User`s manual

Watlow Controls Communications Guide 26
Mark
Represents the transmission of data bit
logic 1 (see logic level). Usually this is the
most negative voltage value in serial
communications.
Master
The device on the bus that controls all
communications. Only the master can
initiate conversation.
Modbus
A software protocol developed by Gould
Modicon (now AEG) for process control
systems. No hardware interface is defined.
Modbus is accessed on the master/slave
principle, the protocol providing for one
master and up to 247 slaves. Only the
master can initiate a transaction. This is a
half duplex protocol.
MMI
Man to Machine Interface typically
performed in software on a personal
computer. Also called HMI.
Network
When two or more devices share
communications lines, the devices are
“networked”.
Node
A point of interconnection to a network.
Noise Immunity
The ability of communications lines to
ignore electrical noise generated in the
communications lines by nearby magnetic
and electrostatic fields.
Odd
This term is used with parity. See parity.
OSI
Open Systems Interconnection are those
which conform to specifications and
guidelines that are open to all. This allows
equipment from any manufacturer, which
claims to comply with the standard, to be
used interchangeably on the standard
network.
Parallel
Communication using this method,
transfers eight bits or one byte at a time
over eight data wires and one ground wire.
This method is eight times faster than
using serial but utilizes more hardware.
Parity
A bit is assigned at the beginning of a byte
to stand for parity. When the ‘1’ bits are
counted, the number will be even or odd.
A parity bit is used to ensure that the
answer is always even if even parity or odd
if odd parity. If the receiving end counts
the ‘1’ bits and the sum is not the same
odd or even, an error is generated. Ex.
00010000 has an odd number of 1s. In
even parity, we would set the parity to 1
so we have an even number of bits. In odd
parity, we would set the parity bit to 0 so
we have an odd number of ‘1’ bits. Parity
is used to detect errors caused by noise in
data transmission.
PC
Personal Computer, coined by IBM when
it introduced its first IBM PC Jr., which
later became PC, XT, AT, 286, 386, 486,
Pentium, Pro, MMX Pentium, and latest
Pentium II.
Peer to Peer
Two devices that can talk to each other.
Both devices can initiate communications.
This may also be called Master-to-Master
communications.
Profibus
Profibus is actually three buses. Profibus
FMS (Field Message Specification),
Profibus PA (Process Automation), and
Profibus DP (Process Periphery). FMS is a
higher level bus intended to operate with
PLCs, Pcs and higher level nodes. Profibus
DP supports three masters. The masters
then operate with field nodes as master-to-
slaves. Profibus PA is an intrinsically safe
bus. The protocol is essentially the same
as DP but the electrical / physical
specifications are modified to satisfy low
voltage and current requirements.