Sinclair ZX80 / ZX81 / Z88 Forums

I'm trying to modify my newly restored Issue 2 Rampack for Hi-Res using Wilf's diode-resistor AND gate over RD and RFSH lines. I've read in other post that it is possible to do it with 1n4148 diodes instead of the germanium ones, may be schottky diodes are advisable but I couldn't get ones near here.
As my pack has IC1 on a socket it was easy to lift RFSH and RD entry pins and insert diodes cathodes into the socket pits.. the I added resistors up to 4.6Kohms (didn't had either ) and used +5v from IC pin 14 (vcc)
And here is the cockroach : Surprisingly computer did startup nicely..
Loaded WRX 1K to test it (just because of loading time and I needed many, several, attempts to finally load) but didn't work.. just a black box without the white circles.. it did work without the pack and behaves exactly the same way as with another non modified pack...,
Then it could be that the gate is not working at all or that diodes are too slow.. ?
I have this cheap Hantek hand held oscilloscope, (20Mhz 2channels) not too fancy but it's been enough so far. As It is only two channels first I tested RFSH (green) vs RD output at diodes anodes, (yellow) As you can see RD output from the gate has some times a short and some times a tall spike over the rising RFSH Input.
The RD showed e similar behavior with the RD output spike being Short over the rise of RD at the input and larger over RD ramp down Picture is similar but bigger over the rise up other signal.
Then I checked both entries together and what I see is that the superimposed curves shows only a very short overlapping, coincident with RD output spikes.. I must admit that it was easy for me to understand how OR diode gate works but I don't completely understand how the inverted diodes and the 5v and resistor works...
Should I use schotsky diodes? Or change the resistor values? I've seen they use a capacitor around RAM CS on some SRAM designs..
(edited some misspelling..)

Ok as it didn't work I used a 78LS08 AND gate. Mounted over IC2 were pin2 is RD and 12-13 are /RFSH. Didn't work either.
Machine is working fine. Ran some ramtest for more than an our OK. But High resolution is not there...

bola_dor wrote: ↑Thu Sep 02, 2021 5:51 am I'm trying to modify my newly restored Issue 2 Rampack for Hi-Res using Wilf's diode-resistor AND gate over RD and RFSH lines.

It’s actually doing an OR function really. The RAM has to respond when either /RD goes low OR /RFSH goes low.
The resistor is to pull up the logic input when neither/RD or /RFSH is low. The value is not critical, as long as it is high enough that the logic input goes high, and either diode can pull the input to a logic low state. And 1N4148 diodes may work (I’ve not actually tried, as I have some schottky diodes signal diodes available).

Sorry, got to go. I’ll continue later…

Mark

1024MAK wrote: ↑Fri Sep 03, 2021 8:59 pm

bola_dor wrote: ↑Thu Sep 02, 2021 5:51 am I'm trying to modify my newly restored Issue 2 Rampack for Hi-Res using Wilf's diode-resistor AND gate over RD and RFSH lines.

It’s actually doing an OR function really. The RAM has to respond when either /RD goes low OR /RFSH goes low.
The resistor is to pull up the logic input when neither/RD or /RFSH is low. The value is not critical, as long as it is high enough that the logic input goes high, and either diode can pull the input to a logic low state. And 1N4148 diodes may work (I’ve not actually tried, as I have some schottky diodes signal diodes available).

Sorry, got to go. I’ll continue later…

Mark

Before I get a stroke, or fall into deep depression..
I think you mean that because both /RFSH and /RD are inverse logic.. but .. do I have to make an (electrically) AND.. don't I?
It would be easier to fit diodes forward to make an OR gate
Anyway 74LS08 is not doing the job either..
I lifted pins 2(rd) 12,13 (RFSH) from IC2. Soldered 12 and 13 to 14 ,that's vcc to RFSH.
Then I bent inwards all unused pins from 74LS08 and soldered them to a thin wire underneath to pin 7 (GND).
Then used insulated thin cable from pad for pin 13 (computer RFSH) and pin 2 (computer RD)to pins 12 and 13 on the 74LS08. Then anodes wire from 78LS08 pin 11 to IC2 pins (rampack RD)...
78LS08 is mounted over IC2 with only VCC and GND soldered directly to it..
What am I missing???

bola_dor wrote: ↑Fri Sep 03, 2021 9:22 pm I think you mean that because both /RFSH and /RD are inverse logic.. but .. do I have to make an (electrically) AND.. don't I?

Yes. Because both inputs and the output are using inverse logic, to make an OR function you use an AND gate.

bola_dor wrote: ↑Fri Sep 03, 2021 9:22 pm Anyway 74LS08 is not doing the job either..
What am I missing???

I can’t see a problem with the hardware mod.

It’s been a while since I played with hi-res on a ‘standard’ Zeddy (I normally use a ZXpand).
Have you tried another hi-res program such as WRX16?

Also relevant is this older topic.

Mark

Thanks you for th explanation.
It looks like IC2 and the 74LS08 are not doing its job anymore.
Something went wrong..

Finally..
Two 1N4148 and a 6. 8Kohm resistors did the job as spected, much easier to fit than a chip..
Now to solder it properly and put it nicely into its case..

OK, I resoldered the mod somehow tightly to put everything into the case.. and stopped working..
It's the 4th 74ls00 (IC2) that I kill in the process..
I have no idea what's happening..
Now I wil use wires. Solder far from the IC, and feed RFSH trhu the pull-up resistor..
Or should I pull-down the gate exit pin? as RFSH +5v goes to both NAND entries

Well, technically with 74LS logic, +5V should not be directly applied directly to an input pin. Instead a suitable resistor should be used (value of 1kΩ or thereabouts). If you are feeding all the inputs to the same gate, only one resistor is needed.

When soldering directly to a chip, keep the time to a minimum, certainly less than three seconds. And although 74LS types are not as sensitive to static electricity as CMOS types, you should still take precautions.

Mark