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Dear Forum, A little bit off topic. I have a HC11F1 system running in which I use the serial port to communicate (RX & TX) with two external devices. I want to be able to switch the 232 signal lines between the two external devices. I have an arrangement using an analogue switch and use two output pins to select which device is connected to the 232 port. Its a bit messy. I have an idea that there are specific RS232 data switch chips available, but don't have a part number. Any suggestions? With thanks, Paul F.Curtin

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Dear Paul: IMHO I think that it would be equally messy to make the switch work on the RS232 side of the link if you use a dedicated IC. I assume from your message that you can't work on the "TTL"side of the RS232 link, right ? This could be a little easier. Regards, Roberto G. Berner 54 11 4308 3500 (tel) 54 11 4308 3700 (fax) 15 5122 6095 ICQ #119529928 ----- Original Message ----- From: PFC1 To: Sent: Thursday, November 07, 2002 6:38 PM Subject: [m68HC11] RS232 data switch Dear Forum, A little bit off topic. I have a HC11F1 system running in which I use the serial port to communicate (RX & TX) with two external devices. I want to be able to switch the 232 signal lines between the two external devices. I have an arrangement using an analogue switch and use two output pins to select which device is connected to the 232 port. Its a bit messy. I have an idea that there are specific RS232 data switch chips available, but don't have a part number. Any suggestions? With thanks, Paul F.Curtin Yahoo! Groups Sponsor ADVERTISEMENT To unsubscribe from this group, send an email to: Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service. [Non-text portions of this message have been removed]

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I did this with a 4551 analog chip (4PDT) between the cpu and the RS232 converters. It only takes one port bit to switch it. Dan In a message dated 11/7/02 1:35:38 PM Pacific Standard Time, writes: > Dear Forum, > A little bit off topic. > I have a HC11F1 system running in which I use the serial port to > communicate > (RX & TX) with two external devices. I want to be able to switch the 232 > signal lines between the two external devices. I have an arrangement using > an analogue switch and use two output pins to select which device is > connected to the 232 port. Its a bit messy. I have an idea that there are > specific RS232 data switch chips available, but don't have a part number. > Any suggestions? > > With thanks, > Paul F.Curtin [Non-text portions of this message have been removed]

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I don't understand. What's messy, a 74HC4066 between the MCU and the MAX232? (If you also add a transistor you'll only need one pin to select the channel.) A single-chip solution (if it exists) would certainly cost a lot more than this. ----- Original Message ----- From: PFC1 <> To: <> Sent: Thursday, November 07, 2002 11:38 PM Subject: [m68HC11] RS232 data switch > I have a HC11F1 system running in which I use the serial port to communicate > (RX & TX) with two external devices. I want to be able to switch the 232 > signal lines between the two external devices. I have an arrangement using > an analogue switch and use two output pins to select which device is > connected to the 232 port. Its a bit messy. I have an idea that there are > specific RS232 data switch chips available, but don't have a part number. > Paul F.Curtin

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Paul -- Although I have not switched RS232 specifically, I have done similar things with I/O selection. My approach would be to use two RS232 to TTL level converters and switch the TTL(HCMOS) side using standard TTL(HCMOS) data selectors/multiplexors. If you can't tolerate separate RS232 chips (they are, large, expensive, and power hungry), then you are stuck with switching on the serial line side which involves + and - voltage swings at higher levels. If you can tolerate the thought, a small dip relay might be best for this. One problem to consider is the need to drive the inactive line to proper 'idle' line conditions. The data lines normally idle in the 'marking' state. If you have implemented any 'handshaking' signals, you must restore handshake status to the proper idle line states before switching the data lines. Often some carefully placed 'weak' pullups/pulldowns can take care of the problem. I thought that comes to mind is to set this up as a multi-drop RS-485 link using standard RS-485 drivers. Then the usual multi-drop enable/disable protocol could effectively select the 'link' for you. I haven't really thought that out, but you should be able to get there from here. It may require some software protocol considerations. Good luck, Bob Smith --- Avoid computer viruses, Practice safe hex --- -- Specializing in small, cost effective embedded control systems -- Robert L. (Bob) Smith Smith Machine Works, Inc. 9900 Lumlay Road Richmond, VA 23236 804/745-1065 ----- Original Message ----- From: "PFC1" <> To: <> Sent: Thursday, November 07, 2002 4:38 PM Subject: [m68HC11] RS232 data switch > Dear Forum, > A little bit off topic. > I have a HC11F1 system running in which I use the serial port to communicate > (RX & TX) with two external devices. I want to be able to switch the 232 > signal lines between the two external devices. I have an arrangement using > an analogue switch and use two output pins to select which device is > connected to the 232 port. Its a bit messy. I have an idea that there are > specific RS232 data switch chips available, but don't have a part number. > Any suggestions? > > With thanks, > Paul F.Curtin > > To unsubscribe from this group, send an email to:

