The SGI IRIS Indigo

 General Information

History of the IRIS Indigo

The Indigo was the first small SGI system that could stand on a desktop without problems. The first versions had a 33MHz R3000 CPU (see my Indigo R3K). The later Indigos had a R4000 processor with 100MHz or even a R4400 processor with 150MHz (see my Indigo R4K). Unfortunately you cannot upgrade an R3000 Indigo to R4000 machine by simply changing the CPU module - you have to change the complete mainboard plus the power supply.

CPUs and Mainboards

Basically there were two types of mainboards and three different CPUs

Mainboard CPU Memory
IP12 (HP1) R3000A at 33MHz 8MB - 96MB proprietary modules
IP20 (HP2) R4000SC at 100MHz 8MB - 384MB PS/2 modules
IP20 (HP2) R4400SC at 150MHz 8MB - 384MB PS/2 modules

As you can see in the table above, the older mainboards (IP12) only supported one type of processor which was running at 33MHz and soldered directly onto the mainboard. The newer mainboards (IP20) had faster processors which were soldered onto a module that could be exchanged. So it is possible to upgrade a 100MHz Indigo to a 150MHz Indigo by only replacing the CPU module.


The memory modules used in the early IP12-Indigos with a R3000 CPU is proprietary and hardly to find and rather expensive. These memory modules were custom designed by Silicon Graphics and were second sourced by some third party memory manufacturers. These modules were easy to recognize since there was a custom chip on the back side of the module. They were used in both the R3000 based Personal IRISs and the R3000 based Indigos. They were available in
capacities of 2, 4 and 8 MBytes. They would typically have a sticker on the backside denoting the size of the module. The Indigo has 12 memory slots and they have to be filled in groups of 4.

The newer IP20-Indigos with R4x00 CPU use standard 72pin PS/2 FastPage SIMMs with ECC - this type of memory could also be found in many older PentiumPro based servers and some other workstations. But note that you need 36 Bit memory modules. You can use SIMMs up to 32MB per module. Again the Indigo has 12 memory slots and they have to be filled in groups of 4.

Graphic Options

There are different graphic options available for the IRIS Indigo. The "entry" option is a simple 8bit graphics card with no hardware acceleration and an ordinary VGA connector - no 13W3 connector is present. All other graphic options are based on the same graphic board using the "express" graphic architecture. The various options only differ in memory, z-buffer and number of geometry processors.

Name Framebuffer Z-Buffer GEs
XS-8 1 module (8 bit colour) no 1
XS-24 3 modules (24 bit colour) no 1
XS-24Z 3 modules (24 bit colour) yes 1
XZ 3 modules (24 bit colour) yes 2
Elan 3 modules (24 bit colour) yes 4

The Systembus

The SGI Iris Indigo has a backplane where the mainboard, the graphcis boards and the SCSI-Devices are plugged in. The mainboard and graphics board use a special bus to communicate to each other and to the SCSI devices.

Additionally the mainboard has two GIO32 slots (respectively two GIO32-bis slots on the R4x00 Indigos) for extensions. The GIO32(-bis) bus offers a bandwidth up to 100MB per second.

Since the two GIO slots on these systems are both attached to the motherboard, or CPU board, it is possible to design a board that takes up both slot spaces. GIO slots have a fixed address space. Slot 0 always occupies a particular address space while Slot 1 occupies a different address space. This is unlike VME where settings on the board itself determine the address the board responds to.


Although the keyboard connector of an Indigo just looks like a standard PS/2 connector it does need a special Indigo keyboard and mouse - see below for further details. The sound input/output connectors of an Indigo are straightforward and the 10MBit ethernet connector is of the older type.

Indigos with the entry graphics option do have a standard VGA connector whereas all other graphic options do have - as most SGI systems - a 13W3 monitor connector and use "sync-on-green" - although most modern PC Monitors support this method, you should assure that it really does by reading the manual before plugging it to an Indigo.

Indigo Keyboard and mouse

An annoying problem is that Indigos don't have PS/2 compatible keyobards and mice, although the connector looks like it. If you don't have a keyboard or mouse, maybe you can build yourself a converter. See this page for a description.

Video Options

  • Indigo Video
    Video input/output option for systems with Entry Graphics. This board is capable of digitizing video in both 8 and 24 bit formats.
  • Galileo Video
    Video option for systems with Express graphics on the Indigo R4000. The Galileo board supports both compressed (full screen) and uncompressed (a 640x480 window) video output. Video input does not allow real time capture, but can be used to capture single frames or a sequence of single frames.
  • Video Creator
    VideoCreator is available as an optional external or internal board for video output in either compressed or uncompressed format (SCSI or VME).
  • Cosmo Compress
    for use with Galileo Video and offers JPEG compression in hardware
  • Chyron Centaur
    Broadcast quality video option made by Chyron. Supports RGB In-/Output and VLAN.

My IRIS Indigos

I own two of these machines. One Indigo R4k and one Indigo R3k. Plus I owned a Siemens relabeld machine, see here.