[robespierre]

Does anyone know how those cables were made? I’d love to take a crack at it!

I believe these are simply 13W3 to VGA cables that use the more common of the SGI pinouts. (But I am open to correction.)

Most of the adapters and cables on eBay and so forth were made for the Sun Microsystems pinout, which is different and incompatible.

From the Hardware Developer's Handbook Release 2.0:

Code:

Indigo, Indigo2, Indy 13W3 display port:

A1:        Red (coax)                           --> VGA DE15 pin 1 (core) and 6 (shield)
A2:        Green (coax)                         --> VGA DE15 pin 2 (core) and 7 (shield)
A3:        Blue (coax)                          --> VGA DE15 pin 3 (core) and 8 (shield)
1 :        Monitor Type 3 (in)                  --> VGA DE15 pin 15 (SCLK)*
2 :        Monitor Type 0 (in)                  --> VGA DE15 pin 12 (SDATA)*
3 :        Composite Sync [CSYNC] (out)
4 :        Horizontal Sync [HDRIVE] (out)       --> VGA DE15 pin 13 (core) and 10 [SGND] (shield)
5 :        Vertical Sync [VDRIVE] (out)         --> VGA DE15 pin 14 (core) and 10 [SGND] (shield)
6 :        Monitor Type 1 (in)                  --> VGA DE15 pin 9 (+5 V)*
7 :        Monitor Type 2 (in)                  --> VGA DE15 pin 5 (DGND)*
8 :        Ground (shield for Composite Sync if used)
9 :        Ground (shield for Horizontal Sync)
10:        Ground (shield for Vertical Sync)

Cable is available with 3, 4, 5, or 6 mini-coax and additional twisted or untwisted wires.
Ideally you would use 3 mini-coax for the color signals, and 2 more for horizontal and vertical sync. But it is also possible to run sync over twisted pairs (with each pair carrying one sync signal and ground; despite the mismatch of balanced vs unbalanced, this works in practice). It depends on what cable you can get. Running sync over untwisted wires is probably not a good idea for signal integrity reasons.

* Both SGI's 13W3 and IBM PS/2's VGA started life with hard-wired monitor ID pins (the pins are either floating or pulled to ground inside the monitor). This gives a total of 16 monitor types in both systems, but the definition of the types is not compatible, so there is no point connecting them. Later on, both companies adopted VESA DDC as the scheme to identify monitors, and if you have a SGI that supports it, you can connect those signals in the cable. They are starred in the table above. VESA DDC is descended from ACCESS.bus, a short-lived competitor to ADB and USB that was used on the Personal DECstation.

Even with a cable like this, which takes horizontal and vertical sync from workstations that support it, there can be issues with some monitors. That is because unless the monitor has a "sync trap", the sync signals that are additionally output on the Green (and maybe also Red and Blue) wires will enter its video amp circuitry and disturb the color balance. The more universal solution is to use a sync-conversion box like the Extron 112xi, which strips sync from the color signals, recovers it from wherever it happens to be, and regenerates it internally.

The quoted post's warning about RealityEngine should also be heeded. Some older workstations, like the Crimson and Onyx, do not supply separate horizontal and vertical signals on their 13W3. Additionally, they put out +10 VDC on the pin that this cable connects to vertical sync, which will potentially damage the display. Your only choice for them is a box like the Extron.