68K Bus arbitration

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Posted by David Lie on March 01, 1998 at 11:39:44:

I'm a bit confused by how this could work. The way I see it
is:

1) When any device wants the bus it pulls BRn low.
2) The CPU returns BGn low regardless of the state of
BGACKn.
3) When BGACKn floats high, the device with the token
pulls it low and releases BRn.
4) The CPU then releases BGn.

Can multiple devices request the bus? If one device
requested it and didn't get it before another device
earlier on the chain requests it, isn't it possible
that both will get BG's and simultaneously go for the
bus when it becomes free? On the other hand, if
only one device is allowed to request at a time,
why the extra BGACK signal?

-DL

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Re: 68K Bus arbitration Michael van Dam 16:32:39 3/03/98 (0)

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Comments:
: I'm a bit confused by how this could work. The way I see it : is: : 1) When any device wants the bus it pulls BRn low. : 2) The CPU returns BGn low regardless of the state of : BGACKn. : 3) When BGACKn floats high, the device with the token : pulls it low and releases BRn. : 4) The CPU then releases BGn. : Can multiple devices request the bus? If one device : requested it and didn't get it before another device : earlier on the chain requests it, isn't it possible : that both will get BG's and simultaneously go for the : bus when it becomes free? On the other hand, if : only one device is allowed to request at a time, : why the extra BGACK signal? : -DL

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