Hi everyone, I have an update on this build:
I got past the problem and now the board executes code from the onboard EPROM. It wasn't a fault of the clock generator, but the problem was a non IEEE696 compliant signal (not surprising: the board is from 1977...).
The problem in detail is that line 53 is tied to GND on my backplane but the board uses it to trigger some input buffer for the Sense Switch Enable function.
I had to pull out pin 2 of IC14 and tie it to 5v.
Now I still have the problem that while the board executes code from the EPROM (checked by filling it with halt opcodes) the board is still unable to access the external console. I'm digging into this, but part of the problem might be a wrong component i have on the board: I used a 74LS04 in place of an 74L04 (IC12), which is used to introduce a delay in a few signals. I'll try to get hold of a 74L04 and then if this doesn't solve the problem, I'll debug some more.
On Wednesday, April 30, 2014 8:17:36 PM UTC+2, monahanz wrote:
Fabio have you tried 36MHz, sometimes you can get away with higher clock speeds -- particulary later edition chips.
On Wednesday, April 30, 2014 11:08:07 AM UTC-7, Fabio Battaglia wrote:
I know this might not be the best place to ask for spare ICs, but the question is somewhat related to the group.
Yesterday I completed my first N8VEM Z80 cpu card, but I have another S-100 z80 board near completion: an Ithaca Audio Z80 CPU rev.2 card.
I have most of the components needed for this vintage board, and what I don't have I replaced with pin-compatible more recent chips, but I'm left with a missing component: the clock generator.
The manual specifies that the 2Mhz version used a plain 8224 coupled with a 18Mhz crystal, and an 8224-4 (actually, an AM8224-4DC) chip with a 36Mhz crystal for the 4Mhz version.
While the plain 8224 is quite easy to get (there is also a 74' clone), the 8224-4 chip required for faster clock is proving to be very elusive: I checked eBay, UTSource and tried a google search for a reseller. I only one in USA with a small stock, but they only sell to companies.
So, do anyone have some pointers on where to look for the chip? Alternatively, if someone has a spare to sell, I'd love to use it to bring my board up. If I'm unable to find one, I'll settle with a 2Mhz board, as building this board is mostly for the fun-factor.