RS485

[DavidL]

I ran out of serial ports again and put an old PCI to 2 serial port adapter in. While setting up the port config, I noticed it had RS485 settings. Did I waste $110 in purchasing an RCS 8 port RS232 - RS485 adapter for my TR40 serial thermostat? The serial adapter could not have been more than $25 from memory. 8 ports is overkill for me right now. No real regrets, just interested if this could have been a solution. Any thoughts?

 

[Guy Lavoie]

It might have indeed done the job. The only difference between RS-232 and RS-485 is the voltages and signal polarity. The main difference is that with a 2 wire RS-485 device, you have to know when to activate the line drivers to transmit data on the wires, and leave the output in high impedance state when only listening for data. It is actually easier to make a RS-485 transmitter along with the uart because it "knows" when it transmits, while an adapter like the one you bought has to watch the data transmit line and suddenly activate the line driver whenever data is to be transmitted and do this quickly, yet release the data lines to listen for a response. It ideally needs to add a slight delay before turning the driver off in case you want to send more data.

 

[DavidL]

Good to know for the next person that buys a serial thermostat (or other RS485 device).