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Dear friends Bob and Paul: I agree with Bob in all concepts. This should be done ( if possible ) on the TTL side. I manufacture a vehicle locator that was designed to talk to a Motorola GPS board and to a modem chip, sharing a single SCI channel. For this purpose I use a 4066 switch because I need fast switching for the SCI channel to both peripherals. A single port line is dedicated to the device selection. It works perefctly well and as Bob says, care should be taken when switching the channel between devices to ensure the proper logic state of the TTL - RS232 line during the switch transient from one device to the other ( proper pullup resistors do the job at the 4066 inputs ). Switching the RS232 side can of course be achieved, at some higher risk, because of higher voltages, + an - voltages in split supply configuration, and HCMOS or CMOS IC inputs exposures to probably long cable lines that can be protected with diodes, etc to avoid latch-up. If you use Bob's relay solution, you must consider the relay make / break delay and bouncing. If you don't need too much switching speed ( t > ~50 ms) a relay can work fine. I don't know about any specific RS232 switching IC. Regards, Roberto G. Berner 54 11 4308 3500 (tel) 54 11 4308 3700 (fax) 15 5122 6095 ICQ #119529928 ----- Original Message ----- From: Robert Smith To: Sent: Friday, November 08, 2002 10:16 AM Subject: Re: [m68HC11] RS232 data switch Paul -- Although I have not switched RS232 specifically, I have done similar things with I/O selection. My approach would be to use two RS232 to TTL level converters and switch the TTL(HCMOS) side using standard TTL(HCMOS) data selectors/multiplexors. If you can't tolerate separate RS232 chips (they are, large, expensive, and power hungry), then you are stuck with switching on the serial line side which involves + and - voltage swings at higher levels. If you can tolerate the thought, a small dip relay might be best for this. One problem to consider is the need to drive the inactive line to proper 'idle' line conditions. The data lines normally idle in the 'marking' state. If you have implemented any 'handshaking' signals, you must restore handshake status to the proper idle line states before switching the data lines. Often some carefully placed 'weak' pullups/pulldowns can take care of the problem. I thought that comes to mind is to set this up as a multi-drop RS-485 link using standard RS-485 drivers. Then the usual multi-drop enable/disable protocol could effectively select the 'link' for you. I haven't really thought that out, but you should be able to get there from here. It may require some software protocol considerations. Good luck, Bob Smith --- Avoid computer viruses, Practice safe hex --- -- Specializing in small, cost effective embedded control systems -- Robert L. (Bob) Smith Smith Machine Works, Inc. 9900 Lumlay Road Richmond, VA 23236 804/745-1065 ----- Original Message ----- From: "PFC1" <> To: <> Sent: Thursday, November 07, 2002 4:38 PM Subject: [m68HC11] RS232 data switch > Dear Forum, > A little bit off topic. > I have a HC11F1 system running in which I use the serial port to communicate > (RX & TX) with two external devices. I want to be able to switch the 232 > signal lines between the two external devices. I have an arrangement using > an analogue switch and use two output pins to select which device is > connected to the 232 port. Its a bit messy. I have an idea that there are > specific RS232 data switch chips available, but don't have a part number. > Any suggestions? > > With thanks, > Paul F.Curtin > > To unsubscribe from this group, send an email to: Yahoo! Groups Sponsor ADVERTISEMENT To unsubscribe from this group, send an email to: Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service. [Non-text portions of this message have been removed]

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You might try using a 74LS257 and switch the devices on the low side (between the MCU and the level shifter).

