The function I have been using to open the serial ports is
RAMSDOpen( SPortSel port ). This is function is used in most of the old serial port code that I could find. It is confusing to me that this function is referred to as the RAM serial driver, when it is in the Mac ROM. With a little tech support from 1988, we discover that the ROM did not contain the driver on early Macs. It had to be loaded from disk.
Just as confusing is that the Mac 128K did not have 128K ROMS (it had 64K ROMs). As a result, when somebody says the RAM serial driver is included in the 128K ROMs, they mean that it not included on the 128K Macintosh ROM. So, calling it the RAM serial driver makes sense from the perspective of an early Macintosh.
This had me baffled for a while because I thought my Macintosh 128K had updated 128K ROMs. I opened it up for a quick peek and it definitely has the old 64K ROMs in it. However, I noticed that it does have a 512K RAM upgrade. So, my Macintosh has 512K of RAM and 64K ROMs. There isn't anything 128K about it!
Fortunately, MPW 3.0 includes a copy of the serial driver ("SERD") resource that needs to be loaded to get RAMSDOpen to work. I dreaded having to load this in my application through obscure Pascal calls, but, it just loads automatically when included in my program's resource fork. Unfortunately, the program still doesn't work. It just does not negotiate the PPP connection properly on the Mac 128K, like it does on the Mac SE.