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A-2 TCP/IP Ethernet Communications for the Series 90™ PLC User's Manual
–
May 2002 GFK-1541B
A
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 collisions are resolved by retransmission.
Channel An association in a client PLC between the PLC application program and an Ethernet
Interface in that same PLC. The ladder program initiates the channel when it issues a
Communications Request (COMMREQ) to its local Ethernet Interface. In turn, this local
Ethernet Interface initiates a
connection
to a remote server and then makes periodic data
transfers between the client and server PLCs. (See also Connection.)
Channel Status Bits The Channel Status bits comprise bits 17-80 (64 bits) of the status
indication area. For SRTP Channels, these bits consist of an
error
bit and a
data transfer
bit
for each of the channels that can be established. For Modbus/TCP Channels, there is a
Connection Open bit and one reserved bit for each possible channel. Status bits for unused
channels are always set to zero.
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 (CRS 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 Block.
Communication Configuration Utility A utility used by the Windows-based PLC
programming software and the Host Communications Toolkit to configure local
communication parameters for connecting to PLCs.
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.)
Consumer In Ethernet Global Data, a device (such as a PLC) that receives a data exchange from
a Producer. (See also Producer, Exchange.)
CRS Word See COMMREQ Status Word.