(There's no video for Macintosh DA-15 to VGA DE-15 video adapters yet. Please contribute to MR and add a video now!)
These are adapters to convert the video signal from your old Macintosh to a suitable output for a more modern VGA monitor.
Both standard VGA and Macintosh video are D-subminiature connectors with 15 pins, and both are commonly called DB-15, but this is a misnomer (likely originating with the DB-25 connector used for RS-232 serial on the original IBM PC) as the "B" in the name is supposed to denote the size of the connector's outer shell.
The difference is that the Macintosh video connector has two rows of pins in a shell wide enough for them to be laid out in two rows (more correctly referred to as "DA-15") while the VGA connector uses a shell narrow enough that the pins have to be laid out in three rows (more correctly referred to as "DE-15").
Common pitfalls from that-ben:
When using those adapters, keep in mind that even if the adapter supports many older resolutions (such as 512x384) you have to first check your VGA monitor's capabilities because it might not support that old resolution. There is also a common pitfall when first trying to use those adapters: The older the Mac, the slower the pixel clock. This is NOT the horizontal/vertical frequency. It's the rate at which each individual pixel is sent to the monitor. For instance, at a resolution of 640x480, the Macintosh II onboard video sends pixels at a frequency of 30.24MHz. Most LCD VGA monitors will support 50Mhz to 75MHz (or faster) pixel clocks, but many do not support slower pixel clocks. This is why you might be staring at a black monitor that does not turn on or sync. Try another monitor. Note that, counter-intuitively, a more modern 16:9 VGA monitor might support slower pixel clock frequencies and an older, 4:3 VGA monitor might not. It's unrelated to age. I have found that all Samsung SyncMaster monitors support a very broad pixel clock frequency range and will gladly accept a very slow 30Mhz video signal from a Macintosh IIci without any glitch/issue.
See also: Interesting WIP list of monitors supporting slow video signals, 15kHz analog RGB signal monitors
Warning: DA-15 connectors are also used for PC gameports, Ethernet AUI ports, and the older Mac Classic data port. DO NOT use these adapters on those ports.
The PDF for download is a manual accompagning the adaptor I bought for my PowerMac in 1996:
The second PDF describes the pins of the Apple video connector.
I had needed some clarification for the Dips switch settings. In the pdf document the tables for the two 10p and wo 6p IBM or NEC adaptors are exactely the same. As it seems not clear if the switch order of the different adaptors are the same, I had a closer look at the 10p settings given in #1 Generic 10-DIP instructions (2/2) that at first look different. But in fact they are similar. Here the rewriting in the same format (only in basic text format):
Rewrite of the 10 Dips switch setting of Screenshot 3: #1 Generic 10-DIP instructions (2/2)
Split sync Composite Sync
VGA 640x480 2367 235
Multiscan 14' 146789 14589
Apple 12' 512x384 13467 1345
Changes: SVGA & VGA separated (SVGA different)
Thie can be downloaded with document 10DipsSettingsMR.pdf.
Must now check how settings MAC/MULTI-SYNC 15DM/DF-10p(M) are different from the others (it sets dip switch 10 that the others setting always leave to OFF).
(2.81 MiB / 2.94 MB)
IBM and NEC/MAC adaptors
(79.45 KiB / 81.36 KB)
Scan Apple2VGA.pdf rewritten
(59.91 KiB / 61.34 KB)
#1 Generic 10-DIP instructions (2/2)
(149.69 KiB / 153.28 KB)
Pins of the Apple display connector
Some adapters won't work well with some monitors, IBM in general.
Emulating this? It could probably run under: Basilisk II