I need to construct a simple RS232 multiplexer. I would prefer a compact SMD solution.

What is the most suitable switch implementation?

Is there anything preventing me from using MOSFETs for this purpose? I am a bit concerned about the threshold voltage.

Are there any other semiconductor alternatives to mechanical smd relays?

  • \$\begingroup\$ What approximate distance do you need to bridge? \$\endgroup\$ – jippie May 25 '12 at 17:17
  • \$\begingroup\$ How many signals are you trying to MUX? THere are quad TTL/CMOS MUx's chips in SMD, but a CPLD for $1 has 22 signal lines. then add 1 RS232 rx/tx chip total cost $3 in 1" sq real estate. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 25 '12 at 18:08
  • \$\begingroup\$ @jippie I need to bridge about 2m of distance at 4800 baud. \$\endgroup\$ – ARF May 25 '12 at 22:01
  • \$\begingroup\$ @TonyStewart CPLD is clearly overkill for me. I only need to switch RX and TX lines using the status lines between two devices. \$\endgroup\$ – ARF May 25 '12 at 22:02
  • \$\begingroup\$ @TonyStewart: he said he wanted an RS232 multiplexer, not a UART multiplexer. You're fired. You're fired anyway for wasting the company's money this way. \$\endgroup\$ – Federico Russo May 26 '12 at 4:44

Yes, have a look at these AD analog switches. You can have SPST or SPDT switches, several of them with on resistances less than 1\$\Omega\$.

I first suggested the ADG849, but Arik correctly points out that this is only limited to 5V operation. I had checked this, but I must have changed my mind and picked a different device at the last moment.

enter image description here

The ADG1436 is a better choice, also because it's a dual switch, so you only need one to switch Rx and Tx. It can operate at \$\pm\$15V, and switch signals to the rails. EIA-232 is \$\pm\$15V maximum, but chances are that you have 12V or, if you use something like a MAX232, only 10V.
The ADG1436 has an \$R_{ON}\$ of 2.6\$\Omega\$, and switches in about 100 nanoseconds. Contrary to a relay, also hardly requires any power: 0.001\$\mu\$A(!) at 25°C.

  • \$\begingroup\$ It was this sort of design that made our Project Ida fail in an in-house demo in 1983 of a T1 ISDN BB WAN with a setup box that locked up when the user got static from touching a glass & dust covered TV with static. better make sure your ESD ingress control is well designed. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 25 '12 at 18:05
  • \$\begingroup\$ Thanks Steven. I remember seeing these switches a while back but had completely forgotten about them. One problem I see is that they do not support over-the-rail operation. RS232 is likely to be larger than the maximum permissible supply voltage. This would be a clear advantage of reed relays - though the low energy consumption is great! \$\endgroup\$ – ARF May 25 '12 at 22:00
  • \$\begingroup\$ @Arik - yes, the supply voltage is not OK, I thought I checked that. I'll see if there are parts with a \$\pm\$15V supply. \$\endgroup\$ – stevenvh May 26 '12 at 4:14

Are you familiar with the CMOS latchup effect, where the device starts smoking from induced SCR trigger effect from stray electro-static discharge (ESD) from someone who grabs a cable end after walking on an office carpet with plastic soled shoes? I have seen this too many times to know that the built-in diode clamp protection is not enough. Why? because ESR rise times are on the order of 10~50 ps (pico Sec) and the diode clamp rise time is ~ 1nS and if the signal line exceeds the supply rail by more than 0.5V you are pooched.

That's why it's best to stick with bipolar robust designed RS232 drivers.

BTW I designed a split PAX unit as a contract for a Canadian telecom data company in 1983 one week, but now a days, you got all these CPLD's and bigger devices that can do the the MUX/ DEMUX much easier. I dont know if they ever pursued my conceptual design, but it earned our company 50K$ for the work.

MOSFETS will work as long as your TVS clamp/filters dont slow down slew rate with an ultrafast recovery diode (Expensive). So I suggest standard RS232 or choose the high amplitude Limited distance Data set drivers for RS232 to go a few Km. But really it makes more sense to use a VPN over ethernet at 10Mbps rates and then MUX-DEMUX to hex RS232 for local traffic using Xilinx etc for the logic/ sync and use ethernet or CRC.

How many ports and what rates? and what distance? Autobaud detect? Flow control by Xon Xoff or RTS CTS DSR DTR? or what? need specs please.

I guess it's safe to say,.. "the rapid rise of TCP/IP relegated many of Gandalf's products to niche status, and the company eventually went bankrupt in 1997."(Gandalf were famous for LDS data sets/modems etc.) You might find some kicking around in surplus.

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    \$\begingroup\$ dv because I can't find the answer to the question among the questions for OP, reminiscences about old projects, and digressions about ESD. \$\endgroup\$ – The Photon May 25 '12 at 17:49
  • \$\begingroup\$ I could find a alternative answer to OP's question. \$\endgroup\$ – Standard Sandun May 25 '12 at 18:04
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    \$\begingroup\$ Thanks for the answer. Unfortunately I am not quite clear where the problem with ESD arises with regard to RS232 muxing. Could you make this clear for a non-expert? \$\endgroup\$ – ARF May 25 '12 at 21:55
  • \$\begingroup\$ That’s ok @The Photon, I had once Designed a system for Gandalf Data systems in the early 80s to concentrate 1024 RS 232 channels, I could share my success with that if any one were interested. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 15 '18 at 2:43

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