Driving 0.5km CAT5

Not over here. But the idea seems worth pusuing. And I'll have to scrounge for suitable ferrite cores from discarded products.

As Jan wrote, not enough drive power. You need to drive that sort of length differentially and the impedance will be around 100ohms for a CAT5 twisted pair. Can you get RS485 line drivers or transceivers in your area? Those are quite popular in noisy industrial environments with long cables.

I believe India sees lots of monsoon thunderstorms so you'd also have to protect against spikes. Most RS485 chips have at least some built in. You can add TVS for extra protection.

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Regards, Joerg

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pimpom Inscribed thus:

What about the transformers from dead modems !

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Best Regards:
                     Baron.

Yep. I meant that kind of scavenging resource. Thanks for the suggestion.

Thanks to you and Jan. As I indicated in my own follow-up to that post, I can see where I failed to consider the drive power needed for a long low-Z line. That's not hard to correct.

No. I can probably get them from another city,.but only after some considerable delay. Most Indian shops understand only OTC purchases and they are unbelievably difficult to deal with from a distance unless you're a regular customer with large frequent orders. They promise anything in a verbal exchange, but cannot be relied on. Emails are simply ignored >95% of the time, and they avoid quoting a price for as long as possible. (Sorry about the rant, but it felt good to let out that perennial source of frustration :-))

That's another thing I didn't mention before. This project is not for a permanent installation. It will be used for a day or two at a time, with months elapsing in between and the installation removed until the next time. And those times will not be in the monsoon season. Of course, there's always the possibility of a thunderstorm occuring unexpectedly, so it's a good idea to take precautions.

I run video over CAT5 wire (about 140 ft) with virtually no problems. I do it single ended, and were I to drop in a couple of baluns I'm sure it would be undetectable from the coax feeds that I am also using.

I'd suggest starting out with some simple baluns to convert your single ended pulse to balanced, and see what happens.

Thanks. Others have also suggested that route. I'll try it out - probably next Monday as it's already past 2:30 am here and I'm going on a family picnic tomorrow.

Signing off for now.

You can also use a TTL-type bus driver (whatever they have) and feed one of the sections inverted. But at the receiving end you'll need some sort of differential receiver because the amplitude after half a kilometer will be low. So the best thing before we all suggest parts you can't get is to ask the shop what they actually have. There are even some simple audio amps you could use here. Out in the boonies one always has to improvise.

Hey, look at the bright side: You still _have_ an electronics shop. Out here (Sacramento, California) the last one closed years ago and even our Radio Shack store has become a cell phone shop. Just imagine, we have a top notch philharmonic orchestra but no electronic parts shop :-(

Then a few TVS should suffice. It can save the day if some guy accidentally plugs this to a battery.

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le

This is all good advice. You probably want the transformer anyway since the grounds will not match at such distances.

Put put 100K or so across so the whole cable doesn't statically run away from you. A totally floating cable out in the open could cause one of the transformers to go ... tzk ... *POCK* ... li'l blue sparkle in the dark.

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Ground one end. Tie cable ground at other end to local ground via

100K. I've seen 60VAC between the "grounds" of two adjacent high rises :-( ...Jim Thompson

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rs485 supposed to run ~megabits at at that range

-Lasse

These are audio transformers (0,3-3 kHz) designed for line side impedances in the order of 600 ohms.

It was not clear how high frequencies were to be carried, so the losses may be larger than expected and a complex waveform may be distorted in such transformers.

Also the receiving end of the CAT5 line should have a terminating resistance about 100 ohms, so the nominal 600 ohm is not so nice, thus the secondary side of the transformer should be loaded with 1/6 impedance compared to the original load impedance in order to present a 100 ohm load on the CAT5 line. Operating the telephone transformer so much out of the design specifications can cause some other problems.

transistors.

In addition, in the idle state, there is only two 100k resistor to ground and hence totally 220 kohm loop resistance between the receiver transistor inputs. Thus, any external differential noise could cause false triggering.

To make the situation worse, in the idle state, the differential stage is in the middle of the transition region, hence, any noise gets amplified and can cause a false triggering.

Use hysteresis on the receiver or at least bias the input stage so that strongly pulled into the idle state.

Removing the capacitors and the 100 k resistors from the transmitter will improve the situation considerably. R2 and R7 should also be reduced significantly.

If short pulses (

Paul Keinanen Inscribed thus:

I thought POTS lines were a lot lower than 600 ohms.

How then did they cope with 56Kbs ?

--
Best Regards:
                     Baron.

It is quite possible that current telephone cables have much lower impedance and even open wire connections might not actually go that high even on audio frequencies, but in the early years of the telephone industry, that was the assumption and since those days the telephone industry has been operated at the 600 ohm impedance levels. The 0 dBm = 1 mW is also used in audio industry as a reference level, since 1 mW is 0.775 V into 600 ohms.

Some early electricity experimenters even assumed (based on the maximum power transfer theory) that the electric generator resistance should be the same as the total load resistance and they used very thin wires in the early generators. This of course dissipated half of the power in the generator windings :-) and only less than half of the generator mechanical input power would be delivered to the load.

Multilevel modulation. In 56 kbit/s up to 7 bit could be transferred in each symbol (cycle) and only from the exchange to the end user (8 kHz sampling rate and 7 bits/sample). Of course, the practical throughput was less than 7 bits/symbol.

Anyway, phone line modes above 2400 bit/s include a frequency equalizer to correct any frequency and phase distortion created by the lines and audio transformers. To set up this frequency equalizer, the training session (with a known bit sequence) was performed before the actual data could be transferred.

Paul Keinanen Inscribed thus:

Yes I was aware of that reference.

Ah. Is that why the data exchange rate was asymmetrical, 56/33 Kbs.

Thanks for filling in the blanks.

--
Best Regards:
                     Baron.

We use RS485 over CAT-5 to 1500' at 450kbps, or so. The data will go even further but we run out of power after that (a Power Over Ethernet sort of setup). We do transformer couple the data (the DC gets tied to the center taps) and do a 100ns pre-emphasis on the transmit side.

pable

rk.

I suppose I should take my own advice and always try to design so I can state the voltage at any node (i.e. I don't like cap dividers), but there are all sorts of transformer to transformer applications out there that don't seem to blow up. Video over twisted pair is a good example.

This 100k across the cable doesn't make sense. The transformer puts a low impedance at DC across the cable. Did you mean to ground?