Hi Neil
The S-100 Utility board design is intended to default to "slot 0" which is the 32KB ROM and 32KB RAM. I take it you cannot see the 32KB ROM with the CPU without code to configure the 8255? If so, then there is a "chicken and the egg" problem. It is the only ROM in an MSX system other than those plugged into the cartridge port.
Is the 8255 port A coming up after reset as an input port? If so, it may be possible to add pull-up/pull-down resistors to force a configuration on the port pins even if the 8255 is not initialized. Unfortunately I cannot see the schematic at the moment so I will have to take a look later today and see if I can think of any ideas.
If you could run code on CPU from a ROM how would you configure 8255 port A? As an output and with what values on the pins. If port A is an output with 0x00 or 0xFF on it then all we need is a SIP for pull-up/pull-down resistors.
Thanks and good luck! I appreciate you taking this project on. I know it is frustrating!
Andrew Lynch
From: nbreeden <nb...@me.com>
To: n8vem...@googlegroups.com
Sent: Monday, January 20, 2014 10:30 PM
Subject: Re: [N8VEM-S100:2272] Re: Jumpers on Z-80 CPU Card
I'm continuing to have issues trying to access the 32K ROM / RAM on the MSX2 Utility board. There seems to be a pretty basic issue with the design. The 8255 A port need to be configured to enable slot 0 where the EPROM and RAM are mapped. To configure the 8255 I need to be able to run code in the EPROM; I can't however access the EPROM because the 8255 isn't configured.
Any suggestions?
-Neil
Interesting request Neil. Never tried that one. What exactly are you trying to do. Do you want to boot from an external ROM at 0H or somewhere else.
There probably is a way to fool the Z80 to boot externally. Remember U18 forces NOP's on the bus (up to the required boot address). I think if you remove it (and the ROM itself) you may end up booting from 0H. However never tried that. Let's know how it works out.
John
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