driven from a switch installed at the rs232 port's cover lid. I'm sorry for any missunderstanding I may have caused, David > Now I'm getting confused > [quoted text clipped - 6 lines] > > hmm... so there is no need to electrically disconnect the transceivers? There's a need to 'switch them over' for functionality but a brief 'shorting out' of one signal to another is unlikely to cause a catastrophe. > > > Alternatively I could perhaps insert some diodes to achieve the same > > > effect? [quoted text clipped - 12 lines] > but here it seems one transceiver must be electrically disconnected > indeed.. Yes. > do I miss something? if driving outputs high from the line instead of > from the transceiver is not a problem, then what? You differentiate betweem 'line' and transceiver ? I don't understand what you mean. > Please, bear with me if I ask dumb questions. I know very little on > electronics. I've some knowledge on digital circuits but thats all. [quoted text clipped - 3 lines] > aha, that's it! Original question was about what would I need for > performing the switch :-) Yes. > Do this component exist in the form of an IC? I meant a 'physical switch'. If you need this to be accomplished under program control then use a similar style relay. > Do exist in some form of > relay that could be activated by a line drived from a microswitch at > the port's cover lid? You could do that, yes. Graham >Diodes won't work. Why not? This comes from an ancient BYTE (or KILOBAUD :-) where someone had one Ter- minal in the living room and one other near the (South-West TC 68oo IIRC) Computer: one Output ------* | | === / \ / \ ----- | -9 Volt | or -/\/\/--*---------> one Input Handshake- | Line | ----- \ / \ / === | | another | Output ----------* The Resistor being about 5.6 to 10 KOhm. just asking, Holger > >Diodes won't work. > [quoted text clipped - 26 lines] > > The Resistor being about 5.6 to 10 KOhm. I fail to see how this circuit would work. I quickly realized diodes where a dumb idea (rs232 signals singing from -5, -15 to +5, +15 volts) but now I think I don't understand anything. Would this rely on the nonstandard assumption of: Mark(1) --> -5 ... -15 volt Space(0) --> 0 volt Instad of the standard: Mark(1) --> -5 ... -15 volt Space(0) --> -5 ... +15 volt I'm aware most receivers will cope with this but.. Anyway, do I have a clue? > > >Diodes won't work. > > [quoted text clipped - 44 lines] > I'm aware most receivers will cope with this but.. Anyway, do I have a > clue? There is indeed an issue with the various 'flavours' of implementing RS232. That's why physical disconnection is the best idea. Graham > >Diodes won't work. > [quoted text clipped - 26 lines] > > The Resistor being about 5.6 to 10 KOhm. Aren't the diodes the wrong way round ? That might indeed work but I'm not familiar with the quiesecnt state of the RS232 lines so played safe. It would suffer at high data rates though. That could be fixed by using some active devices though. Graham >> This comes from an ancient BYTE (or KILOBAUD :-) where someone had one Ter- This "ancient" means 1978 to 1983... >> minal in the living room and one other near the (South-West TC 68oo IIRC) >> Computer: >Aren't the diodes the wrong way round ? Yess, indeed. > That might indeed work but I'm not >familiar with the quiesecnt state of the RS232 lines so played safe. It would >suffer at high data rates though. ^^^^^^^^^ Remember: ""Dumb video Terminal" about 1980 ==> ca. 9600 Baud... > That could be fixed by using some active >devices though. >Graham Holger PS: Sorry, I do not have that special issue in my collection. I firmly believe in one of the articles of Steve Ciarcia's "Circuit Cellar" from BYTE, but I cant't find it. > >Aren't the diodes the wrong way round ? > > Yess, indeed. Sure? In rs-232 the "mark" (the logical 1) is always a negative voltaje ( anywhere from +3 to +15 depending on where you lookup the info). The "space" (logical 0) is a positive voltaje in the same range. As it seems most rs232 equipment can cope with the space being indicated by 0 volt. In that case I could understand how this circuit works; with the diodes placed as drawn. As far as I understand, seting the diodes the other way round wold require mark being positive voltage and space 0 volt. That for sure would not be rs232 signaling by any means. Then... what do I know? I could be completely wrong. I expose my