PowerMac 4400 at 50 MHz Busclock

A Hardware Odyssey

After reading the 6500/5500 and 4400 Developer Notes it looks to me, that there all based on two ASIC`s.

    1. the PowerPC to PCI Bridge and Memorycontroller called "PSX", P/N 343S1202-02.
    2. the I/O and DBDMAController called "o`Hare", P/N 343S0173-b.
The only difference is the name of the PCI Bridge ASIC. 
In the 4400 Developer Note it is called "PSX" but in the 6500/5500 Developer Note 
it is called "PSX+". maybe an Apple Marketing Gag?

 
So i soldered out the 40 MHz Xtal (Y2) and attach an external 50 MHz clock oszillator to the W40C06 (U11).
After PowerOn, the 4400 does the Bootsound and then.......nothing!
maybe a L2 Cache Problem? i put it out and........the 4400 runs without any errors, at 250 MHz!!!!!
I tested two L2 Cache Modules. An IDT7MP6071 rev.04 and an IDT7MPV6283 rev.01 without success.
Without any Logicboard Documents there is no chance. I try to get Schematics and other Docs from Motorola and Apple about the Tanzania, but itĄs all so very much highly confidential secret. Psssssssst :-). What a joke

Hey, It`s done

The black Man is the TAG-RAM !

On the IDT7MP6071 rev.04 CacheDIMM is used a IDT71216 240K CacheTAG.
It`s organized as 16K x 15 bit and had an MATCH-Time of 12 ns. 
That`s  far too slow for 50MHz Busclock. 
If you could change the TAG-RAM to a 8 ns Part, everything is fine.
Or see, if you can get a CacheDIMM with 8 ns TAG.
Maybe, the 8ns Part costs Apple a cent more.
The following statement is from an Motorola Engineer:
One final problem is the main memory (DRAM) timing. If the firmware still thinks the bus clock is 40 MHz (25 ns), it won't program enough access time (measured in clocks) at 50 MHz (20 ns). There are resistors to tell the firmware what the bus speed is, so that it can program the correct number of clocks into the PSX/PSX+ to get the required 60 ns access time. For the StarMax, this means removing R29 and installing it in the R28 location for 50 MHz operation.
I donĄt test it yet because my PM4400/250 works fine at the default Settings.

MacBench 4.0 Results

! CAUTION !
don`t blow the Mac

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