Memorymoog CPU replacement (and Ram)

[gencomprodukts]

Hi Everybody
I was wondering which Z80 would perfectly fit as replacement.
I've seen different cpu frequency speeds in several MM.
Some units has a 4mhz Z80 cpu (Z80A), others 6mhz (Z80B) and even 10mhz (seen a MM in a video with Z84C5010PEC Ram80, i don't know what "RAM80" means and there's not a lot infos. In a thread i've seen that the cpu should be Z84C000X X=speed in mhz and not Z84C50).

Mine is a MM+ with a NEC D780C (2.5mhz only after a check).
Anybody knows if the MM was originally designed to runs with a Z80 at 2.5mhz or higher, for instance at 4mhz as as mentionned on this link ? http://www.muzines.co.uk/articles/moog-memorymoog/5879

Unfortunately there's nothing about it in the service manual and i'm wondering why there's so much units (non LAMM) with higher frequency/clock rate CPUs.

Could i replace this old NEC cpu at 2,5mhz with a faster one (yes but which one? 4/6/8/10Mhz) without any damage for the Digital board ? Could i even hope to get some advantage in speed ?

Same thing for the RAM, some people has TC5517APL (that's my case) and others has TC5517APL-2 (a bit faster in Nanoseconds).

Thanks for your help !
Cheers.

[analogmonster]

A clock rate of 2.5 Mhz just means that you can't use a 2 Mhz replacement, but a 4 Mhz or faster. But this only means that the processor COULD run faster, not that it runs faster just because you use it. The processing speed ist dependent on the clock rate, not the Z80 variant. AFAIK there are Z80 models with 2, 4, 6, 8 Mhz, others I don't know, but if the CPU board runs with 2.5 Mhz, you need the same or next higher clockable processor as a replacement / substitute, no 2 Mhz version.

[gencomprodukts]

A clock rate of 2.5 Mhz just means that you can't use a 2 Mhz replacement, but a 4 Mhz or faster. But this only means that the processor COULD run faster, not that it runs faster just because you use it. The processing speed ist dependent on the clock rate, not the Z80 variant. AFAIK there are Z80 models with 2, 4, 6, 8 Mhz, others I don't know, but if the CPU board runs with 2.5 Mhz, you need the same or next higher clockable processor as a replacement / substitute, no 2 Mhz version.

Thanks very much for your explanation ! I finally found something in the service manual but it's unclear (see below). Maybe can you help me to understand. Does the CPU board runs at 4mhz or 2mhz ? It seems at 4mhz, right ? So maybe changing my old 2.5 mhz CPU with a 4Mhz or higher as some users did would be a good idea. And i even could expect to get some benefits, for instance in speed.

"CLOCK CIRCUITRY.
Clock inverter U14, crystal Yl, resistors Rll capacitors C8 and C6 form a 4 megaWtz oscillator that drive fiip-flop U16. The flip-flop is connected as a divide by two circuits to insure that the waveform will be a square wave at the required frequency of two megahertz."

[analogmonster]

Does the CPU board runs at 4mhz or 2mhz ? It seems at 4mhz, right ? So maybe changing my old 2.5 mhz CPU with a 4Mhz or higher as some users did would be a good idea. And i even could expect to get some benefits, for instance in speed.

Ah no, it is just the crystal which oscillates at 4 Mhz. The circuit diagram of the CPU board and the textual explainations show it. The frequency which is connected to the clock input pin of the Z80 is 2 Mhz, as noted in the circuit diagram. The clock circuit is more complicated than necessary. The crystal based oscillator consist from the crystal itself, two inverters and some other passive stuff. This formes the naked oscillator. It runs indeed at 4 Mhz. But the flip flop and another following inverter create a pure clean TTL leveled square wave from the 4 Mhz oscillator frequency, but divided by two to obtain a 50% duty cycle symetrical square signal. And these 2 Mhz drive the Z80 (and not to forget the dependent MPU bus system and peripherical ICs!). So I don't recommend to speed up the frequency by using another crystal (and another Z80).

Nowadays a simple crystal oscillator IC does the job of the complete Memory Moog oscillator circuit. I used these oscillator ICs for my own Z80 developments. Much more easy to build and simpler boards are possible then. Moog did not update their boards though.

