this post was submitted on 27 Sep 2025
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This video made me wonder what is the most PCI slots ever found in a single system.
During the dialup era, the most reliable modem I ever used was the 3Com 3CP5610A, which to be fair is probably much the same hardware as in these external models (just packed onto a PCI card).
It's just an awful lot harder to cram a dozen into a single PC when it's one modem per PCI slot.
(It was a great modem because it appeared to the system as a hardware serial port, so was 100% Linux compatible without crazy driver hacks unlike most "winmodem" devices of the day)
PCIe 1x -> 4 serial port cards exist. Multilane PCIe slots can be bifurcated to the point that, in theory, you could have as many cards as the CPU had PCIe lanes. The biggest number I've seen for number of PCIe lanes in one system is 256 (in a dual-socket Epyc server), which means it could theoretically support 1024 serial modems and a connection speed of 57 Mbps.
The practical limit is gonna be a lot lower though, because the biggest actual motherboard I've found had 3 PCIe 16x slots, 2 PCIe 8x slots, 1 M.2 (x4) slot, 5 SlimSAS (U.2 x4) ports, 1 external serial port and 1 internal serial header, for a total of 88 accessible PCIe lanes and thus 162 modems. That would equate to 9 Mbps of bandwidth.
Per the video though, the problem clearly is Windows' ability to enumerate the serial devices and assign COM* port numbers to them. Linux might do a bit better there, although looking at my Ubuntu systems the hard-coded maximum serial port count is 64 (adjustable by recompiling the kernel, at least).
In any case, that is only an attempt (as they say themselves) to maintain period accuracy. If you ignore that, you'd probably have a much easier time with USB modems connected to either a bunch of hubs, or a handful of multi-port PCI(e) USB cards.
Also - and we are getting extremely off-topic here - individual Epyc CPUs allow 128 PCIe lanes, but 48 or 64 are used for the inter-socket link in a 2S system, leaving a maximum 128 or 160 available as external connectivity. A moot point anyway as PCIe switches exist, and can be found in all manner of hilarious expansion chassis that could allow hundreds of PCIe cards even on a desktop system: https://shop.bressner.de/en/products/hpc-solutions/gpu-computers/gpu-expansions/gen5-4u-pro-16-slot/
Hey! I used to work for the company that makes that expansion system, One Stop Systems. :)