Please note that the creator of this guide will unfortunately be unable to add to it. It is being preserved here and any future questions would be responded to by the community - Mod Team
Since the Turbografx/Duo/PC Engine series of systems can get a bit confusing, I did up a quick guide that should answer most questions. Additionally, the compatibility guide at PCEngineFX is very useful (http://pcenginefx.com/main/nec_compatibility_guide.html).
(actually that is an Arcade Card Pro in the PC Engine above)
In the US, the released systems were the:
-Turbografx-16 (RF out only. An upgrade module for AV was available)
-Turbografx-CD add-on (which includes the IFU of course, and the System 2.0 Card (called the System Card))
-Turboduo (combo cartridge/CD unit with the Super System Card 3.0 upgrade built-in)
-Turbo Express (handheld TG-16. No, you cannot attach the CD unit)
*note: TG-16 controllers are NOT compatible with the Duo or any PC Engine system. You can get a conversion cord, though. Turboduo controllers work on all PC Engine systems and vice-versa.
System BIOS Cards (they are backwards compatible):
-System Card (aka. System 2.0/2.1) -had 64K of RAM
-Super System Card upgrade (aka. System 3.0/3.1) -this added 192KB of RAM for 256KB total
In Japan:
-PC Engine (RF only)
-PC Engine Core Grafx (same thing but with AV-out and a different colour scheme)
-PC Engine Core Grafx II (same thing as the Core Grafx, but with a different colour scheme)
-PC Engine Shuttle (same thing as Core Grafx, but looks way different and has no CD-ROM connector)
-PC Engine GT (same as the Turbo Express; a handheld PC Engine)
-PC Engine LT (sort of like a GT, but with a flip-top screen like the GBA SP. It can only be powered with an AC adapter. You CAN hook it up to a CD-ROM!)
-PC Engine Super Grafx (upgraded PC Engine with more RAM and other improvements. Only 7 games took advantage of this; 5 of which ONLY work on this unit).
-PC Engine CD-ROM2 add-on (came with the IFU or sold seperately. It also came with the System Card 1.0, I believe). You need a special attachment to get this to hook up to the Super Grafx or LT.
-PC Engine Super CD-ROM2 add-on (same as above, but had the Super System Card 3.0 upgrade built-in and looks completely different than the original unit. Matches the Core Grafx II colour scheme)
-PC Engine Duo (combo cartridge/CD unit with the Super System Card 3.0 upgrade built-in)
-PC Engine Duo-R (same as the Duo, but looks different and lacks a headphone jack)
-PC Engine Duo-RX (same as the Duo-R, but has a slightly different colour scheme and comes with a 6-button controller instead of the standard 2-button)
System BIOS Cards:
-System 1.0 -had 64K of RAM. Basically useless except it is required for Altered Beast CD.
-System 2.0 -updated the system BIOS
-System 2.1 -slightly updated the system BIOS again. If you have the 2.0, you probably won't notice a difference.
-Super System 3.0 -had 256KB (1.5MEG) of RAM total. GET AT LEAST THIS!!!!!
-Arcade Card Pro -added over 2MB (17.5MEG) of RAM. Only a few later releases used this. Sapphire is the most well known. All the Neo Geo fighter ports used this as well, and blow away their SNES/Genesis counterparts. The video quality with this blows away anything on the Sega CD.
-Arcade Card Duo -added 2MB (16MEG) of RAM. Can only be used on the Duo line of systems and the Super CD-ROM2 add-on, as they already had 256KB (1.5MEG) RAM built in. The Arcade Card Pro also works on these systems, so this would be a useless card if it wasn't much cheaper than the Pro.
All CD games are region-free. Cartridge games are region locked (and Arcade CD games as a result, as the Arcade Card/cartridge is required for them). There are converters (all expensive) available to play PC Engine games on a Turbografx, but not the other way around.