How Does DH+ Compare to Ethernet?

Summary

When it comes to industrial communications protocols, one of the inevitable questions is: How does it compare to Ethernet? It’s a fair question, and, because it requires an answer, here’s my take on it.
 

Baud rate

Ethernet is exceptionally fast and gets faster every day. 1GB and 10GB are currently supported with 100GB on the horizon. DH+ supports a set of much lower baud rates; 57.6, 115.2K and 230.4K baud. Unlike Ethernet, there are no switches allowing devices with slower baud rates to participate in the network. All DH+ devices must use the same baud rate.
 

Cabling

CAT5 and CAT6 cabling are typical cables for Ethernet systems. CAT5 consists of four twisted pairs of copper wire. It’s probably the cabling you used at the office before you started using wireless connections. CAT5E and CAT6 are enhanced cabling types that provide higher bandwidth and up to 10Ghz speed. DH+ uses very simple cabling with one twisted pair surrounded by a shield.
 

Cable length

CAT5 is typically rated for less than 400 feet, far less than the 10,000 feet of trunk cable supported by DH+. This is the one area where DH+ has an advantage over Ethernet though Ethernet backbones render that benefit superficial.
 

Supported controllers

DH+ supports all the original Allen-Bradley PLCs, the PLC2 and the PLC3. Some controllers in the PLC5 line supported DH+, others support Ethernet while still others support both.

Number of devices 

DH+ networks are limited to 64 devices while Ethernet networks are virtually unlimited.
 

Device addressing

Ethernet uses the standard dot decimal addressing while a station number between 0 and 63 is used to address a DH+ device. It is illegal to have duplicate station numbers on either network.
 

Cost

With a limited number of possible customers, DH+ was a pricey solution in its day and, at first, only used by the biggest of Allen-Bradley customers. Ethernet, on the other hand, is a mass consumer product used by millions and it is inexpensive because of the commercial volume.
 

Network architecture

The network architecture for Ethernet is much more sophisticated than the network architecture for DH+. Ethernet uses a layered approach to communication using a physical layer, a data link layer, a network layer and an application layer. DH+ use few layers (physical layer, link layer and application layer) and the DH+ layers are much less sophisticated than Ethernet layers.
 

Network communications

Ethernet and its switch infrastructure allow maximum network throughput with many devices being able to send messages simultaneously. DH+ is more like the original version of Ethernet where only one device can talk while the rest listen.
 

Network mastership

Ethernet itself does not use the concept of a network master though some Ethernet application layer protocols do. With Ethernet, any device can send messages to any other device at any time. In DH+, there is a strict protocol where only the device holding the token can send messages. Devices receiving messages hold their response until the token rotates to them, at which point they issue the response to the message previously received.
 
Because DH+ is many years older than Ethernet, it's not not nearly as fast, sophisticated or flexible. However, it supported the manufacturing applications of its time very well.