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A couple of more thoughts. Funny how things happen. About an hour after posting the following, I found that I needed a copy of the MAX232 data sheet for a current design. Browsing through the data sheet reminded that Maxim has a whole family of TTL to RS232 level converter devices. You might be able to do this all with the MAX248 _if_ you can afford the real estate and expense. It comes in a PLCC-44 package due to the pin count. :-(( The MAX248 has 4 A-side receivers, 4 B-side receivers, 4 A-side transmitters, 4 B-side transmitters, and separate enables for each group of 4. Weak pull-ups and pull-downs are built in. It seems to me that an email contact with a MAXIM engineer is in order. In fact, I would not be surprised to find an application note already on their web site that addresses this requirement. In regard to relays. Small PCB mount DPDT relays switch in under 5ms. Happy switching, Bob Smith --- Avoid computer viruses, Practice safe hex --- -- Specializing in small, cost effective embedded control systems -- Robert L. (Bob) Smith Smith Machine Works, Inc. 9900 Lumlay Road Richmond, VA 23236 804/745-1065 ----- Original Message ----- From: "Roberto G. Berner" <> To: <> Sent: Friday, November 08, 2002 12:18 PM Subject: Re: [m68HC11] RS232 data switch > Dear friends Bob and Paul: > > I agree with Bob in all concepts. This should be done ( if possible ) on the TTL side. I manufacture a vehicle locator that was designed to talk to a Motorola GPS board and to a modem chip, sharing a single SCI channel. For this purpose I use a 4066 switch because I need fast switching for the SCI channel to both peripherals. A single port line is dedicated to the device selection. It works perefctly well and as Bob says, care should be taken when switching the channel between devices to ensure the proper logic state of the TTL - RS232 line during the switch transient from one device to the other ( proper pullup resistors do the job at the 4066 inputs ). Switching the RS232 side can of course be achieved, at some higher risk, because of higher voltages, + an - voltages in split supply configuration, and HCMOS or CMOS IC inputs exposures to probably long cable lines that can be protected with diodes, etc to avoid latch-up. If you use Bob's relay solution, you must consider the relay make / break delay and bouncing. If you don't need too much switching speed ( t > ~50 ms) a relay can work fine. I don't know about any specific RS232 switching IC. > > Regards, > Roberto G. Berner > 54 11 4308 3500 (tel) > 54 11 4308 3700 (fax) > 15 5122 6095 > ICQ #119529928 > > ----- Original Message ----- > From: Robert Smith > To: > Sent: Friday, November 08, 2002 10:16 AM > Subject: Re: [m68HC11] RS232 data switch > Paul -- > > Although I have not switched RS232 specifically, I have done similar things > with I/O selection. My approach would be to use two RS232 to TTL level > converters and switch the TTL(HCMOS) side using standard TTL(HCMOS) data > selectors/multiplexors. > > If you can't tolerate separate RS232 chips (they are, large, expensive, and > power hungry), then you are stuck with switching on the serial line side > which involves + and - voltage swings at higher levels. If you can tolerate > the thought, a small dip relay might be best for this. > > One problem to consider is the need to drive the inactive line to proper > 'idle' line conditions. The data lines normally idle in the 'marking' > state. If you have implemented any 'handshaking' signals, you must restore > handshake status to the proper idle line states before switching the data > lines. Often some carefully placed 'weak' pullups/pulldowns can take care > of the problem. > > I thought that comes to mind is to set this up as a multi-drop RS-485 link > using standard RS-485 drivers. Then the usual multi-drop enable/disable > protocol could effectively select the 'link' for you. I haven't really > thought that out, but you should be able to get there from here. It may > require some software protocol considerations. > > Good luck, Bob Smith > --- Avoid computer viruses, Practice safe hex --- > > -- Specializing in small, cost effective > embedded control systems -- > Robert L. (Bob) Smith > Smith Machine Works, Inc. > 9900 Lumlay Road > Richmond, VA 23236 804/745-1065 > > ----- Original Message ----- > From: "PFC1" <> > To: <> > Sent: Thursday, November 07, 2002 4:38 PM > Subject: [m68HC11] RS232 data switch > > Dear Forum, > > A little bit off topic. > > I have a HC11F1 system running in which I use the serial port to > communicate > > (RX & TX) with two external devices. I want to be able to switch the 232 > > signal lines between the two external devices. I have an arrangement > using > > an analogue switch and use two output pins to select which device is > > connected to the 232 port. Its a bit messy. I have an idea that there > are > > specific RS232 data switch chips available, but don't have a part number. > > Any suggestions? > > > > With thanks, > > Paul F.Curtin > > > > > > > > To unsubscribe from this group, send an email to: > > > > > > > > > > > > > > > > Yahoo! Groups Sponsor > ADVERTISEMENT > > > To unsubscribe from this group, send an email to:

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