Hi all, looking for any m68k gurus that may be hanging around to help me figure something out.
Im looking at a HP Jet Direct card that has a 68EC000 on it, along with an ethernet controller. The ethernet controller can take over the bus to DMA packets in to/out of memory.
Ive scoped out the arbitration signals, and I feel like Im going crazy.
The datasheet says that BR/ is used to request access to the bus, and the CPU will respond by asserting BG/ once it has given up the bus.
On this board, however, the BR/ signal remains constantly high (yellow trace in the attached image) and it would appear that the BGACK/ signal is asserted in place of BR/.
Does anyone know if this was a valid means of bus arbitration with the 68k? It seems counter to everything that is in the datasheet, unless it has an error and has swapped the BG/ and BGACK/ signals, but then BGACK/ is never being asserted either...
Not 100% sure what to make of it, but there are a few tricks being played on this board so it wouldnt surprise me that they are playing a trick with bus arbitration too...
The sequence of signals are:
HOLD_ is asserted by the ethernet controller (AM79C90), this feeds into an ASIC on the board which would then appear to assert BGACK_ to the CPU which then asserts BG_ back to the ASIC which in turn asserts HLDA_ back to the ethernet controller which is now the bus master.
Appreciate any insight.
Thanks!