JesperMP said:
Geospark said:
There are two options for cabling a Data Highway Plus network. Using daisy chaining from node to node, or using a Trunkline with Droplines from Station Connectors.
The quick and easy way is daisy chaining and by the sounds of what you're saying they used the easier way on this one?
Nothing wrong with it if it's done right. DH+ is quiet robust when installed correctly.
JesperMP said:
The way you write that makes it sound as if daisy-chaining is less 'proper' than trunkline+droplines.
I am pretty certain that daisy-chaining is the most proper way to setup a DH+ network...
Yeah I can see how it sort of reads that way looking back Jesper, but I actually meant quite the opposite. I probably just didn't word it too well?
I was simply trying to deduce which method they were most likely to have used here based on the fact that it sounds like it wasn't installed that well. I reckoned they used the easier way of Daisy Chaining out of convenience, more so than them thinking it would prove to be the more reliable method in the long run. That's why I then made sure to point out that just because I'm calling it the "quick and easy way" does not mean I'm saying it's no good. Again, when installed correctly, DH+ is a very robust network. A colleague of mine often calls it "bomb-proof". Sorry for the confusion.
The Plc Kid said:
...I have about 25% of it done and I have already found 4 extra resistors and 1 spur.
After reading your latest update, they definitely didn't install it well, or, it grew legs over the years. 4 extra resistors & 1 spur and you're only quarter way there. Those spurs are notorious, as it's just too tempting to run a single cable from a mid position on the network to an added node. Then they try to redeem themselves for doing so by adding extra resistors on that spur, thinking that will make it ok. I've even seen installations that had a resistor in each Phoenix plug on the network.
The network can usually continue to function somewhat, but spurs or bad terminating resistances will from time to time give you certain issues, like lost packets, timeouts, nodes temporarily dropping off the network, etc.
JesperMP said:
...I remember way back that for installations with DH+ problems, AB recommended removing the 'grey terminal boxes' that was used to insert droplines in a DH+ network and convert to 100% daisy-chain.
One reason why the quick and easy way of daisy chaining is the better method, is it eliminates a lot of the extra terminations involved in the Trunkline/Dropline method.
The Station Connector(1770-SC), or 'Grey Box", is basically a heavy gauge steel junction box. It has a strip of five standard run-of-the-mill PVC connectors screwed to the back of the box. Depending on where these boxes were mounted in plants, the connectors or box itself could corrode, giving high impedance. Poor termination to begin with without testing the network was often a cause of issues. They'd just fire up the network and it "worked". But loose connections, cable braiding, foil or drain wires touching each other or the steel box could cause issues. The flying earth lead from the box might not have been grounded properly, or at all, or the drain wire series capacitor was missing or looped out.
Another type of Connector was the 1770-XG. These were even trickier. This was a Connector Kit which gave you various plugs for different interface devices. You used a T-Connector to join the two Trunklines and the Dropline together and you had to solder on male and female BNC type plugs. While usually only intended for quick disconnection at a node to connect on a programmer or such, I have come across them just hung in mid air or clipped to a wall or cable-tray with no mechanical protection and no grounding at all. It was rare to see them though. Those plugs were slow to make off and had to be done right.
If someone plugged of a Dropline from a node, it could act like an antenna attracting noise. Whereas, if you plugged off the daisy chained plug from a node, not so. Each Grey Box had to be counted as a node, even if no Dropline was connected to it so this restricted the number of possible nodes. If you had 20 nodes with 20 Station Connectors, and 5 Station Connectors with no nodes connected, that counts as 45 nodes of the total 64 used up.
These were some of the reasons why AB recommended ditching the connectors/boxes and using daisy chaining instead.
A company I once contracted to some years ago sent me on an AB training course for all Data Highway networks, DH, DH+, DH-485, DH II. It was very useful and I still have the need to call on that knowledge, even up to recently when I commissioned an old DH+ and DH-485 network that was moved from one country to another with some machinery.
I had a couple of problems on that job that I'd like to share in case it happens someone else.
Problem 1. A DHRIO box was dropping off the network every once in awhile. It was picked up because a signal that should come on at certain times was not, so I watched it for timeouts or retry attempts and sure enough it was taking a couple of attempts to go through, or sometimes would timeout. As it was an end-of-line node, I checked the resistor and a 150ohm had been installed. The contract electricians had put 150ohm at both ends as the baud rate is set 57.6K. This would normally be the correct setup for this baud rate. But a little known fact is that if your DHRIO is installed as an end-of-line node, while on a 57.6K baud network, it actually performs correctly when an 82ohm resistor is used. It's just another little anomaly that exists, but you have to know about it to solve it. That course taught me that one.
Problem 2. The DH-485 network would crash around 7pm every evening when turning off the laptop and wrapping up to go back to the hotel. After plugging everything out and getting the nodes back up one at a time, the network would crash again within minutes? I eventually realized that my 1747-UIC was still plugged into the DH-485 port of the 1747-AIC Link Coupler at one of the nodes that I was using to connect to the network. Even though the USB lead was plugged out of my laptop and the UIC was dead, the network didn't like something inside it when not powered up. I unplugged it and the network recovered. I went to another AIC at another node and plugged in the dead UIC and it pulled it down again! I had three UIC's to hand and tried them all, they all did the same thing. That's why it was happening every evening as I was packing up. I now had to plug out the cable from the AIC while the UIC was still powered on from the laptop before unplugging the USB lead.
It may just happen when connected to AIC Link Couplers, not sure. I'm assuming there must be an impedance in the UIC while it's powered off that pulls the network down? I would have thought there are UIC's left connected, while dead, to DH networks all over the globe? Could this possibly affect all DH networks? Maybe someone else could try if they have a similar network to hand?
The UIC is a SER A REV D FRN 1.3 using the RS-485(DH-485) port with a 1747-C13 SER B cable.
Here's a checklist I made from that course for Data Highway issues and from dealings with AB over the years:
Are you using the recommended Belden cable?
i.e. 9643 twinaxial.
What baud rate is the network set for?
i.e. 57.2K, 115.2K, or 230.2K baud
Are you using the correct terminating resistors for that baud rate?
i.e. 150ohm
1/4 watt for daisy chain @57.6K - 115.2K baud
150ohm
1/2 watt for Trunk/Drop @57.6K - 115.2K baud
82ohm for 230.4K baud
82ohm for DHRIO @57.6K - 230.4K baud
Is the installation clean?
i.e. No short, open, or high impedance circuits.
Are you using daisy chain or Trunk/Drop, but not a mixture of the two?
Are there any star connections(spurs)?
Have you checked the wiring?
i.e. Blue, Clear and Drain not mixed up anywhere?
Are your cable lengths within spec?
i.e. Trunks not over 10,000ft, Drops 10-100ft.
If Daisy Chaining, loops under 10ft should be avoided, such as 2 nodes in 1 enclosure. These can cause signal reflections.
Are the cable routes segregated properly from any potential sources of electrical noise?
Only after all these check out ok, with exception to the last question, would I then consider analyzing the network with a scope or protocol analyzer.
G.
