I am searching for information and experience about using the function ROM socket in a CBM REU with a C-128. I own a 1750. I soldered in a 28-pin chip socket at the bare spot on the board and have been experimenting with it for function ROM use with my C-128D. So far I can’t get a function ROM to work in the REU socket. The same ROM works in the internal socket. (I made sure to disable the internal ROM before trying the external.)
Here are the notes I have on the jumpers inside the REU:
J1 = how much RAM / how dense. I believe it’s cut for 1700/1750 and connected for 1764.
J2 = How big of a function ROM for socket—connect one direction for 16K, the other for 32K
J3 – J5 = ROMH, ROML, and ROMHL. I think this indicates where in the memory map the ROM will appear:
16k ROM at $8000 thru $BFFF (ROML)
16k ROM at $C000 thru $FFFF (ROMH)
32k ROM at $8000 thru $FFFF (ROMHL)
The function ROM I want to use is 32K. Using the info above, I added a solder bridge on J2 for the 32K direction and added a solder bridge for J5. Depending on which function ROM I use, it either locks up or does not start properly.
Has anyone else used the external function ROM socket in the REU successfully? Any info would be most appreciated.
Hi again--it turns out that the info I posted above was correct, at least for the version of the REU that I have, with the rectangular logic chip (vs. the square chip). The issue was that the function ROMs I was using to test were coded to work only in the internal socket. I obtained an image of the Servant function ROM and it worked in both locations. Hat's off to the creator for including code in the startup routine to check which socket the ROM was in and acting accordingly.
Speaking of function ROMs, does anyone reading this know the history of the GEOS 128 v2.0 function ROM images that are floating around the Interwebz and also occasionally available on an EPROM chip from a few eBay sellers? There seem to be two versions of this image, one with a 1571 drive as default and one with a 1581 drive as default, in English and possibly German, for a total of four images. I am curious whether it was a commercial effort from Berkeley or elsewhere or if it's considered to be a hacker effort.
The reason I ask is that a couple of friends and I verified that these images will work only in the internal function ROM socket. Fortunately, a one-byte change creates an image that will work only in the external function ROM socket. If the person who wrote this code is active in the retro community, I thought it might be worth contacting him or her. It's possible that s/he doesn't know that there is an external function ROM socket configuration...
Similarly, the the BASIC 8 v1 and BASIC 8 v2 function ROM images that I have will only work in the internal socket. I haven't checked into what's necessary to make them work in the external function ROM socket.
