Safe way to bypass a transistor (temporarily)

[Sanxion]

Hi all,

Could anyone explain how I can bypass PST518B in the reset circuit for an Amiga 2000?

This may seem like an odd request, so please allow me to explain:

I have been building a new motherboard (rev 6.2) and I don't have that transistor, I have ordered it but it may take days to arrive and I am curious if I can temporarily do without it to test the build. The earlier version of the motherboard didn't actually use the transistor so I am hoping it can be bypassed for now until it arrives. I have attached the schematics for both versions of the motherboard.

Thanks.

[shapirus]

It looks like the images were cropped so that the trasistor was left outside. Or maybe I just can't see it :).

[macboy]

If you mean PST518B, that is not a transistor, it is a reset controller. It can assert the reset line when the +5 V rail is below about 4.4 V. Because of the open-collector output, which is wired-or connected to the '339 open-collector output, you can actually just leave the component out. The rest of the reset circuit will function just like the old version without this.

[Sanxion]

Thanks Macboy.
What do you mean when you say: It can assert the reset line when the +5V is below about 4.4V?
Also, if it can be left out, what is its purpose?

[Andy Chee]

Look at this diagram.

Notice how the voltage is a bit sluggish reaching 3.3V?  That slow ramp up/down time could cause unpredictability in your system.  The reset controller creates a nice square edge, triggered at about 3.0V (it's not labeled on the diagram, I'm just visually estimating). 



In your case, the supply voltage is 5V and it will dropout at 4.4V.