Hi Edi, The distance depends on how you are going to use the cable. If you use it well, I think you can easily do 2500 meters.
How to do it well: It is good to terminate a cable with its characteristic impedance. If you don't, waves/pulses in the cable will reflect and give ringing / reflections. Terminated, you do not have to check if the cable will act capacitive or inductive: your electronics will see a resistor. And the length of the cable does not have so much influence anymore. What remains is the attenuation. To have a low attenuation, you can use a polypropylene cable. Belden has good cables. Use a 100 ohm or 120 ohm cable, twisted pair or shielded (but I do not think you will see much interference at the antartic) and connect a resistor parallel at the ends of the cable. Nowadays there are very good drivers for RS485 that can do more than the old 175/176 transceivers. 2500 meters should be no problem. If your cable breaks, you have the problem that you loose one termination. But I have seen systems as described below that continued to operate. When you want systems to continue to operate you must either use a very low datarate when the cable breaks, or you can also use software for this: send on a regular base messages to the slaves. They work as watchdog - if a slave sees no messages anymore, it should close the relay you wrote about (starting with the last node, so you need different timeouts for different nodes). It must do this until the system is recovered. You could also see a break of the cable by using the voltage levels. There are several ways to solve this problem. As you run on bettery's and want to save power on the 100 or 120 ohm resistor on the RS485, you can use this trick: connect a capacitor in series with the resistor. It should be large enough to act a a short-circuit for the frequency's you use, but it will be open for DC and thus save power. ===== About the temperature range: At the temperatures you write about, you MUST use electronic parts that can handle low temparatures, industrial parts, not commercial! Especially the crystals, cpu's and battery's. Use thermoset cable, no thermoplastic as that will be difficult to handle when it is cold, and more fragile. ===== About a protocol: there are many. I will wite a bit about one. It is copy-paste of another message I once wrote. It does NOT mean it is THE one for you, but may give you an idea of what you may want/need: I have developed electronics, hardware and software using RS485 in Bitbus networks. Bitbus uses twisted pair cable with a characteristic impedance of 120 ohm, terminated with 120 ohm at the beginning and end of the cable. Next part sounds complicated, but is in fact not difficult to implement as 8051-based processors are available with built-in firmware that completely handle the protocol (and give you a multitasking operating system if you want that) and enable you to write only your application. You just give a "send message" command etc, the chip does the rest. Here the some info: Bitbus is a master-slave network exchanging SDLC message frames. 248 bytes max. net data length per message. Bitbus stubs and prolongation is possible with the use of repeaters. Media: Twisted pair cable (one pair, 120 Ohms characteristic impedance) with ground wire and screen. Repeaters can be used. Electrical: Differential pair 0/5V as defined in RS485. Protocol: SDLC bitsynchronous self-clocked NRZI with opening and closing flags, address checking and 16bit CRC check word. Data rate: 62,5kBit/s, 375kBit/s or 1,5MBit/s. Slaves: 28 per segment with repeaters after one segment, 250 maximum. More stations per segment with modern RS485- transceivers (ALS/LBC). Data rate with more than one repeater: 62,5kBit/s only. Extension: 300m per Segment at 375kBit/s, 1200m at 62,5kBit/s - but note: that was with old-fashioned parts, you get higher rates and distances with modern RS485 transceivers. Connector: 9pin Sub-D-connector. I know a place where they went 12 kilometers with repeaters with 65 KBd. Bitbus is not a popular one like profibus or CAN, but it is a reliable one, and totally open, no royalties. You can find it in radio/television stations, nuclear power plants, a large accelerator in Switzerland.. etc. ===== I think you have a wonderful project here, making equipment for the antartic. I love doing such projects. Have fun and success! And feel free to ask any questions. Kind regards, Pieter Hoeben Date sent: Tue, 09 Dec 2003 06:04:19 -0800 > The RS485 is defined to a maximum lenght of 1200m at 400kbps. > I don't need 400kbps, but up to 2500m of cable. > Is it possible to extend to this lenght with 9600bps? > > Are there other standards for data transmission? > I have two wires (plus power and ground), one master and up to 20 slaves. > The maximum distance between two slaves is 500m. > A multimaster bus would be nice, but is not a must. > > The network should also work if the cable is broken (up to the break). > With RS485, I would switch the termination with a relais. > > What else... > Oh yes, the system is battery powered and should work fron 25�C to -40�C. > > Any ideas? > > Edi Im Hof > Author: Edi Im Hof ______________________________________________ Hoeben Electronics Phone: +31 6 51590081 Ronkert 44 Fax: +31 13 5096025 5094 EW Lage Mierde Private: +31 13 5096200 The Netherlands E-mail: [EMAIL PROTECTED] http://www.hoeben.com ______________________________________________ -- Author: Pieter Hoeben INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). 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