Suitable cable for RS-485 (Alpha Wire 2466C / Belden 8723)? - Page 1
Forum section: electronics / projects

author: Lomax
author_rank: Frequent Contributor
timestamp: September 01, 2017, 02:12:10 pm
content: So I need to buy a 30m / 100 foot reel of cable for use around my boat, for carrying RS-485, 1-Wire and NMEA signals (also RS-485, IIUC) - sometimes along with 12V (low) power. This calls for a single twisted pair plus two more cores for power, which if also made up of a twisted pair would allow me to use the same cable to alternatively carry two separate data signals. I've spent some time looking for a reasonably cheap option which has a small overall diameter, with fairly large area conductors made from multi-stranded tinned copper, and either a common shield or each pair individually shielded. The best I've been able to to come up with is the Alpha Wire 2466C / Belden 8723 which I can get for £35 + VAT from Rapid:https://www.rapidonline.com/alpha-wire-2466c-sl005-data-transmission-cable-22awg-2-pair-30-5m-reel-02-0882But this cable has a listed impedance of 60 Ohms, while I believe RS-485 calls for a 100 Ohm impedance cable.1) Will this cable work well in the intended application, despite not matching the impedance?2) Do you have any suggestions for a similarly priced cable which will work better?Gratefully,Lomax

author: ajb
author_rank: Super Contributor
timestamp: September 01, 2017, 05:06:47 pm
content: What's the maximum data rate? And if you're only buying a 30m roll, then your max length is probably, what, a couple of meters? RS485 can be quite resilient over short runs and/or modest signal rates. Does NMEA incorporate error detection? I would probably be concerned first and foremost with finding a cable and connectors that meets your environmental requirements, particularly UV/moisture.Some care should also be taken in laying out your data connections and power distribution, particularly in the vicinity of high current loads. You don't want the the shield of your RS485 cable becoming the return for your bilge pumps or something.

author: Lomax
author_rank: Frequent Contributor
timestamp: September 01, 2017, 05:21:46 pm
content: Thank you.Quote from: ajb on September 01, 2017, 05:06:47 pmNot sure to be honest, 230kbps?Quote from: ajb on September 01, 2017, 05:06:47 pmBit longer, but yeah, the boat is only 15m bow to stern, so that would be the maximum length.Quote from: ajb on September 01, 2017, 05:06:47 pmNot sure, but I don't think so. It's NMEA-0183 we're talking about btw.Quote from: ajb on September 01, 2017, 05:06:47 pmUV is not so much of a concern as the cabling will mainly be used "indoors", but I do want tinned copper conductors for better corrosion resistance.Quote from: ajb on September 01, 2017, 05:06:47 pmAgreed. It's a little complicated, since the on-board computer runs off its own UPS, andsomeRTUs will also be powered from it. For example, I have two Novus DigiRail 4ch digital counters which do not have any insulation on the serial port. Still trying to wrap my head around exactly what the implications of this will be, but it has no bearing on the choice of cable.

author: Kjelt
author_rank: Super Contributor
timestamp: September 01, 2017, 08:51:13 pm
content: You could use UTP or STP twisted pair cable.Choose the 100% copper version.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: September 02, 2017, 12:55:59 am
content: Quote from: ajb on September 01, 2017, 05:06:47 pmNMEA has CRC.

author: Lomax
author_rank: Frequent Contributor
timestamp: September 02, 2017, 01:25:25 pm
content: Quote from: Kjelt on September 01, 2017, 08:51:13 pmConsidering it's for a boat, with a noisy alternator etc, I reckon shielding is a must, so STP in that case. But it's not an ideal candidate for a number of reasons; conductors tend to be single core, so it will be stiff and unwieldy, and cross sectional area tends to be small, so not great for carrying power, and the conductors are not tinned, so no extra corrosion resistance. And unlike the cable I have been looking at (see datasheet in initial post), it doesn't have individually shielded pairs, which might prove valuable in my intended use, and it it's not a perfect match impedance wise either. And, it's not even going to be a lot cheaper. In other words: thanks, but erm, njaeQuote from: ahbushnell on September 02, 2017, 12:55:59 amThanks, good to know; I did a quick search when the question came up but wasn't able to find a clear answer.Doesn't anyone have any comments on the cable I found?

