HLDA should be deasserted, that puts the processor in a DMA mode where it releases all the control signals.
I/O W is asserted low when OUT is high and the low WR pulse comes along. It's both status and timing, coming out of the 8228. You would probably do well seeing if MEMR and MEMW are asserting. If the program is running off due to e.g. bad RAM, ROM bitrot, etc. you may never see the I/O lines assert. Thanks, Jonathan ------- Original Message ------- On Sunday, July 9th, 2023 at 12:12, Rob Jarratt via cctalk <cctalk@classiccmp.org> wrote: > > > Actually I have just found a more detailed description in the Intel 8080 > Microcomputer Systems Users Manual 1975, but it still doesn't tell me > exactly how it works. It isn't completely clear to me if it needs HLDA to be > asserted for I/O W to be asserted. > > > -----Original Message----- > > From: Rob Jarratt via cctalk cctalk@classiccmp.org > > Sent: Sunday, July 9, 2023 4:39 PM > > To: General Discussion: On-Topic and Off-Topic Posts > > cctalk@classiccmp.org > > Cc: Rob Jarratt robert.jarr...@ntlworld.com > > Subject: [cctalk] VT100: Datasheet for Intel 8228 > > > > Hello, > > > > I have a non-functioning VT100. I think I may have isolated the problem to > > an > > > Intel 8228 chip (or 88228, the schematic says 8228, the part is marked > > 88228C). Certainly, the part gets a bit hot and it doesn't seem to be > > outputting anything on the I/O W pin (pin 27) despite activity on STSTB > > (pin > > > 1), DBIN (pin 4) and WR (Pin 3). There is no activity on the HLDA input > > though, but I am not sure if that is required because I think the firmware > > is > > > just trying to send its status to the keyboard LEDs. > > > > I can find a brief datasheet for the 8228 but it doesn't tell me the logic > > for > > > producing the I/O W signal, so I am not sure if it is behaving as it > > should. > > > Does anyone have more comprehensive information on how the 8228 is > > supposed to work? > > > > I have dumped the ROMs and been able to capture the ROM reads and they > > match the disassembled code, so I think the 8080 CPU itself is working. > > > > Thanks > > > > Rob
