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TRS-80 Model 1 L2 ROM board question.

bitfixer

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Apr 6, 2011
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Hello again,
Have my model 1 almost working now, with exception of the L2 ROM board. This is a 3-chip board, version 1.2 roms. If I have the romulator z80 connected and replacing the address space 0x0000-0x2FFF, It boots and runs.
I am wondering about how the ROM select actually works for the L2 configuration. My board has all shunts connected on Z3, which seems to match what is correct for L2 ROMs.
Pins 7 and 8 of Z3 are connected together, and this goes to pin 9 on Z74, which is also connected to a wire going to pin 12 on the 74LS42 on the ROM board, which is one of the inputs to the decimal decoder which selects individual ROM chips on the board.
What I don't understand is if Z3 7 and 8 are directly connected together, that means 2 different outputs from Z21 74LS156 are connected together if all shunts are in place. These are different outputs from the 74LS156 decoder which will never have the same output value at the same time, so how does this work?
When looking with a scope at pin 9 Z74 / pin 12 74LS42, the signal doesn't look like a healthy logic signal, and has some small peaks at roughly 2.5V, some up to 5V with varying amplitudes.
I'm wondering if this is something indicative of a bad 74LS156, or possibly having all the shunts in place is not right?
Any insight welcome, can provide more details or pictures etc if useful.
Thanks.
 
These are different outputs from the 74LS156 decoder which will never have the same output value at the same time, so how does this work?

The 74ls156 is open-collector, meaning it can only pull a signal down, not drive it up. So it would technically be a legit design to tie two outputs of it together with a pull-up resistor. That would effectively make the final output the logical “AND” of the two.
 
Thanks, makes sense. That may point toward the 74LS156 itself having some problems. The output signal on those connected lines looked pretty bad, lots of activity near the middle of the logic voltage range.
 
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