🚀 go-pugleaf

In article <1992Sep11.090525.8360@bernina.ethz.ch>, stettbac@isibee3 (Juerg Stettbacher) writes:
|>
|> Three stateable address and control buses generally need pull ups, to
|> prevent from accidental accesses. I suppose that's true.
|> 
|> But what about data buses. Can a floating data bus cause any troubles?
|> (Oscillating, switching currents .. ?)
|> 

There are three reasons for terminating a bus:

1) Improve the speed and the reliability of a bus system by avoiding
   refections. Impedance mismatch between the bus line and the open
   end of the bus line can cause signal reflections. So you have to
   slow down your bus or put on the open ends terminators with the
   bus impedance. The impedance of a VMEbus is about 20-100 ohms,
   and it is terminated with about 194 ohm. The experienced designer
   thinks thats close enough.

2) As you mentioned, the control lines can be used by several bus masters.
   There are two possiblities: 3-state and open-collector bus control
   lines. With 3-state lines there have to  be a default state, if no bus 
   master takes the bus. With open-collector lines the terminator is a
   'pull-up' while all bus master have only the possiblity to 'pull-down'.

3) CMOS components should not have an open input. A CMOS component have
   a very high input resistance so static electricity can damage the
   component. The input stage of a CMOS gate consists of two complementary
   transistors and a floating input can cause both transitors to work
   analog, that means open half (digital: one is open, one is closed). 
   This cause a high current from Vcc to GND through the cuircut, which
   can be damaged. The expirienced designer never trust an on chip protection.
   Most buses a TTL-compatible and don't use CMOS receiver. An
   exception is the Sbus (SUN-Sparc), with is designed for CMOS.
   So the SBus controller may terminate the Sbus with 10 kohm to Vcc
   and a SBus slave may include a 100 kohm resistor between the signal
   and Vcc.

Jens