Archive for April, 2007

Andover Continuum 1.73

Published by pchipkin April 23rd, 2007 in Andover Continuum. 0 Comments

BACnet clients must respect the servers report of supported services

In BACnet when a client connects to a server it has the responsibility of enquiring what services the server supports and then only using the supported services. In BACnet a server should be driven to a failed state if it receives a message requesing a service that isnt supported.

There are always grey areas in specs. Despite the plugfests, despite the testing … there will always be little gotcha’s. Isnt that why there are plugfests in the 1st place.

We recently supported a site where the server device reported that it did not support COV. Despite this ,the Andover Continuum software sent COV subscription requests. This caused a failure on the server device which was not able to handle them correctly.Normally the Andover Continuum respects the service capability of the server device but in this case additional programming had been configured in a program block which used subscriptions. These program blocks appear in this configuration not to have checked the supported services list before using one. Normally the server device responds to requests for unsuported services correctly – with an rejection message.

The problem was hard to diagnose for the following reasons;1. The problem manifested itself by allowing one server device to be recognized and its points were seen but when an identical (except for device instance id) was added then both servers failed.2. An attempt was made to emulate the problem offsite. The same controller and software version were used. The offsite test did not use the same configuration. Offsite , we were able to bring in the service devices and see the points. However, there were no program blocks and thus the problem did not show up.3. We obtained communication logs from site. The logs did show problems but these were a manifestation of the failure in the service devices caused by the COV subscription. Thus these errors were red herrings.4. The subscription message is sent early after the system is reset. Thus the subscription message was not captured in the logs.

Conclusions;

  1. It is probable that this same problem exists in all versions since 1.73 since we have not seen a correction notice.
  2. There is no substitute for capturing full logs from a site. Logs should be captured using external devices to avoid placing any additional load on the devices already installed on the network. Using an external device can be as simple as using a USB-485 converter. Chipkin Automation System has software tools for capturing BACNet MSTP logs. They are available free of charge. Call or email us for a free copy.

 

© Chipkin Automation Systems 2007

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What is RS485, EIA-485

Published by Steven April 13th, 2007 in RS485. 3 Comments

EIA-485 (formerly RS-485 or RS485) is a specification for the physical layer of a network that uses the difference in voltages between two wires (Three wire) to conveys data.

One polarity of voltage indicates a logic 1, the reverse polarity indicates logic 0. The difference of potential must be at least 0.2 volts for valid operation, but any applied voltages between +12 V and -7 volts will allow correct operation of the receiver.

RS-485_waveform

EIA-485 does not specify or recommend any data protocol is only a specification for the transmission of the ones and zeros. It is up to the receiver to convert the ones and zeros to a specific data protocol such as BACnet MSTP, Metasys N2 by JCI, ect

Since EIA-485 uses a voltage differential it can be used across large distances. (4000 feet or 1200 meters) and it resists electromagnetic interference from motors and welding equipment. When using a network across large distances the transmitter will use the max potential differences (+12 V and -7 volts). The resistance of the twisted pair cable will drop the voltage difference as it travels down the wire. When the receiver reads the voltage on the other end there could be as little as 0.2 volts difference between the two wires. 0.2 is the minimum recommended voltage difference that receivers use to detect a change in polarity.

The recommended arrangement of the wires is as a connected series of point-to-point () nodes, a line or bus, Ideally, the two ends of the cable will have a termination resistor connected across the two wires. Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. Termination resistors also reduce electrical noise sensitivity due to the lower impedance, and bias resistors are required. The value of each termination resistor should be equal to the cable impedance (typically, 120 ohms for twisted pairs).

multidropped network

For more information on RS485

© Chipkin Automation Systems 2007

Metasys® as used in this document is a trademark of Johnson Controls, Inc.

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RS485 Cables – Why you need 3 wires for 2 (two) wire RS485

Published by Steven April 12th, 2007 in BACnet MSTP and RS485. 45 Comments

RS485 needs 3 conductors and a shield. Many people say its a two wire network but it is not.

Two conductors are used to carry the RS485 Differential voltage signal.
The Shield is connected to earth/ground at one end only and provides shielding against induced noise.

So why the 3rd conductor ?
The driver sends data by modulating the differential voltage. The receiver must sense and decode the differential. There are limits to the voltages the transmitters and receivers can work with. These limits are specified by the code. They are -7Volts to +12Volts. What happens if you have two devices and a ground potential exists between the two devices of 24 volts ? You can see that one of the devices will be operating outside the specified voltage range. While you might expect that all the electrical equipment in an installation is ultimately connected to the same ground in practice this is rare especially in cold climates where building architecture and frozen ground can conspire against you. That why you need the 3rd conductor – to connect the ground (of each RS485 driver) to the same reference. Now we don’t care about ground potentials.

