[quote pid='1611' dateline='1653053036']
Hope these symptoms mean something to someone. I also have some photos of the screen if anyone thinks they'll help? Scope is on the bench, ready to go.
Cheers!
Chesh
[/quote]
Glad you haven't given up yet, photos are always a help, as they say a picture speaks a thousand words, but in case with a blank screen it won't tell us much at all.
Ironically, I have started having similar issues whit my set-up that I any trying to emulate your problems, mostly I have the MB8342 on Premium mainboards. The one I'm trying to use has the Colour board option, which for anyone who has deal with these adds a whole extra level of problems with additional connectors and sockets all potential points of trouble.
I have been able to confirm that the BN55 will boot to Monitor if the M key is held down just the same as the BN5443.
Anyway, the big difference between running the TEST ROM / BASIC and the Boot ROMs is how the Memory is used.
The TEST ROM and Basic are run from the ROMs themselves in the same way as the SRAM ROM based Microbees do.
With BN5443 or BN55 ROMs they are copied from their 8000h location to (I believe Bank 1) location 6000h. Then the ROM is switched out and Banks 0 & 1 RAM switched in. Control is now passed to the ROM image which is now located at E000h.
Any issues with the RAM and / or bank switching will corrupt the ROM image and prevent booting OK.
One of the first routines after passing the control to the ROM image in RAM is to initialize the Video driver 6545, which looks like this is not happening. Without an initialize 6545 there is no keyboard scanning and no output to the screen.
Port 50 latch IC16 is responsible for the RAM bank switching via the PAL IC26, however a problem with any of the DRAM chips could cause a multitude of issue.
Ewan asked about the jumper / board changes to the Mainboard as they are set up different between the SRAM models and the DRAM models. I assume as your system was working at one point you have the changes correctly set up for the DRAM. As Ewan said the screen RAM addressing at F000h is one of the big differences. This comes from the Main board for the SRAM models but from the Coreboard for the DRAM Coreboards. Another big difference is the !JLATCH and the RESET functions.
Hope this helps a little, I haven't given up on my experimenting to see if I can throw any extra light your current problem.
Ernest
Quote:Thanks again for all the help!
Hope these symptoms mean something to someone. I also have some photos of the screen if anyone thinks they'll help? Scope is on the bench, ready to go.
Cheers!
Chesh
[/quote]
Glad you haven't given up yet, photos are always a help, as they say a picture speaks a thousand words, but in case with a blank screen it won't tell us much at all.
Ironically, I have started having similar issues whit my set-up that I any trying to emulate your problems, mostly I have the MB8342 on Premium mainboards. The one I'm trying to use has the Colour board option, which for anyone who has deal with these adds a whole extra level of problems with additional connectors and sockets all potential points of trouble.
I have been able to confirm that the BN55 will boot to Monitor if the M key is held down just the same as the BN5443.
Anyway, the big difference between running the TEST ROM / BASIC and the Boot ROMs is how the Memory is used.
The TEST ROM and Basic are run from the ROMs themselves in the same way as the SRAM ROM based Microbees do.
With BN5443 or BN55 ROMs they are copied from their 8000h location to (I believe Bank 1) location 6000h. Then the ROM is switched out and Banks 0 & 1 RAM switched in. Control is now passed to the ROM image which is now located at E000h.
Any issues with the RAM and / or bank switching will corrupt the ROM image and prevent booting OK.
One of the first routines after passing the control to the ROM image in RAM is to initialize the Video driver 6545, which looks like this is not happening. Without an initialize 6545 there is no keyboard scanning and no output to the screen.
Port 50 latch IC16 is responsible for the RAM bank switching via the PAL IC26, however a problem with any of the DRAM chips could cause a multitude of issue.
Ewan asked about the jumper / board changes to the Mainboard as they are set up different between the SRAM models and the DRAM models. I assume as your system was working at one point you have the changes correctly set up for the DRAM. As Ewan said the screen RAM addressing at F000h is one of the big differences. This comes from the Main board for the SRAM models but from the Coreboard for the DRAM Coreboards. Another big difference is the !JLATCH and the RESET functions.
Hope this helps a little, I haven't given up on my experimenting to see if I can throw any extra light your current problem.
Ernest
