Q: Can I upgrade the CPU on my Rainbow?
Yes, you can. The first and simplest upgrade is to a NEC V20 chip. The V20 is almost perfectly compatible with the 8088 except that it slightly faster. In order to successfully install one, you need to update the boot ROM on your Rainbow. The following text is taken from a text copyrighted by Carl W. Houseman in 1985 and outlines the ROM modifications:
The rom modificatons should work for versions 04.03.11A of the 100A, and 05.03A of 100B/100+ boot firmware (the first number before the "." identifies the boot rom). However, subtle differences may exist in the boot rom without an external version number change, so before removing the rom, check for the correct version as follows:
- Start DEBUG at the MS-DOS prompt
- Enter one of the following DEBUG commands, depending on your system:
100A: 100B/100+: D FA00:1FFE 1FFF D F400:3FFE 3FFF - You should see:
100A: 100B/100+: FA00:1FF0 91 B2 F400:3FF0 47 A0
(extra spaces in the display and trailing characters not shown here)
If you have the correct version, you'll need a blank 2764 (100A) or 27128 (100B/100+) eprom for the copy, plus access to the following equipment:
- A prom programmer/duplicator which allows specific prom locations to be changed for a duplicate operation
- A small flat blade screwdriver
- A chip extractor (preferred to a screwdriver for 100B/100+)
- The Pocket Service Guide (EK-PC100-PS-002) or Technical Manual Addendum (EK-RB100-TM-001)
Disassemble the system and remove the boot rom, being very careful with the socket in 100A systems as noted in the technical documentation. Set up the prom programmer to program the locations with data as indicated (all hexadecimal):
| 100A: | 100B/100+: | ||
| Location | Data | Location | Data |
| 043F | 64 | 072F | 64 |
| 067D | 20 | 0B36 | 20 |
| 1FFE | 2B | 3FFE | 1B |
| 1FFF | 70 | 3FFF | 88 |
When you replace the 8088 with a V20, replace the boot rom at the same time. The modified rom will not work with the 8088 installed.
As an addendum, for those who do not have any published hardware references, the boot ROM (or ROM 0) is the ROM closest to the front of the computer, or furthest from the Rainbow's external ports.
Suitable Solutions released a board that allowed an upgrade to a 286. The expansion board, called the Turbow-286, was hard to find and plugged into the memory expansion board's port on the motherboard.