Ever wonder why you blew a 485 device when you connected your laptop or computer ?
Its this problem – there is a ground potential. Thats why its good practice to connect your laptop’s 485 ground conductor before you connect the differential conductors.

Can you get away with 2 conductors ?
Yes. Thats why lab or factory tests suddenly stop working when installed at a site. In your lab or on your desk you can be sure the devices are all commonly grounded. Now if you measure the difference between the ground of a RS485 driver on one device and another device you will find zero volts.

Can you get away without the shield?
The value of the shield is controversial. If you are using a twisted pair and you don’t ruin the twists by unwinding them by more than an inch or two at each end then the shield probably is of little value. But, most cables come with a shield. If its not easy to terminate the shield or you cant be bothered then at least have your design drawings tell the installer to to coil and tape the shield drain wire so that you can use it if you want to.

Where can you purchase 3 wire 485 cable ?
I don’t know. Purchase two twisted pairs with an overall shield/drain. Use one pair for the differential and join the conductors of the other pair to make your ground wire.

What size conductors ?
The bigger the better. Most installations are done with 24awg but remember the higher the baud rate the greater the signal frequency and the greater all kinds of captives and inductive effects. If you can, get bigger conductors.

What kind of cable ?
Choose one with designed to present a almost constant nominal impedance because it will make the addition of terminating resistors easy – just read the cable jacket and get a resistor of the same impedance. Most cables listed for 485 use are designed to present an almost constant nominal impedance. Almost constant means a cable who impedance is reasonably independent of length.

What about biasing ?
If a RS485 driver is idle then what ? The line is disconnected from the driver in its idle state which means that the + and – are floating. What happens if for an instant there appears a differential voltage of more than 0.2 Volts? Well a differential greater than 0.2 is considered a signal and hence constitutes data noise. Solve this problem by pulling the lines to no voltages when they are idle. To do this connect them to ground or some other voltage using pull up/pull down resistors. Nice vendors include this. Nicer vendors provide a choice of biasing resistors selected by switches/jumpers. The reason you might not always use the same value is that the cumulative effect of many device’s biasing resistors may make it impossible to signal at all. How do you calculate the value of a biasing resistor ? How would you know where to connect it ? Good luck getting answerers to these questions. An easier question to answer is this: How do you know if you need to bias the idle state ? If you have a scope you can see what the idle state floating voltages are ? Don’t forget you can only measure this when the device is idle.

For more information on RS485

Please contact us, we can resolve your issues with RS485
© Chipkin Automation Systems 2007

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Open submission – Write for us

Published by Steven April 11th, 2007 in Site. 1 Comment

We are looking for people to write short (400+ word) articles about industrial and building automation, Industrial communication protocols, BACnet, Lonworks, Modbus, Metasys N2 by JCI, Tridium, Continuum etc

All articles will have an author statement at the bottom that credits you for your work. A lot of employers search the internet for applicants’ names before hiring them. Quality articles with your name attached will help you find the job that you are looking for.

Ideally we would like to find a semi permanent writer that can publish 5-10 articles a month. A writer that can work independently, that can select there own subjects and do there own research.

Submission guidelines

  • All submissions must be
    • Written in English
    • The original work of the author. (We want original articles, we will check for plagiarism)
    • Factually correct at the time of submission (All articles are fact checked)
    • Longer then 400 words bigger the better
    • Relate to industrial automation, industrial protocols, building automation, (if you are confused ask, if you have a suggestion ask)
  • All submissions are subject to editor approval.

Subjects for articles
We are looking for articles on the following subjects

  • What is Modbus and what is its advantages and disadvantages over other protocols
  • What is Andover continuum and what is its advantages and disadvantages over other protocols
  • How does BACnet MSTP token ring works.
  • What are something that effects BACnet MSTP performance.
  • What is the most commonly used industrial protocol and why
  • A list of HMI’s and there advantages and disadvantages
  • Cheap home automation that will save you money

Any suggestions for subjects would be appreciated.
A list of accepted articles can be found on our website
www.chipkin.com/articles/

Payment
Accepted submissions will be paid $10 CAN for each article. You can submit as many articles as you like. All payments will be done via Paypal and payments are made with in the first week after submission.

Questions
If you have a question about wither a subject is appropriate or other question that you might have feel free to contact us articles@chipkin.com

Metasys® as used in this document is a trademark of Johnson Controls, Inc.

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