As the clock rate of the Memory Moog is 2 Mhz, a replacement with each type of Z80 should be possible. I worked with 8 Mhz boards as well, but 2 Mhz is absolutely ok for Z80 cpus. To be on the safe side I would use 4 Mhz Z80 replacements within the Memory Moog just to be sure that the clock frequency can be processed, as I don't know how precise the crystals are which are used by Moog. So a 4 Mhz Z80 will work fine anyway, but according to the Moog circuit with more or less 2 Mhz. No speed up will happen.

[gencomprodukts]

Does the CPU board runs at 4mhz or 2mhz ? It seems at 4mhz, right ? So maybe changing my old 2.5 mhz CPU with a 4Mhz or higher as some users did would be a good idea. And i even could expect to get some benefits, for instance in speed.

Ah no, it is just the crystal which oscillates at 4 Mhz. The circuit diagram of the CPU board and the textual explainations show it. The frequency which is connected to the clock input pin of the Z80 is 2 Mhz, as noted in the circuit diagram. The clock circuit is more complicated than necessary. The crystal based oscillator consist from the crystal itself, two inverters and some other passive stuff. This formes the naked oscillator. It runs indeed at 4 Mhz. But the flip flop and another following inverter create a pure clean TTL leveled square wave from the 4 Mhz oscillator frequency, but divided by two to obtain a 50% duty cycle symetrical square signal. And these 2 Mhz drive the Z80 (and not to forget the dependent MPU bus system and peripherical ICs!). So I don't recommend to speed up the frequency by using another crystal (and another Z80).

Nowadays a simple crystal oscillator IC does the job of the complete Memory Moog oscillator circuit. I used these oscillator ICs for my own Z80 developments. Much more easy to build and simpler boards are possible then. Moog did not update their boards though.

As the clock rate of the Memory Moog is 2 Mhz, a replacement with each type of Z80 should be possible. I worked with 8 Mhz boards as well, but 2 Mhz is absolutely ok for Z80 cpus. To be on the safe side I would use 4 Mhz Z80 replacements within the Memory Moog just to be sure that the clock frequency can be processed, as I don't know how precise the crystals are which are used by Moog. So a 4 Mhz Z80 will work fine anyway, but according to the Moog circuit with more or less 2 Mhz. No speed up will happen.

Once again many thanks for all these informations & explanations ! Very clear and usefull
Well, i'll try with a 4mhz Z80, though no speed up should happen. Higher frequency rate should not be so dangerous for the synth since i've seen à lot of Z80 at 6/8 & even 10mhz installed on the computer board, but i don't want to take risks

Regarding the RAM chips, do you know if replacing the Toshiba TC5517APL by the TC5517APL-2 (A bit faster. 200ns) could be relevant ?

Thanks.

[analogmonster]

Higher frequency rate should not be so dangerous for the synth since i've seen à lot of Z80 at 6/8 & even 10mhz installed on the computer board, but i don't want to take risks

Just for the Z80 you are right. But time constants, frequency generation or autotune might rely on the 2 Mhz as well, so I don't recommend to change it.

Regarding the RAM chips, do you know if replacing the Toshiba TC5517APL by the TC5517APL-2 (A bit faster. 200ns) could be relevant ?

Well, if the pinout is identical there should not be any problem. But I don't know whether the processing speed is determined by the RAM chips. Even if the data bits are provided faster at the data terminal pins, the Z80 has it's own timing diagram for the I/O operation terminal pins. See also Z80 timing diagrams.

[gencomprodukts]

Higher frequency rate should not be so dangerous for the synth since i've seen à lot of Z80 at 6/8 & even 10mhz installed on the computer board, but i don't want to take risks

Just for the Z80 you are right. But time constants, frequency generation or autotune might rely on the 2 Mhz as well, so I don't recommend to change it.

Regarding the RAM chips, do you know if replacing the Toshiba TC5517APL by the TC5517APL-2 (A bit faster. 200ns) could be relevant ?

Well, if the pinout is identical there should not be any problem. But I don't know whether the processing speed is determined by the RAM chips. Even if the data bits are provided faster at the data terminal pins, the Z80 has it's own timing diagram for the I/O operation terminal pins. See also Z80 timing diagrams.

All right, again thanks a lot for all, especially your knowledge & time
All the best !

[analogmonster]

The same for you