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The following text may make IEC485 seem like a poor solution, however it is a cheap and robust technology that is simple to interface to a broad range of equipment.
Multi drop in contrast to point to point IEC485 networks are not trivial to implement. There are many engineering factors to take into account. If anyone involved in a project mentions RS-485 then it is imperative to initially ascertain that they in fact refer to IEC485 which is the standard.
1. Equipment compliance.
2. Equipment interoperability.
3. Cable requirements.
4. Network Termination.
5. Network Biasing.
6. Baud Rate
7. Surge Suppression.
1. Equipment compliance.
In order to ensure point 1 is met, It is best if the equipment is independently certified to meet IEC485 on some commercial equipment the manufacturer will have already done this.
2. Equipment interoperability
A Factory Acceptance Test Plan (F.A.T. plan) should incorporate a test that covers interoperability of the IEC485 equipment at the electrical layer with special notice given to framing errors.
3. Cable requirements.
Underground cable should be gel filled, above ground cable that is exposed should be UV stabilized, Arial cables should be supported. In general good cabling practices should be employed. If surge suppression is an issue cable paths should be sufficiently separated. There is no point have 3KV of opto isolation if your 300V rated insulation is the only physical barrier between the cable and the protected environment. Obviously cable with transmission characteristics that suit a differential mode communications transciever should be used.
4. Network Termination.
This will be determined by the cable characteristic impedance, usually a resistive element (resistor) is sufficient however capacitors may also be needed to tune the bus to the best state, the use of a CRO will help determine the best values to minimise overshoot and ringing of the bus and also reduce to a minimum the load placed on the transmitter.
5. Network Biasing.
Is outside the scope of this simple document, suffice to say it requires investigation when designing a 485 network.
6. Baud Rate.
485 is not like ethernet there are no pre determined speeds at which it is to operate, so one party’s 485 network may be 20 times faster than another’s and result in interoperability issues. If bad design prevails this will be picked up when the F.A.T. Plan is followed. (as a note I’ve also used 10/100 fiber optic GBICs that will not communicate with pure 100 GBICs both from HP), (I’ve also seen 100 GBICs from HP indicate a LINK when connected to 1000 GBICs now I suppose that technically there is a link present but no data is going to traverse that link while the GBIC mismatch exists).
7 Surge Suppression.
Is almost always overlooked, sometimes it may be asked during a project to supply a letter stating that your equipment is surge protected. BEWARE this is basically stating that your equipment is bullet proof, the project then won’t need to implement costly surge suppression as you will be kindly asked to replace your damaged equipment. 485 is long distance communications from a surge point of view. There will be surge protection issues in most installations. The cost of a solution to these surges may range from $100 for opto isolation to $200,000 dollars for lightning towers and earthing mats. From a technical point of view there are two tasks Isolate and Shunt the surge to earth. Make it difficult for a surge to get into your equipment and then give the surge an alternate path. Use Multistage surge protection such as Avalanche Diodes, Metal Oxide Varistors, Gas Arrestors and Opto-solation. I do not recommend Polymeric Positive Temperature Coefficient Resistors for current limiting in surge protection.
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