Somewhere in the dim dark past I remember reading that there is a limit of the number of switches you can have in a Rockwell ethernet network. Or maybe just an ethernet network. By number of switches I mean how many times the signal can go into a switch and out again. ie to get 2500 metres than you have to have 25 switches. (TTL or time to live rings a bell.) Edited the bracketed bit. TTL refers only to routers. I am sure someone can clarify this for me.
Also remember a compactlogix is now cheaper (and a lot more powerful) than an SLC.
What I was thinking of was the 5 4 3 rule which is not valid with switches.
Here is an excerpt from
http://www.rhyshaden.com/ethernet.htm
that explains a bit more.
4.5 Fast Ethernet (802.3u) 100BaseTx
Fast Ethernet uses the same frame formats and CSMA/CD technology as normal 10Mbps Ethernet. The difference is that the maximum delay for the signal across the segment is now 5.12 microseconds instead of 51.2 microseconds. This comes from the fact that the bit time (time to transmit one bit) is 0.01 microseconds and that the slot time for a frame is 512 bit times. The Inter-Packet Gap (IPG) for 802.3u is 0.96 microseconds as opposed to 9.6 microseconds for 10Mbps Ethernet.
Fast Ethernet is the most popular of the newer standards and is an extension to 10BaseT, using CSMA/CD. The '100' denotes 100Mbps data speed and it uses the same two pairs as 10BaseT (1 and 2 for transmit, 3 and 6 for receive) and must only be used on Category 5 UTP cable installations with provision for it to be used on Type 1 STP. The Copper physical layer being based on the
Twisted Pair-Physical Medium Dependent (TP-PMD) developed by ANSI X3T9.5 committee. The actual data throughput increases by between 3 to 4 times that of 10BaseT.
Whereas 10BaseT uses
Normal Link Pulses (NLP) for testing the integrity of the connection, 100BaseT uses
Fast Link Pulses (FLP) which are backwardly compatible with NLPs but contain more information. FLPs are used to detect the speed of the network (e.g. in 10/100 switchable cards and ports).
The ten-fold increase in speed is achieved by reducing the time it takes to transmit a bit to a tenth that of 10BaseT. The
slot-time is the time it takes to transmit 512 bits on 10Mbps Ethernet (i.e. 5.12 microseconds) and listen for a collision (see earlier). This remains the same for 100BaseT, but the network distance between nodes, or span, is reduced. The encoding used is
4B/5B with
MLT-3 wave shaping plus
FSR. This wave-shaping takes the clock frequency of 125MHz and reduces it to 31.25MHz which is the frequency of the carrier on the wire.
The round trip signal timing is the critical factor when it comes to the distance that the signal can run on copper UTP. The cable has to be Category 5 and the distance must not exceed 100m.
The IEEE use the term
100BaseX to refer to both 100BaseTx and 100BaseFx and the
Media-Independent Interface (MII) allows a generic connector for transceivers to connect to 100BaseTx, 100BaseFx and 100BaseT4 LANs.
There is no such thing as the 5-4-3 rule in Fast Ethernet. All 10Base-T repeaters are considered to be functionally identical. Fast Ethernet repeaters are divided into two classes of repeater,
Class I and
Class II. A Class I repeater has a repeater propagation delay value of 140 bit times, whilst a Class II repeater is 92 bit times. The Class I repeater (or
Translational Repeater) can support different signalling types such as 100BaseTx and 100BaseT4. A Class I repeater transmits or repeats the incoming line signals on one port to the other ports by first translating them to digital signals and then retranslating them to line signals. The translations are necessary when connecting different physical media (media conforming to more than one physical layer specification) to the same collision domain. Any repeater with an MII port would be a Class I device. Only one Class I repeater can exist within a single collision domain, so this type of repeater cannot be cascaded. There is only allowed one Class I repeater hop in any one segment.
A Class II repeater immediately transmits or repeats the incoming line signals on one port to the other ports: it does not perform any translations. This repeater type connects identical media to the same collision domain (for example, TX to TX). At most, two Class II repeaters can exist within a single collision domain. The cable used to cascade the two devices is called and unpopulated segment or IRL (Inter-Repeater Link). The Class II repeater (or
Transparent Repeater) can only support one type of physical signalling, however you can have two Class II repeater hops in any one segment (Collision Domain).
Regards Alan Case
