Hi,
I have a network of five CR800s, namely 1 nearest to 5 furthest, each with MD485. This network is connected to a pc running Logernet with additional MD485. Furthest logger is about 1000m from the pc connected MD485. Actually CR800 (1) & (2) we're setup in 2013 with MD 485 connections to the pc, all worked well until I join CR800 (3), (4) & (5) to the existing network with MD485.
1) My problem is CR800 (1), (2) & (3) cannot be connected but (4) & (5) can be connected. When I take out MD485 connection between (2) & (3), then I can connect back to (1) & (2).
2) Then I tried trouble shoot at (3), because I got no more access to (1) & (2), using additional MD485 connected to my laptop pc running Logernet. Take out MD485 connection between (2) & (3), then connect with my laptop via another MD485 to (3), I can connect (3), (4) & (5). I presumed my setup at (3), (4) & (5) are correct for all CR800s & MD485.
3) Then I tried, connected back to (2) & (1) backward, I can connect all (2) & (1).
Pls advise why I can't connect all five loggers.
Best regards,
Thein
* Last updated by: Thein on 11/5/2015 @ 6:28 PM *
Are all of the dataloggers assigned unique PakBus addresses? I would suggest using the Network Planner to lay out the network.
Hi Jtrauntvein,
Thank you very much for reply. Yes, I did assign unique Pakbus address to each loggers, for CR800 (1) pb address is 5, for (2) 7, for (3) 13, for (4) 14 & for (5) 15. I used network planner to lay out the network for (1) & (2) in 2013. And now also I used it to add (3), (4) & (5). MD485 baud rate is 115k and CS/IO is SDC7 9.6k for all loggers all MD485s. RS232 baud rate of MD485 at Loggernet end is 115k also as assigned by network planner. But I do not have termination at the far end logger no (5) as I did not know how to terminate.
Is it a concern with MD485 cable lengths as a few ups and down, in reality total MD485 cable length may exceed 1.2km. Will it be help if I placed MD485 repeater at logger no (3).
*Correction for my earlier post, the loggers I used are all CR800 not CR1000, I edited back already, sorry.
Best regards,
Thein
* Last updated by: Thein on 11/5/2015 @ 6:29 PM *
As you say you are on the limits of max length for an MD485 network. There is also a risk that the ground potentials of the systems are different with such distance involved. Making RS485 networks work at extreme cable lengths can be a bit of an art. Here are a few tips.
1) try reducing the baud rate over the RS485 part of the link, perhaps down to 38.4 kb/s or slower. (The SDC speed does not matter so much and can be left at 115 kb/s or to match the RS485 speed, it should not make much difference)
2) install a 120 ohm termination resistor across the A/B lines at the two extreme ends of the network. This reduces and reflections and also helps kills any noise pickup. (These are rarely required for most MD485 networks, but is considered good practice by RS485 network designers.)
3) make sure that all the reference ground connections between the MD485s are connected together. You can check the potentials are not too large by disconnecting the ground referencing wire temporarily and measuring the potential between the end of the wire (connected to the rest of the network) and the MD485 ground. If the potential is more than a few volts you may need to add a 3rd party isolating repeater.
4) make sure all the cable is twisted pair, high quality cable and that using wires from the same pair for the A/B lines. Ideally the cable should be shielded.
If the cable is not so good quality you may have to boost the signal, perhaps by using two MD485s connected back to back via the RS232 port, or using two connected to one logger which is set to route traffic between two RS485 networks. If however, there are signs the problem may be due to differences in ground potentials along the cable, you may have to use an isolating repeater, from a third party.
* Last updated by: aps on 11/6/2015 @ 4:49 AM *
Hi Andrew, Jtrauntvein,
Thank you very much for your replies. I'll do all the things as your advices.
1) Last Friday we added one MD485 at logger (3), which made we can now connected to logger (1), (2) & (3) from the pc at the start end if we take out MD485 connection to (4) from (3). But if we put the connection back, we can connected to (4) & (5) but (1), (2) & (3) cannot.
