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I have two PCs (very old, without usb but rs232 port) that talks via RS232. They send to each other very small bit of info ( one character every 10sec). Now I need to connect additionally two sensors that send data to first PC. I assume no collision for all devices. I have tried to make all devices TTL lvl using MAX232 and it works only for two devices sending data to PC1 (e.g. PC2 and sensor1). When I connect sencond sensor (like on schematic) then there is no communication at all. I guess the MAX232 input in PC1 goes overload for 3 devices. Question - Do you know how to make it right for 3 devices? Maybe tying up Tx pins of both sensors on RS232 lvl and then put it to one MAx232? Any solutions with no use Rs485? (I have no handshakes..) three devices sending data to one (no collision)

EDIT: Here is schematic that would do the trick. Reply provided by Wouter van Ooijen. enter image description here

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    \$\begingroup\$ Are disabling the drivers for the devices that are not actively sending information? If one device is idle and pulls TX down, then the other devices can't send. \$\endgroup\$ – jippie Feb 16 '14 at 8:46
  • \$\begingroup\$ Idle state in TTL is +5V. Also interesting is that IF I shut down power for max232 in sensor2 it still doesnt work. So even if MAX232 is turned downt, it still overloads the line... \$\endgroup\$ – szymz Feb 16 '14 at 9:08
  • \$\begingroup\$ Is idle state +5V or open collector/drain? Check Wouter's answer with the AND port, addressing basically the same issue as I am trying to pin point. \$\endgroup\$ – jippie Feb 16 '14 at 9:48
  • \$\begingroup\$ You could possibly do this with just 3 diodes at RS232 levels. certainly there is little to be gained by converting the single data path to TTL and then back to RS232. The others can be wire ORed together with less chance of converter problems. Three more diodes and a small capacitor to store negative idle voltage fed through a resistor would make it very close to standards compliant. \$\endgroup\$ – KalleMP Apr 25 '17 at 5:11
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I am sure I answered essentially the same question a week or so ago, but I can't find it :(

Use an RS232-TTL converter for each device. Now your problem is reduced to combining TTL level signals. The (TTL) TX from your PC can simply be connected to the (TTL) RX lines of the 3 devices.

A TTL-level asynchronous signal is 1 when idle, so you can combine the 3 device (TTL) TX signals using an AND port. Output goes to the the PC's (TTL) RX line.

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  • \$\begingroup\$ You mean AND gate? well then, thank you for an answer. I'm going to put schematic that explains it for completing the solution. \$\endgroup\$ – szymz Feb 16 '14 at 10:40
  • \$\begingroup\$ I added schematic as I see it. In my case there is no problem using RS232-TTL converter for each device. \$\endgroup\$ – szymz Feb 16 '14 at 10:45
  • \$\begingroup\$ @szymz I think it was my mistake, I understood a different connection. I think your edit circuit will work fine. \$\endgroup\$ – alexan_e Feb 16 '14 at 10:53

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