5150: Motherboard Dip Switch Settings

MAIN 5150 REFERENCE

Dual inline package switch (or “DIP switch”) are small “switches” (like light switches on a wall) to control the flow of electricity. In binary computing, typically the presence of an electrical flow is interpreted as “1” and the lack of flow is interpreted as “0.” For switches that are used often, one would prefer large “lever switches” that are easy to flip (like in the front panel of the famous Altair 8800), but take up a lot of physical space. But for seldomly changed options, DIP switches are essentially the smallest switches that can still be interacted with by human hands (albeit with a toothpick or an awl tool).

The IBM PC 5150 has two groups of these dip switches, located between the power supply and expansion slots, behind the disk drives. These set things like the NUMBER OF DISK DRIVES, DEFAULT TYPE OF VIDEO (MDA vs CGA), and AMOUNT OF MAIN MEMORY.

The specific arrangement and corresponding meaning of the switches are explained in the IBM (5150) Technical Reference manual:

Here is my attempt to summarize this information:

SWITCH 1 (the more “central” group of switches)

SWITCH INDEX…Definition
1ON = no disk drives
OFF = 1 or more disk drives (see DIP 7-8 on this same switch group)
2(reserved for processor slot)
3-4Amount of main memory on system board
5-6Type of monitor being used (or the one to default to using on startup)…
1 1 (no monitor)
0 1 (default to 40×25 television or monitor)
1 0 (default t o 80×25 monitor resoluition
0 0 (show both types of adapter)
7-8If DIP 1 = 0, then DIP 7-8 should also be 0 (no disk drives)
If DIP 1 = 1, then DIP 7-8 should be…

1 1 (1 drive) (means 7 ON, 8 ON)
0 1 (2 drives) (means 7 OFF, 8 ON)
1 0 (3 drives) (means 1 ON, 8 OFF)
1 1 (4 drives) (means 1 ON, 8 ON)

The SWITCH 2 DIP switches is a little harder to explain, as it depends on Model A (64KB) vs Model B (256KB) type of 5150 motherboard. You’d have to hunt through different revisions of the IBM manuals to determine this. So, an existing online resource for this information is available here:

http://minuszerodegrees.net/5150/misc/5150_motherboard_switch_settings.htm

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