Looking for circuit suggestions or examples to boost logic level
I'm working with a serial device that resides on a bus. The problem I'm having is that it won`t recognize or respond to the logic pulse on the bus. The bus is driven by 5 volts but as more devices are added it pulls the logic high level down. The devices use opto isolators and can draw any where from 10mA to 35mA. The particular device I'm working with won't recognize the logic pulse if it isn't at least 4.5 V high. If I disconnect some of the devices on the bus then my device will then work.
I know that the main controller driver could be beefed up to provide more current but that is not an option. Nor is it an option to modify the device I'm working on to work at a lower logic level.
As a proof of concept I kludged together an npn transistor to drive a high side p-channel mosfet fed by a 5V supply and i got it to work. I feed the existing signal to my circuit and then my circuit then feeds the bus. However the waveform is very noisey even though it works.
What I'm looking for is any suggestions to solve this.