Pull-up resistors may be used at logic outputs where the logic device cannot source current such as 7407 TTL device, an open collector buffer, simply outputs whatever it receives as input, but as an open collector device, the output is left effectively unconnected when outputting a " 1 ".
22.
The bus is driven at every transmitter by an open collector transistor driver, and isolated at each receiver with an opto-isolator-though cable harnesses carrying the communication signals may also carry 24-volt power and ground signals to devices, meaning the devices may not be isolated from each other as they share the same power bus.
