BusWorks
®
900EN-S005 Ethernet Switch User’s Manual Ethernet I/O
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Acromag, Inc. Tel:248-624-1541 Fax:248-624-9234 Email:
http://www.acromag.com
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To better understand the operation of an Ethernet switch, you need to
differentiate it from a hub. An Ethernet hub (or repeater) is a device that
simply connects Ethernet nodes. Any message at one hub port is repeated
on all ports. That is, hubs forward data packets they receive from a single
station to all hub ports. As a result, all port devices connected to a single
hub will share the same bandwidth. Then as nodes are added to the
network hub, they compete for this finite amount of bandwidth (at 10Mbps or
100Mbps). This can cause data collisions to occur and makes network
determinism impossible, particularly on busy networks. Determinism is a
term that is used to describe the ability to guaranty that a packet is sent or
received in a finite and predictable amount of time. In the past, lack of
determinism is the main reason that Ethernet has had problems being
accepted for use in critical control applications, as most control systems
have a defined time requirement for packet transmission, typically less than
100ms.
A switch (or switching hub) is an intelligent device that is used to more
efficiently connect distributed Ethernet nodes than a hub. Unlike a simple
hub, a switch provides
targeted
data transfer, as it will forward a data packet
to a specific port or network segment, rather than all ports, thus freeing up
bandwidth. The ability to target a packet to a specific port increases network
throughput and helps to eliminate
the collisions that historically make
Ethernet non-deterministic.
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Switches act as intelligent repeaters to increase network distance.
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Switches split networks into separate collision domains at each port.
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Switches provides determinism by reducing collisions.
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Switches increases network bandwidth/throughput.
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Switches can provide supplemental error checking.
The Acromag 900EN-S005 is a five-port, Ethernet switch that combines
integrated buffer memory, five MAC’s (Media Access Controllers), five PHY’s
(Ethernet Physical Layers), and a sophisticated switch engine for small
home, office, or industrial applications. It is packaged in a rugged enclosure,
suitable for DIN-rail or panel mounting. It operates over a wide temperature
range and is designed to withstand the effects of harsh plant floor
environments. It also carries Class I, Div 2, Group A, B, C, and D ratings,
allowing it to operate in the presence of explosive gasses. The 900EN-S005
also performs additional error checking on data packets to help ensure the
integrity of forwarded data. A wide input switching regulator (isolated
flyback) provides isolated power to the circuit and the unit includes a
redundant power input, for hot standby back-up power should the main
supply fail. Each port of this switch functions just like any other Ethernet
device. It is able to receive and decode Ethernet frames, test for frame
integrity, plus assemble and transmit Ethernet frames.
With Ethernet, any device can try to send a data frame at any time. If two
devices happen to send a data frame at the same time, then a collision may
occur. The arbitration protocol for carrier transmission access of the
Ethernet network is called Carrier Sense Multiple Access with Collision
Detect (CSMA/CD). With CSMA/CD, each device will first sense whether
the line is idle and available for use. If it is, the device will begin to transmit
its first frame. If another device also tries to send a frame at the same time,
then a collision occurs and both frames are discarded. Each device then
waits a random amount of time and retries its transmission until it is
successfully sent.
HOW IT WORKS
Background - Hubs
Contrast - Switches
Introducing The Acromag
900EN-S005
