What is Modbus?
What is a Babel Buster?
How do I monitor power equipment with Modbus?
How do I monitor dry contacts?
How do I set up an SNMP Trap?
Which Control Solutions product should I use?
What about management software?
What is RS-485?
What about Modbus TCP?
Why is it called a Babel Buster?

If you are the IT guru that's been given the task of monitoring everything in your data center, this page is for you. You already know how to monitor servers and routers. But you need to monitor a few more things, too, like backup generators, transfer switches, etc. How do you make them talk SNMP?

There are a lot of things to be found on this web site. Some of them will be foreign to IT people, and quite frankly, IT people don't have much reason to care about some of these products. What we do on this page is try to zero in on those specific items that are of most interest to our data center customers.

The things you are most likely to want to monitor are either discrete sensors, including dry contact alarm outputs from other equipment, or they are intelligent devices with a Modbus communications port. These two possibilities cover the vast majority of monitoring requirements in a data center beyond servers and routers.

What is Modbus? Modbus is a communication protocol that is simple enough to be supported by asynchronous serial communication ports running at 9,600 baud (or any standard async rate). The most widely used Modbus protocol is known as Modbus RTU, which most often runs over an RS-485 serial line. The RTU version of the protocol is raw binary data with a checksum at the end of the packet. ASCII is another less used version where all data is represented by ASCII character pairs per byte of data. The more recent variant is Modbus TCP, which is essentially RTU encapsulated in a TCP packet.

Modbus is popular because it is simple, it is inexpensive to implement and support, and it is a widely used standard with no license strings attached. Modbus is used to communicate data points between machines or between machines and servers. Each data point is officially a 16-bit integer register, although many devices support IEEE-754 floating point implemented as a pair of registers. (There are also provisions for a 1-bit data entity.) Each register has an address. The Modbus register address that points to an integer is very much analogous with the SNMP OID that points to an integer. There is literally a one to one mapping between SNMP OID's and Modbus registers in Control Solutions products.

Modbus is a master-slave protocol. There can be only one master on the Modbus network. The "master" polls all of the "slave" devices periodically. The master can read or write registers in the slave device, therefore reading inputs or controlling outputs on the slave device.

What is a Babel Buster? If you got this far, odds are you may be interested in somehow converting Modbus to SNMP. However, Babel Buster is more than just a protocol converter. And although we refer to this group of products as "gateways", it is technically not a gateway either. It is actually a self contained mini-server, and is a Posix server. On the Modbus side, it autonomously polls one or more Modbus devices collecting and storing its data. On the SNMP side, the collected data is available for retrieval by an SNMP Get, and "threshold rules" are constantly monitoring selected data to see if values have crossed a given threshold. If the threshold is crossed, this constitutes an "event" which in turn can do several things including generate an SNMP trap.

How do I monitor power equipment with Modbus? Many of Control Solutions' products include a Modbus RTU master, including Babel Buster SP. Configuring the Babel Buster means telling it what register(s) to read in the slave device(s). These "maps" are simply a table you enter through one of the web pages in the Babel Buster. An example is illustrated below.

We are going to tell Babel Buster to read a slave device whose device address or unit ID (4) is set to 12. The equipment manufacturer's Modbus interface documentation tells us we should read holding register (1) number 284 (3) and it will be an integer (2). This register is scaled X10, meaning a register value of 100 really means 10.0 degrees. Therefore we enter a scale factor (5) of 0.10. We are going to store our native copy of this data in our local register (6) number 1015, and we will give it a label (7) of "Oil Temperature" for our reference.

The only thing that was up to us here was picking local register 1015 to store our copy of the data. The rest of the information about which register to read comes from the equipment manufacturer. You will need to obtain documentation from the equipment manufacturer that provides their Modbus register mapping. The document itself may come under a variety of titles, but what you want to ask for is simply the "Modbus register map".

Once we have done this, and Babel Buster is online and the equipment is online, we should be able to look at the MIB View of system data, and see something like the example below. If you leave the scale factor at 1.0 above, and store the result in an integer register, which maps to an OID in the integer data table, you can read raw integer data with an SNMP Get. If you scale the data and save it as floating point, the SNMP Get will return an ASCII string (DisplayString) with the ASCII representation of the number.

How do I monitor dry contacts? Several Control Solutions products interface to physical I/O devices such as temperature sensors or switch contacts. You simply connect the contact to be monitored to an input, configure that input via the web page(s), and its data becomes available in the MIB. You can set up threshold rules to generate traps based on the contact opening or closing.

Which Control Solutions product should I use?

You should consider Babel Buster SP if you want to monitor a Modbus device via SNMP. You should consider the other products listed below if you want to monitor physical I/O points. You should consider AddMe Jr. or AddMe III if you want both a Modbus master and some physical I/O at the same time.

Product comparisons: 

The register map capacity in Babel Buster SP is rather large; however, the device is resource limited such that attempting a MIB walk from the enterprise level will run out of resources if the table is not set at a practical limit of 300-400 elements. You can set the size of the tables on the device's network configuration page.

For more product information specific to SNMP monitoring, go to the SNMP page!

What about management software? Control Solutions does not produce a front end network management software package. A number of our customers use HP OpenView, and there are a number of other options. If you are looking for options, Google "SNMP management software" and you will get more options than you probably want to look at.

One thing that is worth pointing out, however, is that each i.CanDoIt® based device is capable of a limited set of features usually left up to the front end software. The i.CanDoIt server can, on its own, send email notifications (including to your cell phone), and is also capable of keeping history log files, which may also be emailed as attachments periodically and automatically.

What is RS-485? This is the electrical specification most often used to transmit and receive Modbus RTU protocol. It is a multi-drop interface capable of connecting multiple slaves to a single master. Each device on the RS-485 network only drives the network when it is its turn. It is not sophisticated enough to do CDMA like Ethernet, hence the requirement for a master-slave polling scheme.

There are some electrical restrictions you should be aware of. Unless otherwise explicitly stated, the network load represented by each device is such that a maximum of 32 devices may be interfaced on a single RS-485 network.

It is very important to observe proper wiring of an RS-485 network. It must be connected in daisy chain fashion from one device to the next. You cannot home-run wiring from devices to a central location. The maximum network "stub" is very limited.

RS-485 is a differential signal that requires 2 conductors, neither of which are ground. You need a third conductor for ground, unless you have a solid common ground between equipment which will result in no more than 7 volts of common mode voltage between devices. Shielded cable is often recommended.

What about Modbus TCP? Modbus TCP is really just Modbus RTU encapsulated in TCP for transmission over Ethernet. Any web enabled Control Solutions product also includes Modbus TCP. Using it to collect data to be accessed via SNMP works the same as with Modbus RTU. You set up the register maps the same way. The only significant difference is replacing unit numbers and serial baud rate with an IP address instead.

Why is it called a Babel Buster? We often find that the sentiment regarding the broad mix of protocols used by various equipment is, "it's a lot of babel". So, having a sense of humor about it, and inspired by a well known comedy movie, we decided to name it Babel Buster. Aside from that, we also think you are more likely to remember "Babel Buster" than "Model 1450" or something equally bland. If you Google "1450" you get 51,000,000 things. If you Google "Babel Buster", you immediately find Control Solutions at the top of the list (in fact the entire first page of hits is nothing but Control Solutions, last we checked).