author: C
author_rank: Super Contributor
timestamp: September 02, 2017, 02:26:33 pm
content: Quote from: Lomax on September 01, 2017, 02:12:10 pmJust wondering if this is a good question.You say "My Boat" and it sounds like you are adding control/status/data.With RS-485, 1-Wire you will need to scan often to get status.Should you be looking to add CAN bus to better handle events?

author: Lomax
author_rank: Frequent Contributor
timestamp: September 02, 2017, 04:38:37 pm
content: Quote from: C on September 02, 2017, 02:26:33 pmThis isoff topic, but I'm curious; what "events" do you have in mind? Real-time navigational data is already handled by the helm instrumentation (compass, GPS, engine instruments, etc), and I have no need to log any of it more than once a minute, in fact even that might be a bit greedy - she's not a speedboat... All "alarm" type functions operate through stand alone devices; e.g. each fire alarm will make a bloody racket all by itself the moment smoke is detected, whether any other equipment is listening or not, and the bilge pumps will start pumping immediately when their individual sensors tell them to, without waiting for permission. Same thing goes for the gas and carbon monoxide alarms. Their activityislogged, but only so that external notifications can be sent (email, SMS), and to keep a history, which again means relatively low polling rates suffice.The only exceptions I can think of are the PIR sensors, whichdoneed to raise an interrupt, in order to trigger cameras, lights etc, but I will simply use GPIO pins for those - unlike the other "alarm" sensors they depend on the onboard computer to execute their alarm actions anyway. Other sensor data is not time critical, and can be polled on a very lazy schedule (tank levels, wind speed, various temperatures, battery voltage, etc).Or in other words: the system has been designed so that all critical safety and navigational systems will remain fully operational even if the control and monitoring system is completely off-line.

author: ajb
author_rank: Super Contributor
timestamp: September 02, 2017, 06:05:26 pm
content: Quote from: Lomax on September 01, 2017, 05:21:46 pmAhh, okay, that's about 3x larger than the first thing that comes to mind when I hear "boat"I think your RS485 will be probably by perfectly fine with the cable you've chosen. At 230kbps, one bit period is something like 900m of cable. Using slew-rate limited drivers will help. I'd be more concerned about the 1-wire interface, and its ability to drive that length of cable, especially if you're using devices that are powered from the 1-wire bus.

author: C
author_rank: Super Contributor
timestamp: September 02, 2017, 06:35:34 pm
content: yes Off topic, but what are you going to use RS-485, 1-Wire for?From you list looks like almost or everything has been done.For an RS-485 bus to be shared, each device needs to know when it is safe to transmit. For CAN bus this is not needed, Any device can transmit with highest priority winning.Lets use this for exampleQuoteWhat is quoted here has some single points of failure. When you use a switch it is a good idea to use the normally closed contacts for normal, then alarm or wire break can be detected. When switch is connected to CAN bus, the single point of failure gets smaller, switch to CAN bus in a lot of cases.With no need to poll & priority, CAN bus lets one sensor talk when needed and many or all connected to CAN bus can hear message. The one point of failure problem is less a problem.Two or more computers can check that each are working properly & inputs and outputs are working correctly..RS-485 is great for moving large amounts of data.CAN is better at small packets of data with priority, Better at Control/status data needs.. This is why it is used in cars, trucks and many other places. It can still move huge amounts of data at lower priority while allowing more important data a higher priority to jump ahead.An Event could just be a new data value like for wind speed or something critical like water in bulge. So while you have bulge switch controlling bulge pump. Messages on CAN bus can have bulge switch detects water, bulge pump is pumping water. And you can have more messages to verify status.