reasoning only so any one can correct any missconception :-) David >> >Aren't the diodes the wrong way round ? >> >> Yess, indeed. >Sure? I have doubts as well... The (intended) function of the circuit: The _input_ of some RS-232 - Device is held to " minus XX" Volt by means of a (suitable) resistor which is connected to some source of "minus XX" Volt. Neglecting the input-current of the RS232-device. One might use a Handshake-signal of the input-devce... One or the other *sending* devices wil be able to pull the line to "plus YY" Volt through the diode. This works well IF the mentioned resistor is neither too low nor too high. This of course depends a little on the input-current of the receiver and the voltage. The circuit comes from the ages of 1488/1499-Chips..! And it works only for the 'normal' TX-Date Line, dosn't it? Of course, funny things happen if both senders will send at the same time :-) >Then... what do I know? I could be completely wrong. I expose my >reasoning only so any one can correct any missconception :-) >David Holger >> >Aren't the diodes the wrong way round ? >> [quoted text clipped - 17 lines] > > David AIUI most equipment can handle mark being 0v. with the diodes as shown the voltage will neve get more positive than -9v  SignatureBye. Jasen >>Diodes won't work. > > Why not? not by themselves... > This comes from an ancient BYTE (or KILOBAUD :-) where someone had one Ter- > minal in the living room and one other near the (South-West TC 68oo IIRC) [quoted text clipped - 20 lines] > another | > Output ----------* that won't work. (you've got the diodes backwards) > The Resistor being about 5.6 to 10 KOhm. seems a little high. Bye. Jasen > >>Diodes won't work. > > [quoted text clipped - 32 lines] > > seems a little high. I thought so too. I also can see some potential issues with using this if the levels involved from the various transmitters are different. Esp if one uses gnd for '0'. Graham >> >>Diodes won't work. >> > [quoted text clipped - 37 lines] > I also can see some potential issues with using this if the levels involved from > the various transmitters are different. Esp if one uses gnd for '0'. 0 is typically +3-12v. DYM gnd for 1 ? SignatureBye. Jasen >> > Instead of anulating the rs232 external port I would like to add an >> > external switch that (probably by means of a relay) would switch TX, [quoted text clipped - 9 lines] > transceiver or the other at any one time while maintaining the other > one isolated from the channel. Does it have to be electrically controlled? You could use one of these: http://cgi.ebay.com/DB9-Female-AB-2-Way-Switch-Box_W0QQitemZ9709748517QQcategory Z39803QQssPageNameZWD1VQQrdZ1QQcmdZViewItem You might have to unwrap the link. But that's not the only one of these that you can get; your local computer store probably has one. Otherwise, any 4PDT relay with "dry switching", as someone mentioned - I think Chris. Good Luck! Rich >> > Instead of anulating the rs232 external port I would like to add an >> > external switch that (probably by means of a relay) would switch TX, [quoted text clipped - 18 lines] > Alternatively I could perhaps insert some diodes to achieve the same > effect? one high one low ould be like both driving a short circuit (if they are of the same type) and they are rated to withstand that condition indefinately, it'd probably be ok. but if you're worried put some sort of OR gate in there. eg: TX1 --+--->|---+ | | +-[1.2K]-+ | | +-[1.2K]-+---- out | | TX2 --+--->|---+ that'd protect the outputs from each other, 1/4 watt resistors will be fine the diodes can be 1N914 or 1N4148 etc. if that doesn't work try 560 ohm resistors instead of 1.2K for DTR and RTS you'll probably want the diodes the other way around. >> As long as you're not interrupting data transmissions I can't see any problem. > > You seem to imply I could just fork the lines in a "Y" fashion and > everithing would be well as long as only one of the transceivers is > communication at any one time. Did I understand right? that's unlikly to work, typically you'll not be able to drive the TX line positive with that setup, and so the receiver may not see any data.. Bye. Jasen > Hi All, > [quoted text clipped - 14 lines] > Thaks a lot, > David Hi, David. If you've only got two devices communicating, I'm not sure why you'd need a switch at all. Given that, physical switching of RS-232 signals isn't too