The chips is still available today by the way on Ebay etc, make sure
you order the DS1216E version.
I now have the freeware TRSDOS drivers for the Dallas Semiconductor
DS1216E RTC is anyone would like them. They also include instructions
for installation into Models III and IV.
This is perhaps the only Model III/IV upgrade still available today!
The DS1216E is available from numerous sources online including Ebay
(must be the "E" version of the DS1216).
Clem