author: Lomax
author_rank: Frequent Contributor
timestamp: September 02, 2017, 11:44:57 pm
content: Quote from: C on September 02, 2017, 06:35:34 pmRS-485 is needed for mainly three things:1) Communicating with some onboard equipment which provides telemetry data over Modbus. Examples of this includes a Mastervolt "Mass Combi" inverter/charger unit, and two Morningstar MPTT 30 solar charge controllers. Without an RS-485 port and a Modbus master on the main computer I would not be able to access to this data.2) Communicating with NMEA-0183 equipment, such as the helm instrumentation (mostly VDO Logic instruments). NMEA-0183 uses RS-485 as its physical layer, and without an RS-485 port and an NMEA parser on the navigation computer I would not be able to access this data.3) Controlling 12V LED light dimmers, which are in charge of the interior lights, as well as some external lights. I have not decided yet whether this will be an off-the-shelf model, or a homebrew based on an AVR MCU - or indeed whether the protocol will be DMX or Modbus. But thephysical layerwill be RS-485.There are other use cases, for example I have been looking at being able to control the ventilation fans, for which RS-485 Modbus also seems the natural choice. Now I know it is an old standard, and most people have probably moved on to newer and greater things (CAN, Modbus TCP, Z-Wave, Zigbee, etc, etc, etc), but Iliketinkering with old tech, and love that I can find used industrial gear on eBay etc which I could never have afforded when it was new. If it's good enough for use in an industrial environment it's good enough for meI have two Novus DigiRail 4C 4 channel digital inputs / counters, and two DigiRail 2R with two 8A relay outputs each. Haven't decided what exactly I'm going to use them for, but I enjoy playing with them and I am confident they will put to good use. One handy thing with a lot of RS-485 gear is that it's well suited for a 12/24VDC environment, with 24VDC being the standard for PLC systems, and at least the Novus RTUs accepting a (specified) power supply voltage all the way down to 10VDC (god forbid my main bank ever drops that low; that would be the end of £1k worth of batteries). As a bonus, I would not have to replace any of it should I decide to move to a 24VDC onboard electrical system in the future (before you jump in to correct another perceived mistake: I am well aware of the drawbacks of 12VDC, thank you).Now there was alooongthread here a while back, where I tried to find support for my design for a very compact homebrew DC PWM dimmer/motor controller communicating over 1-Wire. I had previously purchased two old AAG/Dallas 1-Wire weather stations, which I had a lot of fun with, and as a consequence had become quite fond of 1-Wire. Its ludicrously low power consumption, simple physical interface and flexible topology appealed to me, particularly since this is a solar powered application. Every mA counts and I'm happy to pay for lower power with slower access. But after much arguing, I allowed myself to be talked out of using 1-Wire for the dimmers, and go with (you guessed it) RS-485 instead, mainly due to the greater signal integrity of its differential signalling scheme. That project kind of stalled after that, but I will be returning to it soon - though I'm currently leaning towards off-the-shelf DMX or Modbus dimmers, for time reasons. Be that as it may, 1-Wire will still fill an important role, as the interface for said weather station, and a number of temperature sensors (there area lotof interesting temperatures to keep track of on a boat). Probably also for humidity sensors, and perhaps a few other things.And just to be clear: this project hasalreadymoved well beyond the planning stages; Ialreadyhave the main computer system up and running, with both Modbus and 1-Wire interfaces being used (courtesy ofthe amazing Monarco Hat). Ialreadyhave the navigation computer listening