2-pin headers on IIfx MLB

[equill]

In addition to the polarized pin pair for the speaker, the IIfx's MLB has three 2-pin headers that are presumably for selection/connection into circuit of summat-or-other.

J106 and J103 are at grid Q6. On the board that I have, J103 only is jumpered. J20 at N17 is also not jumpered. No combination of IIfx, header, pins, jumpers or baisez mon cul in Google has netted me any info. about the purposes of the header blocks. Can anyone oblige, please?

de

 

[trag]

In addition to the polarized pin pair for the speaker, the IIfx's MLB has three 2-pin headers that are presumably for selection/connection into circuit of summat-or-other.
J106 and J103 are at grid Q6. On the board that I have, J103 only is jumpered. J20 at N17 is also not jumpered. No combination of IIfx, header, pins, jumpers or baisez mon cul in Google has netted me any info. about the purposes of the header blocks. Can anyone oblige, please?

I don't know what they're for either, but I did find that the machine wouldn't boot with one jumper configuration and a particular memory installation I was using. However, I didn't take notes and I didn't experiment further. I was too busy getting my SIMMs to work.

I just noted it down as a project for future exploration.

Based on that tiny snippet of info, I imagine that at least one of the jumpers controls something to do with the RAM.

 

[equill]

Thank you, trag, for your response. You may be familiar with the IIci, in which header W1 at (roughly) H7 is marked ROM SELECT. Because there is/never was a ROM SIMM for the IIci, the jumper must be in place permanently to use the soldered-in ROM. There is room to suspect that one of the headers in the IIfx does the converse, by selecting the ROM SIMM, given that there is no onboard ROM. However, that could be entirely spurious reasoning.

The IIfx that I have came in immaculate condition, as if it had been recently cleaned as well as having been carefully maintained during its active life in the mid-90s. It is just conceivable, although none of the jumpers is marked as such, that the ROM-activating jumper was misplaced during/after cleaning. Even the certainty of that, however, would not explain the functions of the other two headers.

This IIfx behaves just as a IIci without its ROM-select jumper does. No PSU activity. No other startup activity of any kind. No voltages appear at the PSU/MLB connector, and especially not a 5V TRKL. Yet the PSU fuses are intact.

de

 

[trag]

Thank you, trag, for your response. You may be familiar with the IIci, in which header W1 at (roughly) H7 is marked ROM SELECT. Because there is/never was a ROM SIMM for the IIci, the jumper must be in place permanently to use the soldered-in ROM.

Let's see. I explored that jumper when I was working with Gamba on the IIci ROM for SE/30 project. I remember in a general way what it does, but I do not remember explicitly what it is connected to, unfortunately.

In general, what that jumper does is keep the Chip Select (CE_) pins on the soldered-in ROM connected to ground.

Specifically, I think that if you remove the jumper and check the continuity, you'll find that one pin is tied fairly directly to GND--either 0 ohms or a few hundred ohms, and the other pin is connected to +5 much more weakly 500 ohms to 10K ohms.

You will also find that the side which is tied weakly to +5 is also tied directly to all the CE_ pins on the soldered-in ROM chips. I think that is ROM pin 22 (with the notch on the chip towards you, pin 1 is the bottom right-hand pin, numbering goes counter-clockwise from there).

When you install a ROM SIMM, you remove the jumper and all the Chip Enable pins on the soldered-in ROM are pulled high (or rather, no longer pulled low), which is Inactive for the CE_ signal, so all those chips go dormant.

Later machines, such as the early PCI Macs (x500, x600) use a much more elegant system. On those machine, the CE_ pins of the soldered-in ROM are routed to one of the pins on the ROM DIMM. They are also weakly tied to GND on the motherboard. So, under ordinary circumstances, the soldered-in ROMs' CE_ pins are tied to GND, held low and therefore active.

When you install the ROM DIMM, there is a circuit on the DIMM which ties the CE_ pins of the soldered-in ROM (remember those signals are routed to a pin on the DIMM) to 5V, therefore pulling the CE_ pins high and disabling the soldered-in ROM.

This IIfx behaves just as a IIci without its ROM-select jumper does. No PSU activity. No other startup activity of any kind. No voltages appear at the PSU/MLB connector, and especially not a 5V TRKL. Yet the PSU fuses are intact.de

I believe that you should get start up activity even without a ROM. Most Macs will power up and do nothing even if the CPU is removed.

However, are you aware that the IIfx requires a working battery to power on? I can't remember which one it is, or maybe it is both. But at least one of the two and maybe both need to be good batteries. If you were already aware of that I apologize, but for now I'm going to rest with that bit of advice.

 

[equill]

It is entirely possible that the headers represent test points for factory setup of the IIfx, given its then non-standard features, which guess is supported by their lack of screened-on identifications (other than header numbers). Thank you for the description of gross ROM control, but it also may be that the IIfx's MLB never included the possibility of on-board ROM. We shall see, in due course.

Yes. Replace battery or batteries is the second thing that I do with a 'new' Mac. The first is unpack it and put it on a table. I was preaching to someone else the other day about the two-battery Macs. The junction of the series pair maintains the clock, and their sum provides the startup voltage. If the batteries do not decline in voltage at the same rate it is notionally possible to get startup but no clock, or clock but no startup.

The seller of the IIfx struck me as entirely straightforward, but perhaps reliant on what the Mac was doing when put aside 10yr ago, rather than on what it did just before dispatch. I shall ask him whether it was booted recently. It is a clean (in and out) and well-preserved Mac, and given that the insides are neither molten nor rotted, the challenge of startup is mine rather than his. Some knowledge about its recent behaviour will be useful, nonetheless.

de