well i should link to johns toy here, erm...(linking disabled?) [ http://www.damnsmalllinux.org/mini.html ] At the time there were no pico psus so what you see is a fanless itx 500mhz motherboard and a powersupply board probably 256 or 128mb ram. circa 2005? pico psu powersupplies came later. I'd built a few of these type machines, using no moving parts using them as kiosk machines. it was revolutionary at the time, an x86 machine running from flash memory, in Johns case, compact flash with an ide adapter. doms, Disk on module, were the next step. we have since moved to sata so these ide type disks are not fast bot with dsl we only need to move 100Mb of data to ramdisk. I still have a ten year old "johns toy" machine running on 64Mb, 128Mb ram called "flash" somewhere in the garage, [ http://sturl.uk/stusjohnstoy ] its still on my lan, but as I also have 6 other dsl boxes as 4tb video file servers it doesnt get much attention but it has an uptime of around 3 years, ....probably a power cut. i took to wrapping them in clingfilm/saran wrap.. [ http://sturl.uk/stustoy ], well, fireproof heatshrink really.
these itx motherboards (1ghz,sata) are currently being sold for $10 with 1GB ram, a 2gb drive will be $20 and a case and psu will be £25. I have taken to to install a new machine i just boot the disk,fdisk to delete and add one bootable partition, mkfs.ext2 /dev/sda1 , scp and unpack my dsltarbomb.tgz, grub; root (hd0,0); setup (hd0);quit and bobs youre uncle, a new machine. the tarbomb is just a backup, the knoppix file and the usual grub stuff.
it came down to eliminating moving parts such as hd platters and fans using slower cpus.
My friend prefers windows 10 and watercooling and is always fixing it.
I think John is a very quiet individual, but i love his work.
I'm retired now but in the past I have repaired systems such as you're describing and they where using DSL. They where small computers used for climate control in stores, it seems they where out of Canada while I was under contract in California to do their work. And that is how I remember DSL.