to NMEA-0183 data from the helm instrumentation, and publishing it as MQTT messages over the internal network. Ialreadyhave a WWAN router integrated, which keeps a permanent uplink and feeds MQTT messages to my cloud server over a VPN. Ialreadyhave a fairly advanced monitoring system build in Node-RED, which contacts me via SMS and email when certain events occur, and which presents the telemetry data on some pretty fucking fancy looking dashboards. It isonly, andprecisely, because I have gottenthis farthat I am now in need of purchasingthe cableneeded to actually install the system aboard the boat. Picture or it didn't happen? Ok then.Quote from: C on September 02, 2017, 06:35:34 pmSo? This is not for a nuclear power plant, or a self flying aeroplane. I understand the limitations of RS-485/Modbus very well, and they are entirely acceptable to me.Quote from: C on September 02, 2017, 06:35:34 pmNo it doesn't. The PIR sensors hold the input pins high, so any cable break will trigger the alarm. Sure, youcouldmaybe find a location where you can access the cable without triggering the PIR, figure out which wires are power and which are the output, supply your own voltage to hold the input high and cut the wire to the PIR without triggering the alarm. BUT COME ON MAN, THIS ISN'T FORT KNOX WE'RE TALKING ABOUT HERE! I'd say if you could manage to sneak into the marina, break into my boat, disable the alarm system (hint: try hooking up a Tazer to some exposed wire - should be a lot quicker) and rob me blind - I'd say you're welcome to whatever bits of rope, life-vests and rusty anchors you can carry. Just don't steal the computer system; I would be coming after you!Quote from: C on September 02, 2017, 06:35:34 pmI'm sorry, but that is just complete and utter bullshit. It really is no such thing.Quote from: C on September 02, 2017, 06:35:34 pmIt is also afarmore complicated protocol than Modbus, with more complicated support circuitry, more complicated drivers needed on the computer side, more complicated documentation, more complicated specifications, more complicated equipment at every level. This is why it is used in cars, trucks and many other pieces of equipmentmanufactured by large multinational corporations, and remains comparatively rare in the hobbyist/homebrew environment. WHAT, YOU THINK I HAVEN'T LOOKED AT IT!? WellI have, and my prevailing impression is that CAN Bus is a can of worms I do not have time to digest.Quote from: C on September 02, 2017, 06:35:34 pmFirst of all, the wind speed is averaged over a time period; I currently only read its pulse counter over the 1-Wire bus once a minute - but thenI am not planning to start my own weather service either, I just want to know if it's "windy", ok? Secondly, the bilge sensors have preciselyoneimportant task to perform: to start the pump that is attached to each one when water is detected. The fact that I can log this happening, and have the boat send me an alert when it does, isa bonus, and of rather limited value at that. What can I do if I get an SMS saying the bilge pumps are running, and another one ten minutes later saying they arestillrunning? Not a whole lot, unless I happen to be on board, in which case I'd probably notice the water spraying out the side of my boatanyway. I might be able to change my travel plans if such a problem persists, but in most cases it would probably take me untilthe next dayto get to the boat. So I have no use whatsoever of millisecond precision here - instead I have to rely on the capacity of the pumps and the house battery bank, the dB rating of the fire alarms, the friendliness of any passers-by, and the alertness of the local fire brigade, to save the boat should any such calamity occur while I am not aboard myself. This sounds old-school perhaps, but that's life in the real world for you.Case closed? Case closed. Unless anyone has anything to addabout the cable I was asking about.Lomax

