The most interesting design feature of these emulators is that the CPU in the emulator pod of most models (at least the EM-180B, EM-188, and EM-189) is also used to run the emulator itself. This is a drawback in the case of the 6809, in which opcodes 14H and 15H send the CPU into a test loop which can only be stopped by a hard reset, but this design surely reduced the complexity and increased the features of the emulator.
The EM-180 series also had a RAM overlay option. This RAM could be put into the target address space, and optionally be made read-only. The RAM overlay came in three sizes: 8K (two 4K banks), 16K (two 8K banks), and 64K (sixteen 4K banks). The 8K and 16K let you choose the address range for each 4K bank, and enable and disable each half, while the 64K lets you individually enable and disable each 4K bank.
The EM-800 is (presumably) an EM-188 with different firmware and a different pod. Opening it up reveals that it uses the same motherboard as an EM-188, so it should work with the 8080 and 8085 pods for the EM-188 after changing out the firmware.
The EM-149 has a Z-80 in the base unit to run the emulator, and instead of the overlay RAM board, there is a board with 7K of RAM. There is no hole cut in the back for overlay RAM address switches.
The one thing that disappoints me the most with this whole series of emulators is the lack of a 6502 version. I'm sure this is because the 6502 wasn't very popular in the embedded systems world. (It probably didn't help any that the 6502 has limited stack memory.) Since I am quite convinced that the hardware in all the base units is esentially the same except for the ROM firmware, I hope to be able to make a 6502 pod and firmware someday. (Yeah, right after I win the lottery.)
I immediately claimed it, and it proved completely trustworthy, even after their old HP monster (most of the bulk was an IEEE-488 bus hard drive in a three-foot high cabinet!) bit the dust. I even found a project where I could use the overlay memory. By the time I left, they were moving away from the 6809, so I asked and was allowed to take it home, with plans to eventually use it with a Vectrex.
Having seen them on ebay a few times, I finally bought an EM-180B which had 64K overlay memory and the targetless clock module (the first I had known of it). I mostly use it with a ColecoVision. I later got another EM-180 console with no overlay RAM or pod for spare parts. Then I got another complete EM-180B with 64K overlay because it came in a way-cool carrying case.
Then I bought an EM-149 without a pod so that I could dump the ROMs, so that I could look at its design, and for spare parts. Chip dates indicate that this unit was manufactured in late 1988.
In early 2005, I got an EM-188 with 16K overlay memory and seven assorted 8080 and 8085 pods.
In mid 2006, I got another EM-189 base unit with 64K overlay memory for parts, and an EM-800 base unit with 16K overlay memory to see its design, to dump the ROMs, and for parts.