Hamlib can control PTT in several ways. One way is to send a serial command to the radio to turn PTT on, and another to turn it off. The most common way is to connect a transistor to the radio's PTT line, then use a programmable I/O line on a PC port to turn the transistor on and off, which drives PTT on or off. Hamlib can use an I/O line on a printer (LPT:) port, or serial (COM:) port. In January 2012 an experimental feature was added to allow hamlib to drive PTT using a GPIO line on a popular USB sound chip.