The 3964R protocol (also referred to as DUST) provides the link layer
functionalities, so it is not a fully functional protocol in a proper sense.
It supports transparent transmission of data blocks on point-to-point
connections, operating in half-duplex mode.
The physical layer operates in asynchronous standard mode (1 start bit, 8
data bits, 1 parity and 1 stop bit) using the ISO 7 bit character code for
control commands (such as DLE, STX, ETX).
There are two variants::
only parity is used to ensure data integrity
in addition, horizontal parity is used to ensure data integrity
The protocol has been very popular in process automation, in particular for
data exchange between devices with Simatic-SPS. 3964R is also used as link
layer for a Sinaut 8FW variant.