So having been bitten by the sad symptom of a dark, black screen after trying to modify Open Firmware settings (and apparently failing) on Old World Powermacs, I wanted to document ways to get the system working again. Part A is steps that can be done with only the Powermac in question, and require little or no additional equipment, but can be time consuming. Part B requires additional equipment, but may be quicker if you have the additional equipment at hand.

Part A.

If your system has built in video, like say the Powermac 7500, it may be simplest to zap the PRAM. Reboot, and regardless of what you do (or do not) see on screen, hold down command - option - P - R. (command is the left apple key, which also has a graphic like a four leaf clover, option is also known as alt). Repeat as needed, it may take several attempts to clear the PRAM completely, it chimes and restarts on each attempt when the command was done in time, and it is reputed to work best from a cold boot (power completely off).

If this got it working, at least to the point of showing video via the built in connector, you can probably troubleshoot from there on as if you are simply installing/using Mac OS.

If this did not work, the next way to achieve something similar is: unplug the motherboard PRAM (aka CMOS in PC terms) battery, unplug the power cord to the power supply. Leave it alone for some time (I recommend at least 30 minutes, but more can't hurt). When you reconnect it, it should have lost any volatile settings and also reverted to various defaults. Another supposedly even more thorough variation of the same procedure is to press the CUDA switch (small, round button, typically found between the CPU card and PCI slots) while the power is disconnected, to drain power faster and more completely from PRAM.

Generic troubleshooting like removing extra drives, PCI cards or RAM may help in extreme circumstances, particularly if you suspect SCSI termination errors.

If the screen is still black after all that, your system is most likely in a state where the Open Firmware variables input-device and output device have defaulted to ttya (the built in modem port).

Part B.

What follows is in two parts, software and hardware:

Software setup:
Requires another computer with a working OS and a terminal emulation program.
Using the example of MintPPC 9 on an Old World Powermac (or if you must use Mac OS, try Zterm):

In Unix/Linux you must have correct permissions to access things like serial ports.
The built in modem port (or modem/printer for systems with only one combined port like Powerbooks) is labeled /dev/ttyPZ0 (that's a ZERO at the end not the letter "O"!)

Here is how you add yourself to the correct group to be able to use it (along with any other ports/files that belong to the same group). Note this requires the use of sudo, so you must have done that first, or already be in /etc/sudoers, and there is already a tutorial on that.



For this addition to the group dialout to take effect, you MUST log out and log back in at least once!

You can check who has permissions to use the modem port:



On my MintPPC 9 install, it's owned by root and can also be used by group "dialout".

Check if you're in the correct group:



At this point I installed gtkterm, but any terminal emulation program that uses the serial port on any OS will do, with the correct settings: (speed 38400, 8 data bits, no parity, 1 stop bit, Xon/Xoff flow control)




Hardware Setup:
By far the simplest hardware setup is connecting from another Old World Mac. You only need 1 of those round Mini-DIN 8 printer cables pre-USB Mac printers use. Plug one end into the modem port on your terminal, the other into the sick Mac, and away you go. Connection to New World Macs can be as simple, but only if you have a serial port, such as the Griffin gPort or Gthree serial port.

Otherwise, for both Mac and PC, you need a USB to serial adapter with drivers for the host OS on that computer, plus a cable with a PC serial connector (DB-9 female) on one end, and a Mini-DIN 8 male on the other.

If you're not tired of this yet, you can then view/modify Open Firmware settings:

printenv to show what they are currently either all the variable or one particular one (printenv input-device)

setenv to modify one (setenv input-device kbd) input-device should practically always be kbd
output device is much trickier, because it varies from one model of Mac to the next!
a good place to start for reference is:
http://www.netbsd.org/ports/macppc/models.html

Another approach if Mac OS boots already is to try to find out within Mac OS what the Open Firmware path to the video is. For me the most consistently successful method to do this is to run Mac OS 9.1 and XPostFacto, which displays these variables when it runs. (IMHO more reliably/accurately than System Disk or BootVars)