The main function of the interface is to adjust the voltage levels of the serial signal from the decoders, to fit the +/- 10 V levels of the RS232 standard. The proposed schematic below also includes a voltage regulator for the power supply of the decoders.
Schematic
Figure 1. Interface schematic.
The signal from the most downsream decoder is connected to X2, and the computer is connected to X3. The optional connector X4 can be connected to the input of the most upstream decoder if you want to control the data rate of the decoders from the computer. Connector X1 is the power supply connector. I have indicated 12 V because that is what I will use, but with the 7805 regulator the input voltage can be in the range from 7 V to 30 V.
The MAX202 can be replaced by a MAX232A, and there are also several other similar components that could be used. The schematic is just an example of how it could be built. I will build it a bit differently, because I intend to use the the reverse channel (RXD in the schematic) to control turnouts and lamps on my layout.