Some of the commonest failure modes for SPARCstation SLC and ELC machines involve failure of the video electronics, but nothing else. Such machines work perfectly well if run headless; disconnecting the keyboard will normally cause the ROM monitor to use serial port A as the console. (This is the port you get if you plug a normal serial cable into the funny Sun two-ports-at-once connector.) You can then park the machine in a corner somewhere and pay no attention to whatever is wrong with the video.
However, this isn't always satisfactory. When run this way, the video electronics still generate heat, and some of the failure modes generate enough noise to bother anyone who has to be near the machine. Since they are normally very quiet (no fan!), this can be a problem. Also, a few of the failure modes raise the unpleasant specter of something overheating and emitting lots of smoke, or maybe even catching fire. This document describes what I have learned about dealing with such machines.
On SLCs and ELCs, everything but the video circuitry is on a single board, so it's not unreasonable to try to run them standalone. If you look down into the machine after removing the CPU board, you will see that it plugs into a connector on a smaller board, on which are mounted the various connectors at the back of the machine, with wires leading off into the interior.
With a bit of effort, it is possible to take the machine apart far enough to get this "connector" board free. By feeding it suitable power, it is entirely feasible to run the computer completely independent of the monitor case.
Note: Everything here is based on a comparatively small sample size. I have pulled apart a dozen or so machines, some SLCs, some ELCs, and I believe the description below reflects all of them. But I have no idea if maybe there are other variants I haven't seen, though it seems unlikely differences would be drastic. And of course, I cannot promise that anything here bears any relation to any machine you may have; if you're doing something based on the information here and anything goes wrong, I disclaim all responsibility.
It all started when I acquired an ELC with fried video. The video did make noise (it sounded like high voltage arcing over) so I wanted to pull the CPU board out of the machine and run it "standalone", as it were. After eventually succeeding with that machine, I went on to others; as I mention above, I have done this to about a dozen machines now, and I have heard from three others who have had comparable success guided by (an earlier version of) this page, so I must have gotten something right.
For the purposes of this page, the ELC and SLC can be considered identical. The only differences I have found are
The CPU board has one card-edge connector to plug into the small "connector" board (and four SIMM sockets, but they're irrelevant to this discussion). Everything else connects to that board. Besides the connectors on the back of the machine (audio, keyboard/mouse, serial, SCSI, and Ethernet), it has three connectors on the other side, the side that normally is inaccessible because it faces the interior of the machine. These connectors are:
The RCA plug carries the video signal for the monitor. If you're running the machine headless, you'll be leaving this one disconnected. I've been told you can treat this as an ordinary composite video signal; I've also been told that this signal is nothing but luminance. I have no direct experience supporting either claim, but I am more inclined to believe the latter.
The six-pin connector drives four wires: two to the speaker and two to somewhere in the video electronics. (I speculate that the latter wires carry video sync signals. One correspondent reports that the points they connect to were marked GND and SYNC; another correspondent says one wire is H sync and the other V sync, presumably referred to chassis ground.) See the pinouts section for the exact pinout. I cut the wires to the video electronics, leaving those pins unconnected, and plugged the speaker in. If you don't care about the speaker, you can just leave this connector unconnected.
The ten-pin connector is power. Three pins carry ground, two carry +5V, one each carries -5V, +12V, and -12V, and the remaining two pins can be left unconnected (they appear to be sense lines, to carry +5V and -5V back to the power supply to allow compensating for connector and wire voltage drops). See the pinouts section for details.
My first thought, when I first started on this, was to extract the power supply from the ELC and power the CPU board with that. I did this, and it worked for about five minutes. Then something went dead, and that power supply has never worked since (it drives about 0.8V on the 5V supply lines). Another person I've corresponded with reported getting white smoke upon trying this. It seems that the power supply does not like running without the load presented by the video electronics. In principle this load could be simulated with suitable resistors, but I have no idea what they would be, and since they would have to dissipate some 50W, they would be large and would get rather hot. Since any commodity PC power supply can drive +5, -5, +12, and -12 over a wide range of loads without any problem, that's what I used: I cut the power connector free of the (now-dead) ELC supply, opened up a handy PC power supply, and soldered the wires to the appropriate points. (I could have done this without opening the supply if I had had a motherboard power extension harness available to cannibalize, since all the voltages are available on the motherboard connector.) Doing this requires taking care to get the wires connected to the right places; I found that the power supply had markings that identified what each wire carried, which made it easy. Since I could deduce the pinout of the connector from that, you don't need to take the power supply out of your machine unless you want to (for example to get the speaker free, something which borders on impossible to do without also getting the power supply out). Fortunately the SLC and ELC appear to use identical pinouts for the power connector.
One of the few incompatibilities I've found between the SLC and the ELC is on the power connector. The connector has a keying tab, which matches with a notch in the mating socket. The difference is that on some machines the tab and notch are narrower than on others. A power plug with a narrow tab can fit a socket with a wide notch just fine, but the converse is not true - you'd have to perform surgery on either the plug or the socket to use a wide-tab plug in a narrow-notch socket. (It's probably easier to remove part of the tab than to widen the notch, especially since the plug is more accessible.) Unless you end up mixing-and-matching parts from different machines, this won't be an issue for you, since the power plug you'll get out of your machine will perforce work with the socket from the same machine.
The speaker is marked as being 50-ohm, 0.2W. Presumably any other speaker of similar impedance would do fine. Note that the impedance probably does matter; I once hooked one up to an 8-ohm speaker and it worked for a while, but it stopped working and I think I may have fried the audio output stage on that machine.
The pinout of the six-pin connector is available in ASCII-graphics and image file formats.
The pinout of the ten-pin connector is also available in ASCII-graphics and image file formats.
I have been sent a pinout of the board-edge connector that interfaces between the CPU board and the "connector" board. However, I haven't yet got it converted into a suitable form to put up here; when I do, I'll replace this paragraph with more info.
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