It is totally possible to multidrop RS232 connections in a bus arrangement if you use the appropriate drivers. Now whilest this configuration is not "standard RS232" it is however a configuration that is more common than one may think. MAXIM makes an IC just for this application that has a ENABLE/DISABLE control for the RS232 driver TxD output driver. The diagram below shows how this chip can be wired up to implement multidrop RS232.



The complete data sheet for the part can be seen at this link: MAX3323E. This part works on a range of supply voltages so it is possible to make this multidrop system work in 5 volt systems or 3.3 volt systems, or even a mix.

The use of multidrop RS232 is still a single ended signalling interface and as such has distance of transmission limitation similar to normal RS232. If you need greater distances than say 50 or 100 feet then you should resort to the use of RS422 or RS485 where differential signalling on impedance controlled lines with terminations is used.

Any multi drop system, whether it be RS232 type, as shown here, or RS422 or RS485 requires an appropriate software protocol for the devices so that only one of the multi-dropped transmitters is enabled at any one time. A master / slave arrangement would provide this capability and a packet protocol is also a very good idea.

Michael Karas


Michael Karas