Sinclair ZX80 / ZX81 / Z88 Forums

A.G.B. was a distributor in France. Does anyone recognize this board or case shape? Or maybe AGB manufactured it in-house


with chips:

• 16K-bit DRAM: STC 4116 4N date code 8426

• Quadruple 2-Line To 1-Line Data Selectors/Multiplexers With 3-State Outputs: Texas Instruments Portugal SN74LS258AN 8323C

• Timer/Oscillator: TI NE555P

• F- 74LS00PC (?)

I think XavSnap may have some answers for this as would new member, jeffounet as it is part of his collection as seen here...

https://forum.system-cfg.com/viewtopic. ... t+plus+%21

Hello,
yes AGB made extensions for both ZX81 (RAM, Sound card, RS232, UDG card...) and ZX Spectrum (Video UHF, joystick, RS232...)

thanks! This extension module is for sale on eBay right now: Extension 16ko pour ZX81 - AGB

The two 74LS258A chips multiplex the 14 bit address from the processor to the two lots of 7 bits that the DRAM need (7 bits for the row, 7 bits for the column of the memory matrix). They act as seven electronic change-over switches (the eighth will be unused).

The 74LS00 NAND gates (four gates, each with two inputs) will be for the address decoding and control logic.

The NE555 is likely being used to help produce the -5V supply rail that the DRAM chips need. One of the electrolytic capacitors, some diodes and a 5.1V zener diode are likely involved with this as well.

Mark

Hi,

Yes it's a french manufacturer:
http://zx81.ordi5.free.fr/CartesZx81/AGB/

"AGB" means "Alain Grégoire Brégeon", two brothers interested in softwares (Alain) et the hardware (Grégoire).
Alain wrote an adventure game "La maison du professeur Folibus" on ZX81, and "Le passager du temps" on CPC.

thanks moggy

Yes, I saw jeffounet's introduction post and it inspired me to look at his wonderful hardware collection.

I found the AGB module on top of Monitor document particulary intriguing. Was it a hardware monitor/debugger or a composite-monitor adapter? It lead me to search, and I found this 16K Expansion for sale on eBay

Moggy wrote: ↑Sat Nov 23, 2024 1:32 pm I think XavSnap may have some answers for this as would new member, jeffounet as it is part of his collection as seen here...

https://forum.system-cfg.com/viewtopic. ... t+plus+%21

What is A.G.B. Monitor?

jeffounet wrote: ↑Fri Nov 22, 2024 11:54 am Hello everyone,
... I am also passionate about the ZX81 (I bought the kit when it was released) and I own, today, 25 with more or less improvements "modifications".
You can see them on:
https://forum.system-cfg.com/viewtopic. ... t+plus+%21
...
Jean-François

Merci Dim_75 and Xav

Those advertisements are fascinating


And now I see what the AGB Monitor is! Some sort of ROM ... so much to explore ...

Dim_75 wrote: ↑Sat Nov 23, 2024 3:08 pm Hello,
yes AGB made extensions for both ZX81 (RAM, Sound card, RS232, UDG card...) and ZX Spectrum (Video UHF, joystick, RS232...)

XavSnap wrote: ↑Sat Nov 23, 2024 7:29 pm "AGB" means "Alain Grégoire Brégeon", two brothers interested in softwares (Alain) et the hardware (Grégoire).
Alain wrote an adventure game "La maison du professeur Folibus" on ZX81, and "Le passager du temps" on CPC.

Appreciated, Mark

Thanks for the details on the chips. This is something I didn't know about ... that you can use an oscillator to drive a DC-DC converter. I was reading an older post of yours that explains it

1024MAK wrote: ↑Sat Nov 23, 2024 6:29 pm The NE555 is likely being used to help produce the -5V supply rail that the DRAM chips need. One of the electrolytic capacitors, some diodes and a 5.1V zener diode are likely involved with this as well.