PACKET-1200 can be monitored in the amateur VHF and UHF bands. This mode uses indirect FM (sub carrier) modulation.

To start PACKET-1200, select 1200 Bd or 600 Bd. The correct shift or center may be set in the Demodulator menu. Standard values for the center frequency and the shift are 1700 Hz and 1000 Hz.

The packet radio protocol is a derivative of the X.25 and HDLC computer network protocols. Packet radio is a synchronous system in which data is transmitted in ASCII character packets.

At the start and end of each block a control character or flag (01111110) is sent. The address can consist of up to 80 characters, but 16 or 24 character addressing is common when using direct connections or a single repeater.

The packet protocol distinguishes between three frame types: I (Information), S (Supervisory) and U (Unnumbered). Payload data transfer uses I frames or very rarely U frames. S and U frames are used for transmission control.

The data field can contain up to 256 characters. All characters and character combinations are permitted since transmission is transparent. A checksum is sent in the FCS field.

The status field contains three fields with connection status information in addition to the call-signs:

Frame types are indicated by I, S or U.

S-frame status messages, e.g., Receiver Ready (RR), Receiver Not Ready (RNR) or Reject (REJ) are displayed.

In the third field the transmit and receive sequence numbers are displayed. This number ranges from r0 to r7 or s0 to s7. The sequence number indicates to the opposite station which packets have been received free of errors.

The large volume of available radio amateur literature is recommended for further study of the packet radio system.