old hardware adaptation - 9 pin D-sub to VGA or USB - Retrocomputing Stack Exchange
9-pin D-sub-to-USB converter?
Possible to use a D-Sub to USB adapter?
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2 rows 9 pins D-SUB 9 (or Cannon 9) connectors were used on PC for serial COM ports (RS232) and in even older days also for IIRC EGA/CGA monitor connection.
My bet is that it's the serial port (you can check with multimeter if GND is on the right place - IIRC pin 5). Serial port uses male Cannon 9 connector on the PC side and female connector on the cable (or device if plugged directly) which matches your description.
Nowadays machines do not usually have RS232 any more, so in such case you need a USB to RS232 or USB 2 COM port adapter first which might be a big problem on W9x with drivers for it and also the cheap ones might not work at all with your device... there were also ISA alternatives. Any decent IDE card had RS232, GAME and LPT on-board but again having ISA on modern machine is luxury (even PCI these days) unless you have a developer machine style motherboard (these are quite expensive but have "all" the old stuff onboard).
Now if you run W9x program on Win7 it means you have 32bit OS version (on x64 you can not run 16bit apps any more) which is fine but problem is W9x drivers are not compatible with win7. And also RS232 does not have any notion of VID/PID so you have to select/install correct driver manually. NEVER USE automatic driver install for it - Windows will almost always choose wrong driver.
If your device is old and discontinued and not very common you will have most likely no chance of getting win7 compatible driver (there are online driver writing services out there just for these cases, but it's not for free. IIRC it was called driverguide or something like that; formerly it was database of old drivers so if you are in luck it might contain what you need already).
So I would try some emulator or virtual machine similar to DOSBOX to run Win9x virtually inside your Win7 (beware that MS Virtual PC does not work on Win7 correctly; it was my favourite up to XP as it was really simple small and free, but after they ported it to Win7 it's unreliable and crashes a lot). There are quite few of them out there like WMware; just google it. After that run your app inside it with the old drivers installed there too. It should work normally as on real machine if the emulator supports COM port forwarding...
If all fails, you could try to reverse the communication protocol yourself as RS232 COM port serial communication is pretty easy (just file access - you do not even need a driver). However you would need some stuff about the device yourself (ideally capture the data between old machine and device during usage) and then either create your own app for using the device or somehow inject the data into the Win9x app (similar to what DOSprint does). However you never described what the device is ...
The fact that Windows thought it was a mouse is a really good sign, and likely means that things are nearly working properly.
First of all, there's no way that Windows would mistake a USB serial port device for a USB mouse. What's happening is, it's detecting the serial port (step one complete!) and automatically running its mouse detection routines. Those work by sending certain control signals and looking for a mouse-like response. Unfortunately, there were a number of serial mouse protocols, and a random non-mouse serial device will often unintentionally send data which will fool the mouse detection. But that means you're receiving serial data from the device (step two complete!).
So the only thing currently going wrong is the mouse driver squatting on your serial port. See this SO question for how to stop that from happening. With that resolved, the host program (which is successfully installed - step three complete!) should be able to configure the port and communicate, with no need for any additional drivers or OS-level configuration.
As others have mentioned, heedlessly plugging a random DE-9 into a no-name USB-serial bridge was a really bad idea. But you got lucky. Nevertheless: That connection is not hotplug-safe. Do not plug or unplug the serial cable while the computer OR the device is turned on or sleeping, or you may actually seriously break something (possibly your computer).
I suspect the answer is "of course not," but would a standard 9-pin D-sub-to-USB converter (like this one) work as a converter for controllers with Atari-style connections? I ask because I would really like an inexpensive way to use my Colecovision Flashback controllers on my RetroPie setup, and most of the confirmed solutions I've found are either more expensive than I care for or too DIY.
EDIT: Okay, the one I linked to is a bad example as it's apparently very unreliable, but, you know, in general would this kind of thing work?
I wanna RGH1.2 my Jasper 360 and right now I'm in the phase of acquiring the materials to do so. While reading the guide that is pinned on this subreddit, I learned that you need a computer with a 25-pin D-Sub connector, and since that requires getting a whole new computer, I was wondering if I could do with a D-Sub to USB adapter.
Would the wiring be different somehow? Would the PC be able to properly read information to access the console's NAND? Any help is appreciated.
EDIT: Just learned that R-JTAG requires an outdated and hard to find kit, only to find out that for RGH you need a chip that is apparently even harder to find. Getting real confusing to mod this 360, why is it so hard?