Which oscillator in this system is driving the bus and can I change out the can?

[chjmartin2]

There are 3 cans on the CPU board and two cans on the busboard. 18.432 and 16 on the Busboard. We then have 24, 14.31818, 4.7727266. It is a Vendex 888-XT.

I am trying to see what can controls the bus and if I can try to switch it out. (I know, I know, all kinds of weird stuff can happen with the timer, etc.) Anyway, from the computer servicing manual:



Side note: It looks like I could trick out the 8 Mhz turbo to a higher rate by clocking up the 24 Mhz crystal if it isn't attached to anything else.

Anyway, seems like the bus clock is run by the 4.77 crystal. So if I ran that at 7.2 (or dare I say 8 or 10) then I might be able to up the performance a bit. Also, any insight on what I break if I up the 24 Mhz, looks like that is a 1/3X for the 8 MHz operation. Bet I could put a 30 in there and try to get to 10 MHz. I'm driving an Inboard so wondering also where that would max out although my guess is that is more tied to the bus clock than the CPU clock. Very odd that bus can be 4.77 and CPU be 8. What am I missing?

Thanks,

Chris

 

[chjmartin2]

And talking to myself again for anybody in the future... I swapped out the 4.77 Mhz for a 10 Mhz... it booted but wasn't stable. I went down to an 8 Mhz and booted. Oddly I got a SLOWER system as a result. I'm using an Inboard/386. So, whatever the 4.77 MHz is doing, it isn't going to help to oscillate that faster. I'll keep experimenting.

 

[chjmartin2]

And ok... I swapped out the others as well and it made no difference to the operation of the machine with the inboard/386 installed. I didn't try it with a regular 8088 processor. Well, I didn't swap out any on the Bus board, just on the CPU board.