DaveLecky wrote: ↑Thu Aug 15, 2019 3:57 pm
Also the DFS chip above the board, is this correct for a bbc b or is it for a bbc b+ .
Originally a BBC model B would have had an i8271 and associated DFS. This was an old chip even when the upgrade was first released, dating from the 8" era, and soon became expensive. Various independent disc upgrade manufacturers made these daughter boards using the WD1770 chip and released an accompanying DFS ROM. Even with the expense of the daughter board these were cheaper. As far as the DFS itself working it is more a case of the DFS matching the daughter board concerned because not every board had the various registers at the same address.[1]
Acorn eventually joined the party and used a WD1770 in the B+ and also released their own daughter board with WD1770 for the BBC B, though I am not sure in exactly what order these were released. Acorn included their own 1770 DFS and some other daughter board manufacturers then came into line and used the Acorn addressing scheme for the registers in an updated version of their board so some of these later ones will work with the Acorn 1770 DFS.
As far as I know the only case where it matters if the machine is a B+ is where the B+ is a 128K machine, i.e. with sideways RAM. The 1770 DFS can have some sideways RAM utils tacked on the end. You can see that here in the HELP message from an emulated B+:
see "SRAM 1.05". It looks like a separate entry but shares the ROM with the 1770 DFS. These sideways RAM utils will expect the B+ sideways RAM addressing scheme rather than something DIY.
[1] In a little more detail the WD1770 integrates a slighly different set of functions than the i8271. The i8271 has drive selection logic built in but the data seperator was external. The WD1770 has the data separator built in but drive selection is external. That means on the daughter board there has to be an extra latch to do the drive selection and in most cases also to control the density select pin on the WD1770. Some boards had the WD1770 at the lower addresses and the latch at the higher addresses and some had them the other way round. Then the assignment of the bits in the latch also vary. This is also discussed in much more detail in the thread about emulating various DFSes in the emulators forum. B-Em emulates Acorn, Watford, Opus and Solidisk but these are others too.
