So I have a very, very old DOS machine with an RS232 port on it and I want to be able to transfer files to it from a Windows 7 machine. For this i bought an RS232 to USB cable.
What sort of software do I need for the devices to see each other, once i get the cable plugged in?
What you're asking for is not exactly straight forward. Even when connecting two computers with both ends being USB it typically requires a bridge component to facilitate file transfer. [1]
Likely, you would need some kind of terminal program or something on the DOS machine to tell it to push data out through the RS232 serial cable and another program on the Win7 side to receive it.
You would likely have a much easier time plugging your DOS machine in with ethernet and doing a simple file transfer. (Assuming it has an ethernet port, though I haven't seen too many DOS machines without one)
MS-DOS 3.1 was the first DOS version to support networking and file sharing. To share files you would use the SHARE program.
My goto application for this sort of activity was kermit. Other alternatives are xmodem or ymodem. Most terminal programs should support one or more of these.
kermit is quite scriptable and works over 7bit and 8bit connections.
You might try PuTTY, which can connect via a serial connection: http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html
