In the past, with the aid of Zozo, I build a
RTC that took advantage of an unused header of the Microteam Exdos clone card. He has modified his Zozotools to work with it, but is an approach that only serves to that Exdos clone card.
Now I will try an internal RTC with the aid of Zozo and Pear. I think it will work exactly like the Meszaros' Clock card, because it will use the same Z80 ports. It would be adapted to all Enterprise computers, because I will take the signals from the internal memory expansion headers, that the 64 also have.
Only three signals must be taken from other points at the EP main-board, /IORQ, /M1 and /RESET, the last not used on Meszaros' project. This is because I am following the schematics provided by Pear, that substitutes some logic gates by a GAL and includes that /RESET signal.
The RTC chip is the same that I used with the Microteam, the Dallas DS12C887, totally compatible with the original chip mounted on the Clock card, the Motorola MC146818, but with the added bonus of being CMos, to have static Ram, crystal and a battery included on the same package. This saves a lot of circuitry, compared with the original chip.
My card, made of strip-board, will measure about 60x50 mm, little enough to fit inside of all EPs. I will put here the details.