What is the most simple and most common ? Still the 5 volt TTL signals.

So, SHACKBUS uses TTL levels to communicate between controller and devices.

SHACKBUS Hardware / electrical:

• SHACKBUS uses 3 wires. Voltage level is 5 Volt (idle) and 0 Volt (data):
  BUSIN (controller to all devices): normal TTL level signal
  BUSOUT (devices to controller): TTL open collector signal with pullup
  BUSGND (common ground wire)
• Data from 'PC' controller (RS232) is converted to the BUSIN line using a simple MAX232 receiver with TTL output. 
• The BUSOUT wire is a 'wired or' line with a pullup resistor. The devices use a open collector npn-transistor or just a diode to output the replies to the controller. Data to PC is converted using a single MAX232 transmitter.