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Re: Sourcing Parts
Hi Wayne,
Z80 CPU has static design (even the original NMOS version, and for
sure the CMOS version), and so it can be "underclocked" as much as you
like - until complete stopping of the clock in either high or low
level. So no problem with running 20 MHz CPU on 4 MHz or 8 MHz.
Just for historic reference, original NMOS Intel 8086/8088 CPUs (and
analogs from other manufacturers) use some dynamic cells and require
minimal clock of 2 MHz. Later CMOS redesign - 80C86/80C88 was fully
static design, allowing slowing down and stopping the CPU clock. It
was popular in early laptops due to this feature (and also due to
lower power consumption of CMOS parts vs. NMOS)
Back to N8VEM, since Disk I/O board doesn't implement DMA and uses
polled I/O, it requires CPU to be fast enough to read the data on the
fly. 4 MHz is usually enough for 720 KB disks, and 6 MHz is enough for
1.44 MB. 8 MHz works very well with regular TTL-LS logic. I think it
will work on higher frequency too. Note that I am talking about CPU
clock here. The oscillator on Disk I/O board must be exactly 8 MHz (or
16 MHz if you're using FDC9268 chip). Other values won't work.
I am personally fan of TTL-compatible CMOS parts (74AHCT or 74HCT),
74AHCT is normally faster than 74ALS and have reduced power
consumption, 74HCT have more or less equal performance to 74LS. Some
people mentioned that CMOS bus drivers/transceivers (74*244,74*245)
don't have hysteresis (a Schmitt trigger) on inputs, a feature which
supposedly should improve noise margin. And so CMOS parts might be
less suitable for buses. But:
- Advanced TTL series, such as 74ALS or 74F don't have hysteresis
either (it looks like this feature adds latency / reduces
performance). 74F were designed specifically for driving buses, and
work nicely without hysteresis.
- I use 74AHCT/74HCT in my N8VEM, Zeta SBC, and XT boards without any
problems.
Perhaps CMOS parts require better termination (TTL might work as
terminators themselves, due to much higher input current), but it
never was an issue for me. The only exception was 74ALS373 address
latch in N8VEM SBC-188.
Note that some parts don't have CMOS equivalents (for example ones
with open collector outputs like 74LS06).
Best regards,
Sergey
On Jun 20, 3:41 am, opticpow <wa...@ingram.net.au> wrote:
> Thanks Nick, less worried now!
>
> Wayne.
>
>
>
>
>
>
>
> On Wednesday, June 20, 2012 8:14:42 PM UTC+10, brizza wrote:
>
> > Hi Wayne,
> > You could send them an email and ask them to add it to your order. i've
> > done that on a few occasions. they are pretty helpful. 6Mhz would be a
> > minimum although 8Mhz is recommended.
> > Regarding your other questions, the 74ls parts work fine for me at 8mhz
> > and the cpu's work fine under clocked, but others in the group are better
> > placed to answer these questions.
> > Regards,
> > Nik
>
> > > opticpow <wa...@ingram.net.au> wrote:
>
> > > Things are now starting to get interesting, lol.
>
> > > I've place my main order of parts already with the 3.6864 oscillator.
> > > Thats
> > > no biggy, ont $1.20. I'm more concerned with all the 74LS parts that I
> > > have
> > > ordered. Will they work at the 6Mhz, or do I need ALS?
>
> > > Second Question, Sergey has 20Mhz CMOS Z80s, can they be underclocked &
> > > run
> > > at 6Mhz?
>
> > > Wow, didn't realise my first non kit build would be this complex. Going
> > > to
> > > be a pretty special feeling getting this one working!
>
> > > Wayne.
>
> > > On Wednesday, June 20, 2012 6:50:16 PM UTC+10, brizza wrote:
>
> > > > Hi Wayne,
> > > > Just in case you are not already aware, you'll need a 6mhz cpu
> > > oscillator
> > > > or faster to get the floppy drives to work properly.
> > > > Regards,
> > > > Nik
>
> > > > > opticpow <wa...@ingram.net.au> wrote:
>
> > > > > Hi Nik,
>
> > > > > Thanks for the info! I has noticed the some of the parts, but was
> > > not
> > > > > sure
> > > > > of their compatibility.
>
> > > > > Much appreciated.
>
> > > > > Wayne.
>
> > > > > On Wednesday, June 20, 2012 1:07:18 PM UTC+10, brizza wrote:
>
> > > > > > Hi Wayne,
> > > > > > Futurlec has substitutes for most of them;
> > > > > > Part Number Description Qty Price (AUD)
>
> > > > > Total
> > > > > > (AUD)
> > > > > > 8250A 8250A - Asych. Communication
> > > > > > Element 1 4.50 4.50 Remove
> > > > > > Z80B Z80B - CPU 1 1.95 1.95
> > > Remove
>
> > > > > > 74LS06 74LS06 - Hex Inverter Buffer/Driver (Open
> > > > > > Coll.) 1 0.50 0.50 Remove
> > > > > > OSC1.8432 1.8432MHz Crystal
> > > > > > Oscillator 1 1.90 1.90 Remove
> > > > > > 628512 628512 - 512Kx8 CMOS
> > > > > > RAM 1 12.90 12.90 Remove
> > > > > > OSC3.6864 3.6864MHz Crystal
> > > > > > Oscillator 1 1.20 1.20 Remove
> > > > > > Total (AUD) 22.95
>
> > > > > > the floppy controller chip can be found on ebay
>
> >http://www.ebay.com.au/itm/FDC9266-FLOPPY-DISK-CONTROLLER-IC-/2707848...
>
> > > > > 6?pt=LH_DefaultDomain_0&hash=item3f0c08a698
> > > > > > network resistors from element14 or ebay?
>
> > > > > > Regards,
> > > > > > Nik
>
> > > > > > > opticpow wrote:
>
> > > > > > > Hi All,
>
> > > > > > > As I've mentioned in my first post, I've been working on
> > > sourcing
> > > > > the
> > > > > > > parts
> > > > > > > for the SBC v2, backplace & Disk IO v3. I am getting most parts
> > > from
>
> > > > > > > Futurlec, ECB connectors from Altronics, and the odd part from
> > > > > Jaycar
> > > > > > > (pickup from local store)
>
> > > > > > > There are a few harder to get parts and I'm look for
> > > suggestions. It
>
> > > > > > > seems
> > > > > > > that I can get them from Jameco & Digikey, but with the shipping
>
> > > > > costs,
> > > > > > > I
> > > > > > > might as well get all my parts from them. The items I still need
>
> > > > > are:
>
> > > > > > > 4Mhz & 1.8432Mhz oscillators - Digikey
> > > > > > > 1k & 4.7k resistor networks - Digikey
> > > > > > > SRAM - Jameco / Ebay
> > > > > > > Z80 - Jameco
> > > > > > > 74LS06N - Digikey
> > > > > > > FDC9266 - Ebay / utsource
> > > > > > > 16550 - Ebay / utsource
>
> > > > > > > I wanted to try and keep down the number of suppliers, otherwise
> > > the
>
> > > > > > > shipping will add up and I might as well just get it all from
> > > Jameco
> > > > > &
> > > > > > > Digikey.
>
> > > > > > > Any advice would be appreciated.
>
> > > > > > > Wayne.
>
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