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Appendix A Glossary
Byte A group of bits, typically 8 bits, operated on as a single unit. A single ASCII
character typically occupies one byte. (See Octet).
Carrier Sense In a Local Area Network, an ongoing activity of a network node to detect
whether another node is transmitting.
Carrier Sense Multiple Access with Collision Detection (CSMA/CD) A bus network in
which the medium access control protocol requires carrier sense and in which
exception conditions caused by collision are resolved by retransmission.
Channel An association in a client PLC between the PLC application program and an
Ethernet module in that same PLC. The ladder program initiates the channel when
it issues a Communications Request (CommReq) to its local Ethernet module. In
turn, this local Ethernet module initiates a connection to a remote server and then
makes periodic data transfers between the client and server PLCs. Channel,
however, is an entirely client function; the server knows nothing of the channel. See
Channel Status Bits The Channel Status bits comprise bits 17-80 (64-bits) of the status
indication area. The first 32 bits consist of an error bit and a data transfer bit for each
of the 16 channels that can be established. The last 32 bits are reserved for future
use and set to zero by the Ethernet Interface.
Client A node that requests network services from a server. A client PLC initiates a
communications request. (See also Server.)
Collision A condition that results from concurrent transmissions by two or more nodes
on the transmission medium.
Collision Domain A single CSMA/CD network. If two or more nodes are within the
same collision domain and both transmit at the same time, a collision will occur.
Nodes separated by a repeater are within the same collision domain. Nodes
separated by a bridge are within different collision domains.
Command Dictionary Provides an alphabetical listing of the LAN Interface commands.
Command Field That part of a protocol data unit (PDU) that contains commands, as
opposed to the address field and information field.
COMMREQ Function Block The COMMREQ Function Block is the Series 90 PLC
ladder instruction used to initiate a communications request.
COMMREQ Status Word The 16-bit CRS word receives the initial status of the Series
90 PLC communication request from the Series 90 module to which the
communication request was addressed. The location of the CRS word is assigned,
for each COMMREQ function, in the common area of the COMMREQ Command
Communications Window A part of the PLC scan that provides an opportunity for the
LAN Interface to read and write PLC memory. The window is executed
automatically once per PLC scan.
Connection An association between a client and server on a network, used to reliably
transfer data between the two. Here, usually refers to a TCP or an SRTP connection,
where the client server, or both are PLCs. See also Channel.
CRS Word See COMMREQ Status Word.
CSMA/CD See Carrier Sense Multiple Access with Collision Detection.