Possible to disable Serial port programatically?

TheWaterboy

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Is it possible to put the serial port into an "Off" state in a Contrologix PLC? I have a high-faluting "Y" cable so that I can run 2 plcs out the same serial port. It works but has the expected problems.
So it would be great if I could place one of the ports at a time into a high impedance state so that I could use the other one.
Even when the PLC is not using the port for anything, its alive and watching the pins. And its impact is obvious.
 
No, there isn't a way of making the RS232 port high impedance. Why not use a switchbox? There are still inexpensive multiport serial switches out there.
 
If there was one that I could switch via PLC, that might work. This thing is up on a hill and I need to switch it from remote sites.
I'll have to see if one exists.
 
IF you are up to building it yourself, Maxim Max232 line has AutoShutdown/tristate and RX enabled while the chip is in Shutdown mode.
Most are powered by 5VDC. Some require NO external CAPS.
Several have multiple channels.

MAX232.jpg
 
Quick ideas that will need investigation

1) does RS232 user mode assist - in user mode you can read the buffer and ignore all input and never write back
2) Gold contact 4 way changeover relays - ie physically disconnect the unused PLC
3) Convert the rs232 into rs485 then do the tristate there then convert back
 
I think door number 2 is the ticket. I cant find a device to purchase.
But I just found that putting the port into full duplex isolates it well enough. I'm not sure why that works but it does so for now that's working.
Thanks all.
 
I think door number 2 is the ticket. I cant find a device to purchase.
But I just found that putting the port into full duplex isolates it well enough. I'm not sure why that works but it does so for now that's working.
Thanks all.

I think that in Full duplex the TX RX lines are ignored until the other handshake lines are set eg DTR DSR CTS RTS so it depends on your wiring.

Gold plated relays are for long term use
normal relays are ok if brand new for a short time

http://www.omron.com.au/service_support/FAQ/FAQ02827/index.asp
 

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