For two microcontrollers on the same board I would consider just cross connecting the two back to back TxD1 => RxD2 and TxD2 => RxD1. If there is any possibility of there being any significant capacitance along the traces between the two controllers or if the two chips are on nearby boards that share the same power supply GND then I would buffer the signals at each end using either 74xxC125 or 74xxC14 type buffers. If the two microcontrollers are more distant or utilize a grounding system where there could be a common mode difference between the two boards or if they are on different power supplies then I would use a differential type buffering system using buffers of the type that are similar to or suitable to use with RS422 type communications links. I would stay away from using RS485 type drivers for the simplest applications of this type because of small bit of extra protocol required to deal with a half duplex communuications path. RS485 type drivers are ideal on the other hand IF you have more than just two microcontrollers OR if you want to plan for expandability to 3 or more devices.

It is my honest opinion that the use of a MAX232 for buffering UART serial communications over a private local connection is not the best implementation because of the added charge pump converters on the chips, the added capacitors, and of course the increased voltage swings. I think it is easier to simply use chips that operate at the same voltage levels as the microcontrollers. Suitable buffers can be found in packages as small as 8 pins for a differential driver/receiver pair. I would confine the use of the "RS232" type buffering to where the signals either go out of the chassis or enclosure OR IF it is necessary for such signals to also interface with other RS232 type equipment. (There are times when the point to point connections between two microcontrollers in a product may need to be opened and connected instead to say a PC hookup. This can be useful for testing, re-programming, calibration, or data log capture).

Michael Karas