The least expensive way to get your C64 online is by building what I call a “Strikelink”, which is basically just a null modem that converts TTL signals to rs232 and vice versa. The parts listed below will allow you to hook your c64 to any usb port on a pc. Software on the PC side makes the “Strikelink” act as a virtual hayes modem at 2400 baud!
You’ll need a ch340g -> Serial/USB Device. You can find them on ebay for a few bucks. For 9600 baud, you’ll need to solder directly to the chip’s rts and cts lines, or find a ch340g -> usb with direct rts/cts out pins.
Connect the TxD pin on the TTL Adapter to pins B and C on the User Port plug (RxD). Connect the RxD pin on the TTL adapter to pin M on the User Port plug (TxD). And connect the GND pin on the TTL adapter to pin N (Signal Ground) on the User Port plug. Install the driver, load up Leif Bloomquist’s BBS Server, and now you have a 2400 baud modem for $15 or less.
To add full 9600 baud capability
Add a wire from M to 5 on user port plug
Add a wire from B,C to 7 on user port plug
Attached a wire from L to 6 on the user port plug
Add the rts/cts lines to the ch340g from the user port (user port rts -> ch340g cts and user port cts -> ch340g rts)
Then use the UP9600 driver on Striketerm or CCGMS 2017
Use the instructions in the Connecting to C64 BBSes section on the left and look for the heading Using StrikeLink or other TTL/Serial Adapter/swiftlink/turbo232 (to use your PC as a virtual hayes modem)
Refer to http://www.hardwarebook.info/C64_RS232_User_Port for a diagram of the C64 User Port