difficult. I have used 4PDT telephone service relays for this successfully. It's better to use pullup/pulldown resistors to ensure the lines stay in a known inactive logic state when the relay contacts open. >From a software standpont, you'll then just have to ensure no one's talking or listening when you do the switch. Good luck Chris > > Instead of anulating the rs232 external port I would like to add an > > external switch that (probably by means of a relay) would switch TX, > > RX,RTS & CTS lines from the port to the transceiver when and back. > > Hi, David. If you've only got two devices communicating, I'm not sure > why you'd need a switch at all. So there appears not to exist any electrical problem! Given that the port has a cover, I can shutdown my transceiver when it opens. So there could be only one communication at a any one time: - original transceiver <--> added transceiver --> and then BT module - original transceiver <--> device connected to port In the second case a modem connected to the port would be feeding potentially high voltages (-15 to +15 volt ??) to my shutdown transceiver (MAX3218) if lines are not switched. Thats no problem for me as long as nothin breaks :-) > Given that, physical switching of RS-232 signals isn't too difficult. > I have used 4PDT telephone service relays for this successfully. It's > better to use pullup/pulldown resistors to ensure the lines stay in a > known inactive logic state when the relay contacts open. I'll do some research on what a 4PDT telephone service relay is prior to comment further on this.. > >From a software standpont, you'll then just have to ensure no one's > talking or listening when you do the switch. Of course you're right Thanks for your reply, David > > > Instead of anulating the rs232 external port I would like to add an > > > external switch that (probably by means of a relay) would switch TX, [quoted text clipped - 31 lines] > Thanks for your reply, > David Hi, David. Manual relay switching of RS-232 lines (especially if you're only going with TxD, RxD, RTS and CTS is relatively easy. You need a small 4 pole relay which is capable of switching small currents at lower voltages. Specifically, you need a relay contact capable of "dry switching", which means a low voltage (yes, +/-12V is low voltage for relays) at low current (<5mA) will still have a good, low ohm contact. You might want to look at the Omron G6A line, specifying 4PDT and *non-latching* version. If you've got 5V to drive the coils, try looking at the G6A-474P-ST20-US-DC5, which is available at Mouser for $7.75 USD in single quantities. http://www.mouser.com/ Check the website and take a look at the data sheet. Again, remember that the receiving lines on each side need a resistor pullup/pulldown to place the line in a known, inactive state while switching is taking place. I used 2.2K resistors, which won't load the line too heavily, but ensure a good RS-232 voltage when the relays are open, and provide enough switching current. Otherwise, you'll end up getting very familiar with clearing and resetting your UART registers. Voice of experience. :-( Good luck Chris Chris, thanks for the very instructive post > Hi, David. Manual relay switching of RS-232 lines (especially if > you're only going with TxD, RxD, RTS and CTS is relatively easy. So did I hope initially :-) > You might want to look at the Omron G6A line, specifying 4PDT and > *non-latching* version. If you've got 5V to drive the coils, try > looking at the G6A-474P-ST20-US-DC5, which is available at Mouser for > $7.75 USD in single quantities. Something like this is what I was thinking about initially. I had a look at the datasheet. I've a 6V max 30mA line available for driving the coil which matches exactly the datasheet. I'd rather to have some margin... The biggest problem is that I'll be trying to cram this thing inside a pda!! I cannot imagine how this could be made to fit.. Well, now that I know the requierements I'll try to find a smaller equivalent. I'm a bit curious. At first you, and also Graham, seemed to neglect the need for fisical switching of serial lines but now both of you agree on this same need. I'm sure I'm missing something.. > Again, remember that the receiving lines on each side need a resistor > pullup/pulldown to place the line in a known, inactive state while [quoted text clipped - 3 lines] > getting very familiar