Posted on August 17, 2015 at 18:30When you need to convert from RS-232 to RS-485 the main issue is the transition from a full duplex interface (RS-232) to a half-duplex interface (RS-485). Assuming only the RS-232 is connected to a controller then you need to use one of the extra RS-232 pins as a direction control. A common way to do this is the RTS pin, which is normally an output (depending on DTE or DCE wiring) and use it for the direction control into the RS-485 converter.
If the RS-485 converter operates at RS-232 voltage levels for the direction (RE and DE) then all you need to do is connect the RTS pin on the RS-232 side to both RE and DE. But if the converter has these pins at TTL levels then you have to find some way to shift the RTS to a TTL level, or pass the TTL level directly to the RS-232 connector (risky as it will no longer be truly RS-232 compatible, plugging in something else may damage the TTL level GPIO pin and the controller).
Aside from the hardware connection you also have to allow for line turnaround, meaning there has to be a gap between sending data and receiving data on the RS-232 side. And you can't do both at the same time, meaning you can't echo back the RS-232 data. You have to send entire messages, not just character at a time.
Interfacing like this actually goes back to early days of networked computers in the 60's (look up the Bell 202 modem and the BISYNC protocol). The RTS and CTS pins exist on the RS-232 side for just this sort of problem, controlling when to send and when to receive on a half duplex connection.
Jack Peacock
