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Old 30 October 2013, 08:53   #5
Precious & fragile things

Join Date: Feb 2009
Location: Victoria, Australia
Posts: 1,946
Originally Posted by Spirantho View Post
Apologies for the thread necromancy, but this may be useful.

I've been trying to fix an A1200 rev 2b motherboard which doesn't play nicely with my GVP 1230 Turbo+ card, with exactly the same symptoms as above. The card is perfect in my revision 1 board.

What's happening is that the /BOSS line is being asserted (sent low) but the /BR one is being left floating. The /BR is then pulled down to /BOSS by R118 and pulled up by another 1K resistor to 5V, making the voltage of /BR 3V. Obviously this is NOT a TTL level! This makes the 68020 not at all sure if it's the only CPU on the bus, and it tries to run at 40MHZ at the same time as the 68030. Not good.
The jumper setting the OP found disabled the 68030 which is why it worked: /BOSS was set high so /BR was a clean 5V.

The rev 1 boards short /BR to /BOSS so on that board /BR is grounded and the 68020 is safely disabled.

The solution? I reckon just short the /BR and /BOSS line on the accelerator. Going to try it later but I think it'll be fine... it'll just be the same as the rev 1 I'm using. I think maybe GVP were just lazy and only grounded one of the lines to enable it, instead of both (because the pins were shorted anyway on the boards made at the time).

Hope that helps someone some day!
A level of 3V will be seen as a logical level high, going from my PIC manuals a logical level 0 is from ground to 0.8V and high is from 2.0V to Vdd or +5V so 3.0 would certainly be high. Although there is a range of ambiguity between low (0-0.8) and high (2.0 - 5.0 ), undetermined (0.8 - 2.0), exactly why schmidt triggers exist.
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