Automation Communication Hardware

These are hardware terms used in PLC and Scada Engineering.

Fiber Optic:
Unlike conventional wire where signals are voltage or current values, in fibre the signals are light conditions, so a logic 1 is represented by the light being On. In general there are two types of fibre – Multimode and Singlemode.
Multimode is the thicker of the two – 62.5microns inner core with an outer core diameter of 125.
Singlemode – 5-10microns inner core and outer core again 125 micron.
The Singlemode fibre is the more expensive, however when selecting fibre you need to consider the technical differences between the two types rather than their cost before you choose.
Singlemode allows light to travel straight down its centre, it also allows for larger signal bandwidths up into the GHertz region. Multimode on the other hand, bounces light along the fibre, this is due to its increased thickness. Reflecting the light off its inner wall down its whole length allows for greater distances being covered than that of Singlemode, however the drawback of this reflecting light is a reduced bandwidth and slower signal frequencies.
The light transmitter can be either a LED or a Laser, the Laser being capable of much larger distances than that of a LED. At the receiving end there is a Light Sensitive Semiconductor which conducts current when the light hits its surface.
One of the advantages of using Fibre is that it is immune to electromagnetic interference, which is a great problem in industrial environments, so transmitting office data around a factory where heavy motors are operating can cause problems to standard electrical data transmissions.
However – while it is immune to EMI, it is effected by vibration and bends in the cable, therefore when running the cable you need to take into consideration not to lay the cable next to heavy machinery, and when storing excess lengths, you should coil the fibre cable in a large loop rather than pushing it all into a confined space. A tight bend in the cable will result in light intensity reduction, so slow and easy bends are advisable.

A network that connects individual LANs that has a higher capacity than the LANs being connected.

Impedance matching device allows coupling between coax and twisted pair.

Category cable
The higher the number ref of the cable (eg Cat5) the higher the speed it can cope with.

Ethernet Switch
This directs a packet to its destination port, whereas a standard repeating Hub will repeat the packet to all ports on the system. The destination address is read from the header of an Ethernet packet. This provides a more efficient method of transmitting data.

Client Server
This is the common way to describe a network setup. The Client is usually the users computer whose software will make requests for information from the server, located somewhere on the network.

This is the result of more than one computer trying to access the network at the same time, Ethernet uses CSMA/CD (Collision Sense. Multiple Access / Collision Detection) to detect this by allowing a computer to send a preamble onto the network and then listening to see if it is quiet. Once it is quiet the computer will begin transmission.

OSI model
This method uses 7 layers to make up how data is transmitted, layer2 is the second final layer which is the Data Link, this handles network access – detecting errors as well as network control, any errors will result in the Data Link re-sending the packet. The IP address is another layer that hold addressing information and some control information that makes up the packet.

Domain Name Server, this provides the IP address for each machine on the network, the administrator sets up the addresses in the server and attaches these to each machines name, when a machine is added to the network it gathers its address from this server. Some networks do not use DNS, which means whoever connects a PC to the network has to also setup the IP address themselves.
File Server
A computer on the network the provides mass disk storage.
Two types – Dumb Hub repeats data across all ports, whereas an Intelligent Hub can use SNMP support.

Multimode fibre
Fibre optic cable with diameter of 62.5 microns, allows transmission up to 2Km. Singlemode fibre is 8microns and can transmit over 2Km

Token Ring
A network where a computer has the right to transmit only if it holds the token, it then attaches the data and passes the token to the next computer in line. Not necessarily a ring, the topology could be a bus or star network.

Hardware address
The physical address, the data link address associated with the device. The address of the NIC Network Interface Card (MAC address), which can be assigned by the manufacturer or by local administration ie Computer name.
Note: NIC can be set for what is called ‘promiscuous mode’ which allows all cards to receive the same transmission, irrespective if where they were initially intended, NOT a good idea!

A Segment refers to the number of nodes allowable under the current LAN, when the maximum number of nodes has been reached a new segment is created, a multi port repeater is used to expand these segments. A 10Base2 segment would normally be 180mtrs and 10Base5 maximum length 500mtrs.

Similar to the PC Bios (Basic Input Output System) but the IBM standard for extending the PC I/O onto a network.

Network layer
Layer 3 of the OSI, plans the routing of packets, it is responsible for addressing and delivery transmissions to the destination

Peer to Peer
A network where no one computer has control over another.

Physical Layer
Layer 1 of the OSI model, the actual connection on the network of the computer. Sometimes referred to as the hardware layer.

Packet Internet Groper – a program that is used to determine if a remote computer is reachable by sending it multiple ICMP echo requests and waiting for a response.

Point of Presence – the connection on a network where the public connect to – Dial up Networking access.

Routing Table
A router will have a table, a list of addresses of other networks or devices and how they can be reached. (when you apply for a domain name, you are first given an IP address, it is this address only that you use until the domain name has been updated in all networks).

Session layer
Layer 5 of the OSI, when a computer accesses a network it is this layer that another computer will access and communicate with, to indicate with each other any network problems.

A high speed fibre optic connection – multiplexes low speed lines into high speed lines.

Subnet Mask
A 32bit number used to mask with the IP address to obtain the network address.

Transmitted data is grouped in blocks of 8bits with a start and stop bit, this informs the receiver that the character has been sent, while this is efficient it increases the packet size because of the added control bits for each character.

Characters are blocked together as a group with synchronization character at start and end of each block. Faster than Asynchronous, but less efficient.

Access or Terminal server
Connects modems and terminals to a network.

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