Hm. (MS-) DOS 5 only. Tricky. DOS 6.x included Interlink for such a reason.
I imagine it is possible to type in a simple Kermit program in either Debug or QBASIC which receives a file via serial port.
Other than that, I don't really know. If the laptop has got PCMCIA slots, perhaps it can boot off a PCMCIA-CompactFlash adapter.
If it was a real computer, I would just take the HDD out and use another computer to copy a few useful programs onto it.

Edit: You can look up the pipe/copy commands of DOS.
It's possible to send data to a port by using COPY. I used that to copy ASCII files to a PenMan plotter (COPY FILE.EXT COM1).
Binary copies are also possible in DOS (COPY /B ?), I heard. Not sure if receiving also works that same way (COPY COM1 FILE.EXT), sorry. 🙁
Edit: I forgot to mention. Abborting an operation can also often be done by pressing CTRL-C.

Maybe you should take a second look at that floppy disk.

Some options to transfer files:
- Via Interlnk/intersvr, as Jo22 said.
- Using laplink or norton commander.
- Using a network card.

All those methods involves using drivers and/or software that you don't have installed, but most of them (DOS 6+Interlnk, Laplink and DOS+Network card) will fit in a floppy. So your best options would be preparing a boot floppy with MS-DOS 6.2 and the software involved, and then boot with that floppy in your laptop.

Why can't you use your floppy?

(BTW, I remember that some serial transfer software allowed it to be "downloaded" or "run" from a connected computer using only DOS instructions like CTTY, COPY /b and the like... I don't remember what software allowed it or how to do that, but maybe you can find some information)

Maybe he doesn't have a floppy drive on another computer.

I'd say networking is the easiest way to go... or if that 486 hard drive is typical 2.5" IDE drive, get an IDE to USB adapter.

I think your stuck.
If a USB floppy drive for your modern PC isn't an option
Replace the laptop HDD for a CF card that can be read on your modern PC.
or IDE laptop USB reader.

Long term solutions in my order of preference but do need drivers
pcmcia network card (I always liked Xircom)
Serial/Nullmodem connection
External CD/ZIp/Jazz/etc Drive

The server part of Interlnk (Intersvr) can copy itself over a serial connection.
You could try Zip instead, it has more options, can be menu driven and can also copy itself to a second machine: https://www.sac.sk/download/comm/zip222.zip

It works fine with the onboard serial port, but not with a USB-serial adapter, it can't establish a connection.

You wouldn't happen to have Windows 3.x on the laptop, would you?

Laptops without Windows 3.x exist?

it's MS-DOS only. I was hoping for some very simple way to receive files over either LAN cable or serial rs232 port (I have usb to rs232). Maybe I could type some batch file to receive files, only need to know how. I'll have a look at some dos commands.

I use FastLynx with a USB-to-RS232 cable (Vivanco branded) connected to a nulmodem cable (simple 3-wire quick hack that I made) and the serial port on my old computer.
I'm reasonably sure that FastLynx contains an option to copy the slave client over to the old computer using just simple DOS commands (something like copy com1 sl.exe).
You might want to download the trial version and check out the options/documentation.
Edit: here is an explanation of the process:
http://minuszerodegrees.net/transfer/fastlynx … nx33_serial.htm
http://minuszerodegrees.net/transfer/fastlynx … sl_transfer.htm