RS232-RS485 converter build

A french company ENTRELEC build it and I thing they have a sale office in portugal. I will searching for it and send you.
Buy one from "": or "": If you want to build one, what you need depends upon what, if any, operating system you are running. Typical desktop OSs do not give the required timing control of the transmitter unless coded at the driver level (typically using RTS to control transmit), and even then it often isn't fast enough (NT, for example can't do the job reliably). If this is you situation (no realtime OS), then you should implement hardware control of the transmit signal. Basically you need a circuit that enables the transmitter on a "start bit" and remains enabled for slightly more than one character time. It's fairly trivial, but certainly more cost effective to buy
National Semiconductor has lots of app notes in the back of the "Analog - Interface" data book which covers RS-232/485. Not sure where those
would be on-line, but you could start at

However you'll find that the RS-232 and RS-485 part of this converter are trivial (2 chips worth $3 each). It's the packaging, power-supply, fault-protection (surge, ESD, etc) that will take your time.

Are you sure you don't just want to buy a cheap one from black-box or other web sources?


Lynn August Linse, Senior IA Application Engineer
15353 Barranca Parkway, Lantronix Inc, Irvine CA 92618
[email protected]
Tel: (949)300-6337 Fax: (949)453-7152
I've done this before w/ a SN75ALS180 (to convert between 485 and TTL) and MAX232 chip (to convert between TTL and 232). Wasn't an ideal design, but it worked! good luck.
Also beware that a big part of the problem with RS232 to RS485 conversion is that you're translating a full duplex medium to half duplex. This means that unless you take specific corrective action, your RS232 side will see both
transmitted and received data, which some (probably most) PC or PLC software has trouble dealing with. Commercial adaptors either use an intelligent adaptor that cancels the 'echo' of transmitted characters, or a handshaking wire to turn the receive interface off while transmissions are being made from the RS232 side.

Tim Linnell
I use linux (kernel 2.4.x), I know that this OS is not considered real time although there is a project to enable real time jobs in linux, but I would like to ask you if this OS uses RTS to control transmit.

thanks a lot!