It is most important, that the expense on the device controllers is reduced to a minimum.

The following points must be observed to keep costs and software expenses to a minimum

• slow speed to make software UART solutions possible
• no duplex (simultaneous TX and RX) required (software UART)
• sufficient time window to compute and transmit the reply string

The only problem with the controller is to react immediately to any controller signals on the bus. The bus input of the device controllers should generate a hardware interrupt and start the serial input routine immediately. At least the first three characters have to be buffered and compared to the device adress. Receive routine has to remain active until a <CR> is signalling the end of the command line.

This job can be done by any 0.50$ PIC microcontroller without problems. However, if the controller is to be used for other real-time tasks of the device, it would be desirable either to use a MCU with internal UART (to keep interrupt rate low) or to separate the SHACKBUS-controller PIC and the main controller and use two separate MCUs.