coax Ethernet

Todd, leaving the end-end terminated transmission line for a T-tapped structure is a matter of impedance matching/mismatching. Whether twisted pair RS485 or ethernet or twisted pair ethernet doesn't really matter.

I remember the TI RS485 transceivers adapted for long and fast transmissions. The 65HVD23D achieves 25MBit@160m and the

65HVD24D achieves 3MBit@500m

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

Perhaps we forget that Ethernet was wired with coax (huge RG-8 stuff the size of your thumb, and then RG-

58, etc.) long before UTP ("Cat5", etc.) ever came along. HOWEVER, it was only defined as daisy-chain, never as any other topology. For many reasons, impedance being only one.

You didn't mention what your data rates are here? If they are low enough you could get away with the same kind of CSMA/CD (carrier-sense, multiple- access/collision-detect) scheme that Ethernet uses, but designed to contend with the kind of reflections, etc. you would get from a random tree of coax.

If you could live with lower throughput, and could design your own transcievers, you could get away with coax joined any old way you wish, because the transcievers would make allowances for the lousy transmission medium. But I don't think you would be successfull trying to use stock Ethernet interface nodes with a random coax tree. Unless maybe your the time between packets was MUCH longer than the packets themselves.

The original Ethernet was not a daisy chain, but a broadcast medium. The coaxial cable was supposed to be ternminated at either end, and every transceiver transmitted to every receiver on that length of coax.

Obviously, you can use it as a star system

The two crucial points about Ethernet were

1) Collision detection 2) Random back-off after a collision was detected.

Collision detection just meant that a receiver monitored its own transmitters output, and signalled a collision whenever the received signal didn't match the transmitted signal, which immediately killed the transmission.

Random backoff meant that the station didn't try to re-transmit immediately, but waited for variable amount of time before attempting to retransmit.

The theory behind this is pretty well understood an works time, but you don't want to load an Ehternet link at anything approaching its maximum capacity - IIRR around 10% is as high as you'd want to go.

That is about 1Mbit/sec on the original 10Mbit/sec hard ware, but that covered distances of over a kilometre. Waht the 100Mbit/sec hard ware looks like I've no idea.

The single coaxial cable can be extended as an un-rooted tree by simple dumb repeaters - bearing in mind that the area that can be covered by an Ethernet is limited by propagation delay and attenuation - or by more complicated bridges that store and forward packets addressed to stations outside a particular net, or packets from outside the net addressed to stations within a particular net.

Either mode of extension allows you to realsie the physical connections as a start system, though it does make the system a little more expensive.

It was quite a powerful system, and remarkably simple.

The is a lot of stuff about it on the web

--
Bill Sloman, Nijmegen

[snip] [snip]

Bill, the poster is talking about a master-slave communication system. This means there are no collisions.

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

Perfectly true, but he needs to understand that the classical Ehternet was not a daisy-chained system, which means that he need some understanding of how it worked and why the protocol worked the way it did.

If you broadcast Ethernet packets ontp the cable from the master station, and only let the addressed station respond, you certainly don't need collision avoidance, but the size and structure of the minimum Ethernet package is determined by the collision dection system, and if he wants to use standard Ehtenet hardware an software as the basis of his system he is constrained by these rules.

--
Bill Sloman, Nijmegen

The trickiest Ethernet wiring relates to gigahertz and 100baseT systems; if you can tolerate 10baseT (10 MHz Ethernet) you won't have that kind of problem at all. For 100m distance, you could use either thinnet (10base2) or unshielded twisted pair (10baseT) wiring without particular problems, BUT I'm not sure the exact wire specs for commercial buildings (cat-5) are gonna work for a ruggedized application like yours.

If the wet wiring can carry two pairs with 110 ohm impedance and suitably low crosstalk, 10 MHz Ethernet will work fine. It might even tolerate your RS-485 wiring. And there were baluns used with 10base2 that allowed single-pair transmission (this is NOT recommended, and it makes full-duplex impossible, but... it's a thought). Distance limits and specific losses in the wiring are unlikely to be critical at 100m.

Hello Todd,

AFAIK neither coax not twisted-pair LAN are suitable for application where you want to just "tie together" cable branches at arbitrary points. Depending on what you mean by "multiple" maybe you could consider dropping down one pair per sensor.

Anything else would be more of a custom solution. Can be done though if the cables are terminated properly at all end points.

--
Regards, Joerg

http://www.analogconsultants.com

Since the classic original 1981 Ethernet was half-duplex anyway, and probably perfectly adequate for this application, the non-recommendation needn't be taken too seriously.

Though this does depend on the speed you want and the protocol you end up using to get it.

--
Bill Sloman, Nijmegen

Ethernet is not a 'single master' protocol, its peer-to-peer with all the collision detection and other overhead that this entails.

Its not a daisy chain, its a single bus with impedance matching terminations on each end.

The classic coax ethernet was good for a theoretical 10 Mbps, but 5 Mbps might be an optimistic upper limit, depending on the number of hosts, packet size, protocol overheads, etc.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
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I have worked with coax Ethernet a lot in the recent past. I even have a small 3 computer dos/win311 network in my home. The big problem with the coax is, that it has a maximum length of 75 meters(~80 yards). On one such line, you can connect several computers,by cutting the line and putting in a BNC T connector. At our institute we had concentrator units for 16 cables in each corridor,which were then connected to ~ 1 inch thick high speed network cable with a high impedance T connection. One high speed cable serviced several concentrators. So your 100 m branches would not work well with coax Ethernet, unless you could bring those concentrators very close to a set of drop down lines. If I would be intrumenting this, I would use twisted pairs in the sensor cables,you need 2 twisted pairs in each one,and connect them to rather cheap router units,

4 to 16 to a unit. Then connect those to a master unit. You would need a smart micro down at the end of the cable, to interface. If you use your data collecting computer as master, requesting data from each sensor package in succession, there will be no problem with data collision,and data traffic can get as high as ~800.000 bytes per second, the other 200.000 being needed for the downward traffic. I really got that speed on dedicated 10Mbit Ethernet, with two computers exchanging packets in handshake mode.