Excalibur 64 - A new build

Managed to get a different version on ROMs for the board. Luckily, I did as up and till now the only version I had was for the Hi-Res board which unfortunately I don't have so if use it here it never would have worked.

For the build looking like I'll go 'all the way' with TTL ICs. There is some tricky timing happening so will be good to keep everything the way our founders intended then mess about with new technology

Still looking for that elusive Hi-Res card and 512k expansion. Here is the Hi-Res! wanted dead or alive. With install instructions please 😁

Ok the clock is running nicely, even with-it being way over on the other side of the board!! Not all ICs are in yet but I'm assuming I'm going to have issues with noise once the board gets busy.

Have also installed the reset circuit as per below with a CMOS Z80 the circuit doesn't work. I think it is due to the mix of TTL and CMOS. The reset circuit using purely 74LS caused the /RESET line to stay low. I replaced the 74LS02 with a 74HCT02 and it fixed the /RESET staying low but the reset button doesn't work. Need further investigation but probably need to replace IC2 with a 74HCT as well. Need to scratch around to see if I have one else I guess I could put a 1k pull up on the /RESET to ensure there is +5v on the line.

Also getting some nasty spikes on the data and address lines, this CMOS/TTL mix is going to be an ongoing problem me thinks. 

The 6331-1 PROM used for the colour palette is not very new. So are a bit expensive and need a special programmer. The average programmer, like the TL-866, don't supply the right programming voltage for it. So being bipolar it needs something more specific and this is it. A manual programmer where you have to flick switches for the address then load the value for that address. This board is very similar to what was published in the early eighties by ETI and probably a lot of other magazines as well. The PROMs are one time programmable so need to take it nice a slow.

Have all the bits so will be building it like it's 1984.

Quick specs of the 6331-1

• Memory Capacity: 256 bits, organised as 32 words × 8 bits.
• Package Type: Narrow 16-pin Dual In-line Package (DIP).
• Technology: Schottky Bipolar architecture using Nickel-Chromium (NiCr) fuses.
• Speed Designation: The -1 suffix indicates a high-speed variant of the standard 6331 chip
• Equivalents:  82S23 or 82S123 from Fairchild, 74S188 or 74S288 from TI , 74S188 or 74S288 from Harris

So, nearly all the passive components are in. Missing some ceramic caps but rough enough for now. Unfortunately I'm missing some key 74LSxx ICs. Installed what I had and the clock circuit works fine. Going to use the OneROM for this build, hope they are ok with the 74LS? If not I can still use 27128 and 2764 EPROMs. Just need to order them from Unicorn or somewhere like that. This isn't a development board so maybe OneROMs are an overkill?

One interesting thing with the EPROMs is the that one of the images is 16Kb and the other is 8kb. Which equates to using a 27128 and a 2764. This matches the memory map I have from a forum ROM1 as $0000-$3FFF and ROM2 $2000-$3FFF, the region from $0000-$1FFF is always active when the /ROM switch is selected. Which ROM appears at $2000-$3FFF (either rom2 or the second half of rom1) is determined by bit 5 of the control register at port $70. This isn't mentioned in the documentation or schematics, so presumably these are later version ROMs from the original?

The schematic shows them as both 2764 and A13 is not connected. I know a couple of people who worked on this computer and developed expansion boards back in the day. I simply call them 'the oracles'. Hopefully one of them can explain how it works. I got away with in my recreation as I just used on big Flash ROM. This is where having a pre-build one would be handy 😒

Oh well, poor me, need to order even more bits. On the plus side, the programmer for the 6331-1 has arrived, so I will be busy building that and programming a couple of one-time programmable ICs. What could go wrong? 😊

Footnote: I'm using the original construction manual for the build (on the left) and has proven to be quite handy.

Nothing amazing to report but all the sockets are done. And, just because I had it I, installed a 74AHCU04 for the oscillator circuit and works fine. But there is some noise so may have to fiddle with the resistor values. But still want to keep it original so will try a 74LS. If it worse then I will need to accept not all of it can be original. Need a clean stable clock. The CPU is on the other side of the board.

Now onto the resistors and caps. Notice on the right? So many resistors and just to make it fun they are nearly all different values ... they are for the D to A used by the composite video out. 

Ok I but my big boy pants on and actually made a start on the Excalibur 64 Original build. My initial idea was to build it stages. Starting with the oscillator, then CPU, ROM/RAM, then move further out to the I/O and finally video.

But I didn't have quite the right ICs for the oscillator. It needed a 74S04 for amplification and phase control and a 74LS161 to divide the 16Mhz crystal. Only had a 74LS04 and a 74HCT161. Discovered these are no good. The oscillator circuit is a  two-inverter series-resonant crystal oscillator. The two inverters provide a  0 degree phase shift. The problem is the 74LS series has much less drive capability compared to the 74S series. I could muck about with the 470 resistors to reduce the load but I want to keep it as original as possible. So will have to leave the oscillator for the next bits order. So figured might as well start installing all the IC sockets. Which is quite therapeutic.

                     This the two-inverter series-resonant crystal oscillator

                I thought it best to conduct a cat scan before declaring I had the wrong ICs

                   Reading her facial expression it's confirmed 😒 need the right ICs

                    And the progress so far - not too exciting but will be!!