RS485 to DB9


Active Member
I have a tr16 that I want to connect to my pc serial port. The tr16 as you are aware is rs485.

I was wondering if there was a way to accomplish without dishing out $80 for a converter from RCS.

Thanks and looking forward to hearing form you.



Active Member
I don't think that one would work. The problem is not have the ports. the problem is the cable that will connect the rs485 to rs232. There are adapters out there from cables to go but they sell these not configured so I would have to know what pins on the db9 I need to set.

The connction that I am looking for is RS485 (Rj11-12 2 wire) to DB9.

I can't figure out the pin structure on the DB9 let alone build mine...but do appreciate the reponses.

Please let me know if something like this can accomplish what I am tryin to do.



I'm not sure on this one (i.e. disclaimer) but I think Guy is saying you can't directly connect RS485 to RS232 even if you had a cable.

I used THIS type of adapter to get to a serial port from a LANTRONIX EPS-1 Print Server to a DB-9 serial port, but the LANTRONIX Serial Output had an RJ-45 jack already there for it's native serial (RS-232) output.

I also use RJ-45 jacks for my phone lines (RJ-11) so I do know that RJ-11 phone plugs will fit into and work with RJ-45 jacks (just can't use the outer pins of course).

Don't know if this helped any. I also have the pinouts that I used for the DB-9 to RJ-45 adapter for RS-232 serial operations if you would like that (Guy and someone else (can't remember who) helped me with this).


Active Member
Thanks. I think I will dish otu the money and get the adapter from RCS. Although the info is great...I am new to this stuff and what if due to my mistake it destroys the thermostat...

Better be safe than sorry.

thanks for all the replies. Anyone who actually had any expericence doing this with tr16 would be greatly helpful.


Guy Lavoie

Active Member
Yes BSR, what I'm implying is that going from RS-232 to RS-485 is not just a matter of cabling. The two specs are using different voltage levels. RS-232 is bi-polar, going from a voltage between -12 and -3 volts for a logical "1" to between +3 and +12 volts for a logical "0". On the other hand, RS-485 is differential, using two signal lines that use opposite signals of 0 or +5v volts. The converter looks for one side to be receiving a signal change and transmits out the other.

I've actually built my own module for communicating with a RCS TR40 stat. This project is based on a microcontroller and uses a 75176 chip to interface to the stat. On the RS-485 RCS stats, you also need to provide it with a seperate +12 volt supply to power the interface. They use optoisolation so its 24 Vac power source cannot be used to power the serial interface.


Senior Member
There's at least one thing you need to keep in mind when you buy an RS-232 <-> RS-485 adapter.

RS-485 can operate over 2 or 4 wires. RS-485 uses two wires to transmit a signal (the balanced signal Guy refers to above). An RS-485 network (or a connection between two devices) can use 4 wires - 2 for transmit and 2 for receive - or it can use 2 wires. The 2-wire systems use the same wires for transmit and receive and the chips on either end control the current mode.

If you buy an adapter for a 4-wire system and need a 2-wire one, it won't have the appropriate hardware to handle this (it's mostly just using a different chip). Likewise, a 2-wire adapter won't be the right hardware for a 4-wire system.

Most HA systems - at least most readily available adapters - use a 4 wire system. It's just something you need to be aware of.

Also, many adapters will use an RJ-11/12 or RJ-45 jack for the RS-485 side. There are no consistent standards for which pins are used for which signals. So you need to keep that in mind, too.