Siemens SIMATIC TIWAY 1 - Manual (Page 66)

CONFIGURING THE HOST COMPUTER PORT
UNILINK HOST ADAPTER 3–8
USER MANUAL
3.3 HOST PORT COMMUNICATIONS
The UNILINK Host Adapter provides a choice of two data link layer protocols
for ensuring data integrity through the physical link; the Byte–oriented Data
Link Control (BDLC) protocol and the Non–Intelligent Terminal Protocol
(NITP). Both protocols include link establishment, message framing, and error
checking functions. In addition, the BDLC protocol provides for link
disconnection and sequence control. The protocol and other host port
parameters are selected with dipswitches. Several factors are important in
deciding which protocol to use.
3.3.1 Choosing a Host Protocol
The NITP protocol was designed to be simple and still provide data integrity.
The major features of NITP are listed below.
D It works in a point-to-point circuit, connecting the host computer to the
adapter. NITP cannot be used in a multipoint circuit because it does not
have the ability to address individual adapters.
D Messages are transmitted in a frame that includes beginning and
ending delimiters, message character count, and a checksum.
: Character
Count
Message
Body
ECC
;
Figure 3–7 NITP Message Structure