with clearing and resetting your UART registers. > Voice of experience. :-( This very same piece of info I'll save to avoid repeating your bad experience :-) Thanks a lot again David > Chris, thanks for the very instructive post > [quoted text clipped - 35 lines] > Thanks a lot again > David Hi, David. A couple of notes here: If you don't have 150mW to spare, you're not going to be able to power a relay coil, especially one with 4 poles. The Omron one specified has pretty low current. Whether you use a relay or a 4PDT rotary switch (e.g. E-Switch Adjustable Stop Rotary Switch, 4P3T Solder Eyelet, Digi-Key P/N EG1958-ND for $5.07 USD ), you're not going to fit this inside some PDA. Depending on the power supplies you have available, your most space-limited option would be to use two quad analog switch ICs that can handle RS-232 voltage and use a micro pushbutton or switch to turn them on/off. You'd have to actually take the time to describe what you have and what you want in some detail for more advice here. I'd prefer not to go through 20 questions before you've taken the time to describe things. No matter what, you can always go with the noble tradition of the external blivet box. If you'll reread your initial post, *you* suggested the mechanical switch. But whether you use relay, mechanical switch, quad analog switch ICs or any combination thereof, don't forget the pullup/pulldowns! By the way, what's "anulating" (first post)? Did you mean "emulating"? Good luck Chris > Hi, David. A couple of notes here: > > If you don't have 150mW to spare, you're not going to be able to power > a relay coil, especially one with 4 poles. The Omron one specified has > pretty low current. I don't think I've that much on this particular 6V line. I've 3V (that's batery voltage) elsewhere but then I'll go 66 mA for the relay coil. That's a bit much, I'd rather "anulate" the port (more on this below) > them on/off. You'd have to actually take the time to describe what you > have and what you want in some detail for more advice here. I'd prefer > not to go through 20 questions before you've taken the time to describe > things. Ok, I'll post a diagram with parts, explanations, etc. on some webspace and come back with a link :-) > No matter what, you can always go with the noble tradition of the > external blivet box. I'd try some modification of the Free2move Bluetooth serial port plug (F2M01) at http://www.free2move.se/prod_spp.shtml. Then I wouldn't mind about serial lines switching at all, just plug it in or out :-) Maybe I'll be forced throught this way.. > If you'll reread your initial post, *you* suggested the mechanical > switch. Please, bear with my poor english (and electronics knowledge). My first idea was a mechanical switch that would drive a relay's coil. Sorry for the poor phrasing > But whether you use relay, mechanical switch, quad analog switch ICs or > any combination thereof, don't forget the pullup/pulldowns! Sure, I'm really aware of the problem know. This will save my day as I won't have any kind of low level access tho the UART from software (reseting registers and the like). I'll only be able to send commands/data on the serial channel. I'll explain every detail jointly with the diagram. > By the way, what's "anulating" (first post)? Did you mean "emulating"? Oh, bad english again.. babelfish comes up whith "annulling" for what I meant. As I see it, I could just reroute TX, RX,RTS & CTS to my tranceiver/BT module conbination, thus leaving a dead (anulled) rs232 port; or I could try to devise some kind of "switching" mean in order to be able to select betwen rs232 cable or bluetoth SPP operation. OK, no more questions for the moment; back to drawing :-) Regards, David > > By the way, what's "anulating" (first post)? Did you mean "emulating"? > > Oh, bad english again.. babelfish comes up whith "annulling" for what I > meant. You may want 'disabling'. -- john John O'Flaherty ha escrito: > > Oh, bad english again.. babelfish comes up whith "annulling" for what I > > meant. > > You may want 'disabling'. Yes, that's what I meant. Thanks :-) > Chris, thanks for the very instructive post > [quoted text clipped - 7 lines] > driving the coil which matches exactly the datasheet. I'd rather to > have some margin... You won't find relays with much greater sensitivity than that. Graham |
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