I know that @Dreamcast has done some work here so perhaps they know the answers to these questions. I've been reading through this page: http://jj1odm.sizious.com/g2bus.html My questions are: Which direction is WR and RD? e.g. when WR goes low, does that mean the Dreamcast is writing to the modem, or the modem is writing to the Dreamcast? What is IRQA used for? How is the two way communication synchronized? For example, if the Dreamcast sets WR/RD (whichever direction it is) and exposes some data on the bus how does the modem know when it's done and when it's safe to write data back? Or are WR and RD only controlled by the modem itself (e.g. checking the read buffer each cycle when not writing?) Does the modem use the 25mhz clock at all or does it run independent of the DC bus speed? Thanks!
The modem never "WRs to the Dreamcast." The modem sends an interrupt request when data is available, and the Dreamcast RDs it. The answer to all of your additional questions, and what I used when creating Moband, can be found here.