author: C
author_rank: Super Contributor
timestamp: September 03, 2017, 12:45:27 am
content: You have stated a lot of good reasons for RS-485 & one-wire.QuoteI might help with this, might not.I would say that CAN bus it simpler then Modbus, But You know Modbus.Try thisFirst simple hardware, It's a switch from a microcontroler with a usart to one with a CAN controller. PIC and Atmel and others build simple microcontrollers with this.In normal logic you can have Open-collector driver and create a wired-or on the wire. The CAN transceiver creates a differential wired-or on CAN bus.A CAN packet is a stream of bits. At low level, bit stuffing is used. At higher level this is hidden. By receiving what is on wire, The CAN controller can turn off transmit when a difference is detected. This gives some streams a higher priority and with the stop transmitting back off leaves only one packet stream. A search of one-wire does something similar.The CAN identifier could be thought of as the data identifier. The address location in memory of a variable is a data identifier.Good luck with your project.

author: Lomax
author_rank: Frequent Contributor
timestamp: September 03, 2017, 12:36:35 pm
content: Quote from: ajb on September 02, 2017, 06:05:26 pmSo am I right in thinking that cable impedance and the risk of reflections interfering only becomes an issue with cable lengths far beyond what I will be using, even if the cable impedance is only half of the 100 Ohms called for?Quote from: ajb on September 02, 2017, 06:05:26 pmAt the moment there is only one physical unit on the 1-Wire bus, which is an AAG Electronica TAI 8515 weather station, containing three 1-Wire devices: a DS18S20 temperature sensor (air temperature), a DS2450 four channel ADC (wind direction) and a DS2423 two channel counter (wind speed). I have this hooked up in the lab over a normal telephone RJ11 lead withtwowires (in other words, the weather station isparasiticallypowered), though it can also be reconfigured to run off a separate +5V/GND power supply. It works very well. On the boat, it will be installed half way up the mast, which will put it at a distance of about five metres from the 1-Wire host interface.Quote from: C on September 03, 2017, 12:45:27 amIf you really want to help, then tell me what you think about my choice of cabling for the RS-485 network I will be building. I willnotbe building a CAN Bus network, so you are completely wasting your time going on about it.I do not careabout the comparative merits of CAN vs. Modbus. As I explained already (and in great detail too), much of the equipmentalreadyon board uses RS-485 to communicate -there is no scenario where I would be able to achieve what I want without making extensive use of RS-485.So you seem to be suggesting that I should add a separate CAN Bus networkin additionto the Modbus/NMEA/DMX networks I already know that I need, because... WHAT EXACTLY!? Because CAN Bus is "better" than RS-485? BUT I DON'T NEED IT FOR ANYTHING! I do not have asinglepiece of equipment which has a CAN Bus interface! You might as well suggest I should be doing SCSI over fibre, since none of my equipment uses thateither.Quote from: C on September 03, 2017, 12:45:27 amThanks.

author: Kjelt
author_rank: Super Contributor
timestamp: September 03, 2017, 02:01:55 pm
content: Fine with your decision and the corrosion and impedance could be good arguments against it but would like to set some points right:Quote from: Lomax on September 02, 2017, 01:25:25 pmYou can get UTP/STP in multistranded as well, there are different RJ45 connectors to use for both wire types.QuoteIts awg26 i believe but have you ever heard of PoE (power over ethernet)?You can run upto 50W easy with one pair of these wires, whole buildings are now utilized with PoE lighting devices of 40W's, but if you have lower voltages then ofcourse lower power because the max. current is the limit.https://en.wikipedia.org/wiki/Power_over_Ethernet

author: Lomax
author_rank: Frequent Contributor
timestamp: September 03, 2017, 05:37:32 pm
content: Quote from: Kjelt on September 03, 2017, 02:01:55 pmYes.Quote from: Kjelt on September 03, 2017, 02:01:55 pmYes. I use it here at home; I have two Zyxel PoE injectors in my telcoms rack, which power two Netgear GS105PE PoE switches, one in my office, to which a printer, a desktop computer and a PoE powered VoIP phone is connected, the other in an upstairs closet where I have a PoE powered WiFi access point and a NAS unit. But PoE gets around the lack of copper by running at 48 volts; a comparable 12V load would draw four times the current.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 03, 2017, 06:29:47 pm
content: for 230k over 30m pretty much anything wiil do.

author: Lomax
author_rank: Frequent Contributor
timestamp: September 04, 2017, 03:26:42 pm
content: Thanks to everyone who's contributed - I have now ordered one 30m reel of Alpha Wire 2466C (2 pairs, individual foil shield) and one 30m reel of Alpha Wire 2461C (single pair, foil shield) from Rapid. Should cover >90% of my signal/data/audio cabling needs. Total cost £80, which I think is quite reasonable.