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Terminate A Circuit Not In Use

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Member level 3
Jan 23, 2008
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Hi everyone, I have a PC that communicates over serially RS-232. The PC is connected by a cable to RS-232 Device 1. I am able to communicate with Device 1 from the PC. I now want to communicate from the PC with another RS-232 device, RS-232 Device 2. I want to be able to communicate from the PC with Device 1 when I want to and with Device 2 when I want to but never communicate at the same time (only communicate with one device at a time).

My goal is not to have to have two separate cables to talk to each Device 1 and Device 2. After communicating with Device 1 I do not want to have to disconnect the cable from Device 1 to the PC and then connect the cable from Device 2 to the PC to communicate with Device 2 from the PC.

Someone brought up to to me that I could terminate the Device that is not being currently communicated with. How do I use 1 cable with termination to the Device that is not being communicated with from the PC?

Thanks so much!

Go for USB to serial converter to make this happen? Connect the serial port to device 1 and connect the USB converter to device 2

or go for serial board add-on card in your PC. So that you will be having 2 serial ports.
The problem you have is that two devices need to transmit on the same wire to the PC. If you are certain that only one device will be transmitting at any given time, you can solve this with 2 diodes and a pulldown resistor on the PC Rx signal, or a diode and 22k in parallel from each Tx line to the PC Rx. The inactive decvice will then function as an active pulldown. The PC will then transmit to both units at the same time, and get an ANDing of the Tx lines from each device.

This will only work if the devices don't answer communication destined to the other device.

The value of the resistor will be a bit depending on the baudrate you use, but 10-22k should be ok.
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