The EM-180 has a 256 byte PROM at U216. If it is missing, what can you do? When someone told me their EM-180 was missing U216, I opened one of mine to check it. But I didn't find a chip there, I found a daughterboard. So I reverse-engineered it.

First, what does the chip do? If you look at the EM-180 schematic (fortunately we have a scan of the service manual), the eight address inputs are connected to the data output of the UART, and the four outputs are connected to the RESET/RUN/RUN BKPT/STEP keys on the front panel. The two enable inputs are connected to a "remote disable" signal, and a "data ready" output from the UART. Clearly, this is taking specific input bytes to the UART, and making them effecitvely press buttons on the front panel! The "remote disable" signal disables it until remote control mode is enabled, and the other enable makes it only work when the UART just received a byte. So the first thing you can do is... ignore it if you don't need to use the remote control mode, since you probably don't have software for it anyhow.

But why is it on a daughterboard? And why is the daughterboard needed? The PROM chip on my daughterboard is a different part number than the schematic (82S126 vs a Harris 7610), but it seems that these are equivalent parts. There is an LM339 comparator, which seems to be used as a buffer to ensure that a good signal reaches the key inputs. There is a voltage divider to provide a 2.5V reference vs the PROM outputs. The R/C parts on the LM339 inputs probably guarantee a proper waveform, perhaps with a minimum pulse width.

Another reason for using the LM339 would be that it has an open-collector output. When you're pushing keys that something else is scanning, you really don't want two outputs driving at the same time.

Now for the wierd stuff. First of all, CE2 is not used. I guess they decided that the R/C parts gave more reliable timing than the UART data ready. Second, the PROM output bits on the daughterboard are wired backwards! There seems to be no good reason for this, but if you want to make a clone of the board, you need to pay attention to which output is which. I think a GAL16V8 would be a much better part to use than a PROM, even though it has four more pins.


This is the output of the PROM, as seen from the motherboard U216 socket.
Note that the high bit is ignored, to avoid issues with parity, etc.

 INPUT   pin 9  pin 10  pin 11  pin 12   key being pressed

2C, AC     1       1       1       0      RESET

2D, AD     1       0       1       1      RUN

2E, AE     1       1       0       1      RUN BKPT

2F, AF     0       1       1       1      STEP

 all       1       1       1       1      (none)
others


The part number of the daughterboard is 170-11190-00A, and the part number of the PROM chip is 340-11202-00.