2) Last Saturday, we added one MD485 each as repeater at logger (4) & (5). But problem still the same, if we have connection between (3) & (4), we can only connect to (4) & (5), if we take out connection between (3) & (4), we can connect to (1), (2) & (3).
Out cables are separately shielded two twisted pairs, one pair connected to A & B, of course A to A, B to B. But I have to confess, I did not connect RG as once we connected we are having noisy data from the sensors. It works well for first two loggers (1) & (2), that we installed in 2013 till now. If this reference ground connection is a big concerned in this network, I will connected back all reference ground.
The things I'm going to do tomorrow will be as follows:-
1) I will reduce down MD485 baud rate to 9600 at (3), (4) & (5) for all six MD485s. Now we have 2 MD485 each at (3), (4) & (5).
2) I will put 120 ohm resister across A & B at far end MD485 which was attached with logger (5).
3) I will double check all cable connections. Fix all RG through out the network (At this point, please tell me should I use one wire or two twisted pair wire to put in RG, as I read some where for MD485 connection one pair need to put A & B, and the other pair need to go to RG). Fix all drain wire to Ground. I will check potential as well.
4) I will make sure the same pair goes entire network.
I'm not sure about isolating repeater. Please advice little bit detail about isolating repeater from third party and where to put. I'll do it, if the problem still persists after all above steps have been done.
I'll report back to you but it may not be finished in one day as we have to do a lot of works. I would like to send my network map to you, if you have other idea you may advice, can I have your email address? Mine is thein at geonamics dot com dot sg. Finger cross..
Best regards and thank you very much,
Thein Htun Aung
Snr. Geotechnical Engineer, Geonamics (S) Pte. Ltd., Singapore.
At first guess, especially with your comments about sensors becoming noisy when you connect the resistive ground, then this may be your issue as it implies there are some significant ground potential differences and they are noisy too. I would look closely at the grounding of the different systems first because it could be one system is not grounded well at all and could even be potentially dangerous!
To answer the question in relation to the reference ground, it does not really matter whether you use one or two wires to connect the grounds as long as you have not gotten the pairs of wires mixed up. It is critical the A/B RS485 signals go down the same pair.
Adding the termination resistors will only affect the length of cable they are on. If you have cut and therefore split the RS485 network using MD485s as repeaters you will have formed several independent networks so if the cables are long may need a pair of resistors on each section.
With regards to RS485 isolators there are many on the market. I cannot guarantee all will work as they generally have to be intelligent or make some expectations about signal timing to deal with the issue of the nature of half-duplex RS485 communications. Some users have used products from B&B Electronics in the past but you can find cheaper products worldwide.
Finally make doubly sure you have the protocol configuration of all the MD485s set to "Pakbus networking" as that reduces the traffic on the network and the risk of data collisions.
Dear Andrew & jtrauntvein,
Thanks for your support. I tried all the options you provided. After, fixing 2 MD485 each at logger (3), (4) & (5) and proper grounding, it briefly ok for three days. I'am about to tell you about it, but on the fourth days the same problem comes back again. It happend after logger (4) was encountered power failure. After the battery has been replaced and power has been restored, I can only connected back to (4) and (5), but (1), (2), & (3) can't with all connected MD485s. When I take out MD485 connection between (3) & (4), I can connect to (1), (2) & (3).
I didn't understand this part,
"Adding the termination resistors will only affect the length of cable they are on. If you have cut and therefore split the RS485 network using MD485s as repeaters you will have formed several independent networks so if the cables are long may need a pair of resistors on each section"
Because I used RS485 ports to connect 2 MD485 at logger (3), (4) & (5). I got no space to put the resistors. Why I did not used RS232 was I did not have Male to Male RS232 cable, also it worked for three days.
The things I didn't do yet is third party RS485 isolater, as I don't know how to fixed and where to fix it.
Thank you.
I am not sure how you connected the two RS485 devices together. If you simply connected one set of green terminals (RS485) on one MD485 to another, this will not achieve anything because the two sets of green terminals on each MD485 are electrically connected, there is no buffering or signal conversion between the two.
