What kind of connection is made between the modem and computer in digital amateur radio systems?

In digital amateur radio systems, the connection between the modem and computer is typically a serial connection. This is due to the efficiency and simplicity that serial communication offers for data transmission. Serial connections transmit data one bit at a time over a single channel, which makes them less complex and more reliable over varying distances compared to parallel connections that would require multiple channels.

Serial connections are well-suited for communication with modems because they can manage the data flow effectively, ensuring that the data sent and received is synchronized. This is particularly important in digital amateur radio operations where there is a need for consistent and accurate data transmission, such as when using digital modes like PSK31 or FT8.

While USB connections are also common in modern setups, they are fundamentally serial as well, but more specifically, they implement a more complex protocol that manages both data and power over a single cable. Thus, while USB can be involved in the connection, describing the connection in a basic sense aligns more closely with traditional serial communication, which is what most amateur radio operators will recognize and use in their setups.

Thus, the correct choice reflects the foundational method typically used in amateur radio digital communications, where a straightforward and reliable connection is paramount for successful operation.