Vintage computers often lack the capability to connect to other computers to include networking. Even if networking hardware and software is available, such connections are difficult and take up resources. Often it's "impossible."
Many vintage computers do have RS-232 serial ports. They are most often DB-9 male connectors on the back, although IBM PC and PC/XT used 25 pin male connectors. Vintage operating systems such as MS-DOS were configured to handle various functions using serial ports.
USB to serial cables have existed for years. They seem like an excellent way to connect a modern computer with a vintage computer. And true, it's possible to make such a connection, although getting the software on each side talking is very hit and miss.Â
This Note will document exploring using IBM PC serial ports under MS-DOS, connected to a modern Windows machine using an RS-232 to USB cable.
The USB/RS232 Cable
I wanted to check things out quickly so I went to Amazon US. My vintage computer is an IBM PC/AT running MS-DOS 6.22 with a 9 pin DB-9 male serial port at COM1.
The cable I chose is a CableCreation USB to RS232 Serial Adapter with PL2303 Chipset, 10 FT USB to DB9 Pin Female Converter Cable as it already had a female DB-9. The 10 foot (3 meter) length helped to reach between computers. The PL2303 USB chipset worked fine with Windows 10, the driver installed automatically. They sell a version with an FTDI chipset but it is $8 more.
I did get a DB-9 male to DB-9 female null modem mini-adapter just in case the RS-232 end needed transmit and receive crossed but this so far has been unnecessary.
Left is the IBM PC/AT 5170. There are two serial DB-9 ports on two RAM expansion cards. You might have to experiment if you have more than one port which is which. On the right I just used a USB3 port but USB2 or even USB1 is plenty fast.Â
Software for the Modern Computer Side
For experimenting, you'll want a terminal emulation program. I happened to have a version of PuTTY available but there are many others.
Go to the Windows Settings and go to Devices. Scroll down and find the entry for your cable. Mine is shown below and says it identifies on my modern computer as COM5.
Open your terminal emulator. Set the communications for your communications port (COM5 for me) with 9600 baud, 8 bits, 1 stop bit, no parity.Â
You may need to set the software to echo CR for LF and LF for CR.
Software for the Vintage Computer Side
If you have issues using COM1 on the vintage computer, try COM2 if it's evident you have two serial ports. An MS-DOS machine can have up to four COM ports, although that's rare.
In MS-DOS you'll need to set the port up using the following command at the DOS prompt:
MODE COM1:96,N,8,1,R
Work in progress, playing with various MS-DOS commands and options.
DIR >COM1:
Should show a directory listing of the vintage computer on the terminal emulator on the modern PC.
This is a good demonstration that the serial connection is working at least one way.
File Transfer Between Computers
- LapLink 3
- FastLynx
- PCAnywhere
- INTERSVR/INTERLNK (tbd)
Your baud rate, bits and parity are likely incorrect on one side. Did you use the mode command on the vintage PC? Did you set the terminal emulator to the same settings?
If you are using COM1 try COM2. Use the mode command mode com2:96,n,8,1 then try again. If not, then you may have a different cable than me that needs Receive and Transmit swapped. Look for a "Null Modem" adapter with the correct connectors but exhaust the options before spending the money.
It depends on the vintage computer. Later computers could do 19200 baud supported in MS-DOS but no higher, there are limits to the serial chips used in older computers. 386, 486, and higher may have higher baud rates. Definitely you cannot use 115200 or higher with very vintage equipment.
This page (Interfacing Modern PCs with USB to Vintage Computers via Serial Ports) was last updated on July 03, 2024.
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