Mac128
Well-known member
Sometimes you just know when a question is targeted to one person ... So, Tom, if you have any input on this I'd be grateful.
The 128K board looks virtually identical to the second generation 128K/512K hybrid. The notable exception being the resistor and I/C network near the RAM array. Presumably this is the multiplexer built-into the digital board vs. being a seperate mini-mux board added to the original 128K when updating RAM to 512K.
An aside: I always wondered why Apple took so long to offer the 512K board when the modifications to the original 128K board were so simple to do. I suppose it makes sense and is far more "elegant" to build the circuitry into the board rather than tack-on a separate circuit board.
Either way, the main difference between the 128K & 512K configuration is several resistors and an IC chip necessary to decode the extra RAM. However, present on the 128K board is a jumper only, which is removed on the 512K board. Any idea what this jumper does? Does it perhaps bypass the 512K decoding multiplexer circuitry? If so, then it would seem this hybrid board would be optimal for my dip-switch modified board to switch back & forth between RAM configurations.
I am want to just re-connect the jumper without understanding what it does at the risk of shorting something.
As long as I'm asking about decoding RAM ... REGARDING THE MAC PLUS:
I have been unable to locate a 64K 150ns RAM SIMM. Assuming I could find one or construct one, would the Mac Plus be able to use it? I know the Mac Plus can use a mere 512K RAM with just two 256K RAM SIMMs, but if you put only 128K RAM in two 64K RAM SIMMs would the Mac Plus use it without issue? I just don't want to keep pursuing this idea if it's not possible.
The 128K board looks virtually identical to the second generation 128K/512K hybrid. The notable exception being the resistor and I/C network near the RAM array. Presumably this is the multiplexer built-into the digital board vs. being a seperate mini-mux board added to the original 128K when updating RAM to 512K.
An aside: I always wondered why Apple took so long to offer the 512K board when the modifications to the original 128K board were so simple to do. I suppose it makes sense and is far more "elegant" to build the circuitry into the board rather than tack-on a separate circuit board.
Either way, the main difference between the 128K & 512K configuration is several resistors and an IC chip necessary to decode the extra RAM. However, present on the 128K board is a jumper only, which is removed on the 512K board. Any idea what this jumper does? Does it perhaps bypass the 512K decoding multiplexer circuitry? If so, then it would seem this hybrid board would be optimal for my dip-switch modified board to switch back & forth between RAM configurations.
I am want to just re-connect the jumper without understanding what it does at the risk of shorting something.
As long as I'm asking about decoding RAM ... REGARDING THE MAC PLUS:
I have been unable to locate a 64K 150ns RAM SIMM. Assuming I could find one or construct one, would the Mac Plus be able to use it? I know the Mac Plus can use a mere 512K RAM with just two 256K RAM SIMMs, but if you put only 128K RAM in two 64K RAM SIMMs would the Mac Plus use it without issue? I just don't want to keep pursuing this idea if it's not possible.