Connecting 4 devices on RS232

communicationmax232rs232rs485serial

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

Best Answer

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.