Epson Multi-mode Data Controller MFJ-1278B - Manual (Page 329)

MFJ-1278B MULTI-MODE PACKET RADIO PROTOCOL
RNR
The RNR frame is used when the buffer space on the receiving side is full.
REJ
Used to request retransmission of frames starting from "x". Missed frames are detected
by receiving a frame number different from that expected.
DM
Sent in response to any frame received other than a connect request (SABM) when the
TNC is disconnected. Sent in response to an SABM whenever the TNC is on the air but
can't connect to the requesting user, e.g., if the TNC is already connected to someone
else or if CONOK is OFF.
SABM
Set Asynchronous Balanced Mode - initiates a connect.
DISC
Initiates a packet disconnect sequence.
UA
Sent to acknowledge receipt of an SABM or DISC.
FRMR
Sent when an abnormal condition occurs, i.e., the control byte received is undefined or
not proper protocol at the time received.
UI
An I frame without a frame number. It is not acknowledged.