Re: [N8VEM-S100:2403] 6502 CPU Board V1 components question

[David Riley <frave...@gmail.com>]

On Feb 16, 2014, at 11:56 AM, Fabio Battaglia <hkzl...@gmail.com> wrote:

> Hi everyone,
> I'm still waiting for the components I need to complete my Z80 V2 board (which are still one week away...), but I noticed I have almost everything I need for the 6502 V1 card (http://s100computers.com/My%20System%20Pages/6502%20Board/Printing%20S100_6502-sch1.pdf) except two items, so I'm going on with this one.
> 
> The first component I'm missing is the bunch of 1.3kOHM resistor networks, and can't find them for sale anywhere with reasonable shipping: I do have loads of 1k3, 5%, 0.25W resistors. Would 5% tolerance be OK if I build the networks myself?
> Second component is C5 capacitor on the output of the 7805 regulator: Is there any reason for it to be 1.2uF, or can I just go with the reccomended value in my regulator datasheet?

5% should be fine; 5% was actually the "expensive" grade of resistor when these parts were new. 1K, more or less, is generally just a nominal value unless it's for a termination network (e.g. for SCSI), in which case you usually want the given value with a 5% tolerance to match the characteristic impedance of the transfer medium.

The recommended value in the datasheet should probably be fine; you don't want an output cap that's too big on a 78xx part, because when you cut off the power supply, you could end up reverse-biasing the regulator if there's a lot of stored energy in the cap, and my recollection is that the 78xx regulators don't have much in the way of protection.  You can always drop a diode from output to input to protect it, though that's obviously not ideal if the PCB doesn't already allow for that.


- Dave