Connecting computers and peripheral devices, Data Highway Plus (DH+) is basically a cable or wire, allowing communication between various devices. This cable, called the network medium, acts as the DH+ link between ControlLogix controllers, PLC and SLC controllers, and other devices that use DH + networks. Altogether, these devices active on this kind of network are called workstations, of which there can be up to 32 only in a single DH+ network.

But the cable is not the only thing a DH+ network needs to be complete; interfaces are needed as well. The Allen-Bradley 1756-DHRIO is just such an interface operating on DH+ networks. It can also work as a remote I/O communication interface module. Today, we will look into this interface and see how we can best utilize it.

How many devices can the 1756-DHRIO support?

The Allen-Bradley 1756-DHRIO can transmit and route a message through up to four (4) communication networks only. The analog card supports routing to up to three (3) chassis for a message. However, please note that the limits mentioned here apply only to message routing. This limit does not apply to the total number of networks or chassis being present in the system.

What types of communication can go through this device?

The 1756-DHRIO interface module is a DataHighway Plus / RIO module, supporting many types of communications. Among them the following four types are most prominent.

  1. Data Highway Plus (DH +) messaging
  2. Control and Information Protocol (CIP) messaging
  3. Remote I/O — the DH+ network is very versatile and allows you to connect to other networks (for example, ControlNet, Ethernet, or other DH + network). The interface allows the networks to send messages between devices.
  4. The interface acts as a scanner using the remote I/O feature. It transmits  discrete data and block streaming data through remote I/O devices, allowing connection to multiple remote I/O adapters.

How does the 1756-DHRIO work?

This is where it gets interesting. Actually, the operations of the interface module is not that much different from older PLCs (such as PLC5 or PLC3) devices — with some modern upgrades added. These upgrades are what makes it such a successful DH+ interface module.

Basically, the Allen-Bradley 1756-DHRIO creates a scan list and adds racks to the CLX I/O list. This way, you can configure it to scan discrete inputs and outputs. Afterwards, the message instruction can be added as triggered by the ladder diagram. Which is in fact the MSG executing the block transfer with the analog card.

However, you should remember that the CLX is different for analog cards in two ways. First of all, this doesn’t come with an I/O config software. That means that the users of this analog card must know how to read the data output. That is to say, you must know how to configure and interpret the data from the card.

Another thing is that there is a restriction upon how many analog modules you can run simultaneously. The present limit is 32 if you want any performance better than the previous kinds of interfaces. This is applicable to standard scenarios; however, special situations can be configured to have better performance yet more analog connections.

What about compatibility?

The good news is that the 1756-DHRIO and 1756-RIO interface modules are designed to operate in tandem with any type of discrete rack on remote I/O systems. That means you can hook it up with anything like 1771, 1794, 1746, multiple drives, PanelViews, various third-party equipment, etc.

The problem is that when you try to implement multiple analog modules or block transfers, it will become much harder to work out the interoperability and will almost certainly increase costs as well. For example, if you have the 1771 I/O, you can replace the 1771-ASB with 1771-ACN (R) 15; and then you can put the I/O on ControlNet. However, as we said, that will put a hole in your pocket.


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