Maxintosh II What is this resistor for?

[reallyrandy]

I have a Mac II with an upgraded PMMU and 20MBs of RAM. I pulled it out of my display cabinet to replace the soldered on batteries with a battery holder and noticed a resistor soldered in the J2 position. Does anyone know what that's for? Here's some pics:

 

[David Cook]

That is R8 100K which pulls RXD+ high (to 5V) on the modem port. This would provide a default signal to prevent electrical noise from being interpreted as incoming data. The modem port is serviced by an interrupt (unlike the printer port in early Macs), and thus false incoming data could degrade the performance of the Mac.

This bodge resistor does not exist on later revisions of the motherboard.

 

[Kai Robinson]

Is that a nubus pal board? If so...

 

[reallyrandy]

Is that a nubus pal board? If so...

A PAL board? I dunno. You mean like PAL or NTSC?
I do know it's a rev A board that has the PMMU upgrade. I bought from a fellow 68ker in 2018. It's been in a display cabinet since then. I just started restoring all my vintage Macs (recapping, BlueSCSIs, retrobriting, floppy servicing, etc.) and discovered these mods on this Mac II. Any info you can glean from the pics is great. (I already replaced the soldered batteries with an Apple battery holder.)

 

[reallyrandy]

That is R8 100K which pulls RXD+ high (to 5V) on the modem port. This would provide a default signal to prevent electrical noise from being interpreted as incoming data. The modem port is serviced by an interrupt (unlike the printer port in early Macs), and thus false incoming data could degrade the performance of the Mac.

This bodge resistor does not exist on later revisions of the motherboard.

Thanks David, I guess since I don't plan on using the modem I can just leave as is.

 

[Kai Robinson]

As in it uses PAL's for the Nubus logic rather than dedicated ASICs - meaning, it can be potentially reverse engineered

 

[reallyrandy]

As in it uses PAL's for the Nubus logic rather than dedicated ASICs - meaning, it can be potentially reverse engineered

Ah, OK that makes sense. I really don't know though. Can you tell from the pics?

 

[Kai Robinson]

Ah, OK that makes sense. I really don't know though. Can you tell from the pics?

It is, i can see a sea of PAL's under the NUBUS slots.

 

[David Cook]

Thanks David, I guess since I don't plan on using the modem I can just leave as is.

Just to be clear, the modem port will work fine. That resistor is part of the board as shipped by Apple. So, don't remove it.