Understanding Modbus RS-232 And Modbus RS-485


  1. Introduction to Modbus
  2. Serial-to-IP Modbus Bridge
  3. Modbus Master/Slave, Registers, & Coils
  4. Remote Monitoring and Control with Modbus
  5. SNMP-Modbus Protocol Converter
  6. Track Generator Output with Modbus
  7. Connect Modbus Controllers to Your SNMP Manager
  8. Central Alarm Aggregation For MODBUS, TL1, and...

Brief Overview of Modbus RS-232 & Modbus RS-485 in Remote Alarm Monitoring.

Modbus Remote Monitoring and Control traditionally uses RS-485 and RS-232 transport for collecting data from Modbus slaves using established protocols. Of the two, Modbus RS-485 is more common than RS-232 due to its support for multi-drop communication. These transports are not in any way limited to Modbus, of course. They're found in a wide variety of other applications. Modbus is just one possible protocol that can be sent via dedicated serial connection.

Modbus is a protocol that (traditionally) uses serial communication lines. These serial lines connect the master to Modbus slave devices for collecting register and coil information. Modbus/TCP was introduced for LAN installations but Modbus over traditional serial networks is still found in many industrial applications. A Modbus master can communicate with up to 247 Modbus Remote Telemetry Units (RTUs) or Intelligent Electronic Devices (IEDs). It uses a unique Modbus address assigned to each RTU.

Differences between Modbus RS-232 and Modbus RS-485 Capabilities.

Modbus RS-232 Allows Concurrent, Two-Way Flow of Data.
Modbus via RS-232 sends data in the form of time-series of bits. It is a standard for communication between data terminal and data circuit termination equipment. Transmission (Tx) and receipt (Rx) for data occurs on different circuits when using Modbus RS-232 lines. That means that data is able to flow both ways at the same time.

Modbus RS-485 Indicates Values Using Differences in Voltage.
RS-485 is similar, but distinct, from RS-232. This two-wire, multipoint connection communicates data by indicating values by sending different voltages across the two wires. These differences between these voltages are related to one and zero values, which make up the Modbus RS-485 communications.

Legacy Gear: Using Modbus to Monitor a Generac Controller in Your Network:

One DPS client from a major telco launched an HVAC-controller project with us. They had standardized on the NetGuardian 480 RTU for all new shelter builds going out. We incorporated support for MarvAir so they didn't have to buy another controller. They wanted us to interface RS485 Modbus to their Generac controller. An alternative, of course, is to use our D-Wire propane monitoring solution to report back to the NetGuardian 480. This client also had interest in battery monitoring.

Modbus for remote monitoring and data acquisition of client-premise devices:

Another client needed remote monitoring and data acquisition for devices they sell to their customers. Their primary business is measuring liquid in tanks (refineries, petrochemical plants, etc.). They have digital inputs (magnetic switches) in their existing management system.

For decades, their customers have put a mechanical device on the tank. The problem is that wiring is expensive to install and maintain. Over the years, they've installed several radio systems, but the radios are typically 900mhz/2.4ghz spread spectrum.With GSM reporting from a modern monitoring device, the options multiplied. We could enable customers to get the data not just in their own office, but also from almost anywhere else in the world.

The tank sensors themselves are analog. There is some intelligence in the enclosure that converts analog to Modbus RS485 or RS232. This client came to DPS looking for one small, GSM-capable, and inexpensive device on each tank. They would also need a central collector of some kind (T/Mon).

As you can see, Modbus projects tend to involve a lot of other protocols and equipment.

Working with Modbus in an increasingly IP & SNMP driven world.

If you work in telecom, you probably have trouble with any remote RS485 Modbus equipment. The world has shifted more and more to IP, T1, fiber, and other modern transport standards. Dedicated serial lines are tough (and expensive!) to maintain. Wouldn't it be nice to route serial traffic over IP? Then you could keep you old-but-still-functional Modbus gear but abandon the aging serial transport.

One excellent new solution to this problem is a remote Modbus-to-SNMP mediator. This is a fairly simple box that takes in serial Modbus (RS-232 or RS-485) and converts the message to standard SNMP traps. These can be received by your SNMP manager. Because the mediator box is modern, you benefit from recent innovations like SNMPv3. Unlike earlier versions of SNMP, SNMPv3 is secured with encryption. For many organizations, this is level of security is needed whenever data will be sent via IP.

If serial transport isn't your problem, but you still want to integrate Modbus into your SNMP umbrella, you can take a different route. This involves using a Modbus mediating master station to convert inbound Modbus to SNMP. This single stream of usable SNMP traps can be easily sent to your preferred SNMP manager. This master-station solution will generally be much less expensive than installing individual mediation devices at each of your remote sites.

Work with a capable manufacturer to get the exact Modbus solution you need.

If you work with any kind of legacy equipment, including serial Modbus, you need very specialized gear to make it work in today's networks. That's where your choice of manufacturer comes in. Some just sell commodities. If you need a very unique solution, you're out of luck.

What you need to get a custom Modbus solution is a manufacturer who can take a proven design and adjust the design to fit your need. Look for a company that does its own engineering and manufacturing. They need in-house teams in those areas to be responsive to your special requests. Otherwise, they'll have a really hard time meeting your specs without an insane minimum order. A company that farms out everything except for their "core competency" simply can't react with the speed that an integrated company can. You need a manufacturer whose core competency is being flexible and giving you exactly the Modbus solution you need.

Are you working on a Modbus project? This SCADA white paper is a great place to start learning:

Related Products:

To see additional information related to a "Modbus HMI", please visit the Modbus page.