For the MD485 to act as a repeater you need to connect 2X MD485s "back-back" with a null modem RS232 cable between the two RS232 ports. You are then splitting the RS485 cable into two shorter sections.
Hi Andrew,
Thanks, I connected one MD485 green terminal to the other MD485 green terminal as follows:
Logger Net's MD485 greem terminal(1) > 1m > Masterlogger's MD485's GT(2)
Masterlogger's MD485's GT(1) > 1000m > Logger(1)'s MD485's GT(1)
Logger(1)'s MD485's GT(2) > 200m > Logger(2)'s MD485's GT(1)
Logger(2)'s MD485's GT(2) > 200m > Logger(3)'s MD485(1)'s GT(1)
Logger(3)'s MD485(1)'s GT(2) > 1m > Logger(3)'s MD485(2)'s GT(1)
Logger(3)'s MD485(2)'s GT(2) > 200m > Logger(4)'s MD485(2)'s GT(1)
Logger(4)'s MD485(1)'s GT(2) > 1m > Logger(4)'s MD485(2)'s GT(1)
Logger(4)'s MD485(2)'s GT(2) > 200m > Logger(5)'s MD485(2)'s GT(1)
Logger(5)'s MD485(1)'s GT(2) > 1m > Logger(5)'s MD485(2)'s GT(1)
Logger(5)'s MD485(2)'s GT(2) --- 120 Ohm 1 W Resister.
I will replaced all 1m MD485 to MD485 connections with RS232 Null Modem Cable. My Male to Male Null Modem Cable connection set up will be as follows:-
1,6 >>> 4
2>>>3
3>>>2
4>>>1,6
5>>>5
7>>>8
8>>>7
9>>>9
1) Is the above Male to Male RS232 Null Modem Cable ok for MD485 serial connection?
2) I will used 120 Ohm 1 Watt resistor at each open GTs of all MD485, is it ok?
3) Is there any additional things need to put it my MD485 net work connection of estimated 1,800m length?
Best regards,
Thein
* Last updated by: Thein on 11/25/2015 @ 10:32 PM *
I am a little confused by your diagram but it looks like you need to split the network at least once, at Logger 3, so you can keep the maximum length of each part of the network <1200m.
In terms of the null modem cable the only wires you need to connect are 2, 3, and 5 (with 2 and 3 swapped). If you connect the other wires it will not matter.
However, perhaps I did not make it clear but if you use two MD485s as a repeater/booster with an RS232 connection between them you cannot also use them to communicate with a datalogger at that point. You can only set an MD485 to use the RS232 port OR the CS I/O port along with the RS485 port.
If you do not have enough MD485s to dedicate two for a dedicated repeater (with no logger and power going into to their power sockets), then you can split the network by connecting two MD485s to one datalogger via the SC12 cable. Set the two MD485s to have different SDC addresses, e.g. 7 and 8 and set the logger to act as a router. Connect one side of the network to one of the MD485s and the other half to the other. You can communicate to that datalogger in the normal way but it will also route the traffic from one half of the network to the other.
In terms of resistors, you normally put one resistor at the two extreme ends of the network, so with a very long network as you currently have, you would put a resistor on the MD485 connected to the PC running Loggernet if it really is the end point. I would try this as a starter before you do anything else, just in case you find this makes the network work as it is (despite being rather too long).
If you have to split the network (I would suggest at Logger 3) you could initially try one resistor at the end of each of the two new halves of the network. If one half of the network seems flakey try adding a second resistor at the other end of the cable to where the resistor is currently installed.
Dear Andrew,
I will try this option, "split the network by connecting two MD485s to one datalogger via the SC12 cable".
At logger(3), the connection will be like this, https://www.dropbox.com/s/yp94ppcltpn6ssz/2015-11-26%2022.54.36.jpg?dl=0
Is it what you meant?
Best regards,
Thein
The diagram is OK. I would try without the 120 ohm resistors at both ends, to start with, as they do load the MD485s a little and increase the power use.
Remember to set different SDC addresses in the two MD485s and to set the logger to act as a router in its own settings.
* Last updated by: aps on 11/26/2015 @ 9:14 AM *