With the 10Base-I interface, Hirschmann is showing how devices can exchange Ethernet telegrams via a passive bus system. The interface enables the simultaneous use of IT technologies and fieldbus components, at the same time it is expanding Ethernet’s area of use to systems which are of a limited size spatially.

The volume of data to be transmitted in industrial communications is rising constantly. And not only in large networks, systems which are only of a small size spatially are also affected. The consequence: In many cases, the transmission systems in use at present are reaching the limits of their performance. Hirschmann has developed the 10Base-I interface for Ethernet especially for this purpose.

Passive Bus Systems – Proven Technology

Many fieldbuses use twisted pair cables as transmission medium. This type of cabling proved itself over many years in automation.
Comparable passive bus systems also exist for Ethernet, 10Base-2 and 10Base-5. However, the coaxial cables normally used (as the transmission medium) are considered outmoded. In the interim, networks with active distributors such as hubs and switches have replaced passive bus systems of this type.

However, there are applications for which active distributors are less suitable – whether for cost reasons or for reasons associated with the reliability of the system overall. These include, for example, connecting components within switchgear or in machines or system components of a very small size spatially.

A Physical Layer for Ethernet, appropriate to industrial use, is now available in the 10Base-I interface developed by Hirschmann, which also makes the transmission technology (developed for fieldbuses) suitable for use by Ethernet. It requires no active distributors at all and permits simple, bus-type cabling. Components such as cables, connectors, T-pieces and electronic modules from the world of fieldbuses can also be transferred, as well as standard Ethernet Controllers.

Web-Based Tools Can Be Used

As a 10Base-I interface transmits the protocols above Layer 2 unchanged, all the advantages of Ethernet communications are also available now for applications which were previously reserved for the fieldbuses.
The interface permits a network size of a maximum of 100 m with 32 subscribers or a maximum of 40 m with 64 subscribers. Cables complying with the Profibus-12 Mbit/s specification are suitable for use as transmission medium.

But how can fieldbus transmission media transmit Ethernet data reliably? This can be clarified by taking a look at the function blocks involved, which are needed for connecting a device to an Ethernet network (see also block circuit diagram). The Media Access Controller (MAC), the Media Independent Interface (MII), the Physical Layer (PHY), the transceiver plus the cable and the bus termination (BA) play an important role. The function blocks MAC and MII come from the Ethernet world. The tranceiver, TP-cable and BA come from the fieldbus world. The central function block, which connects the two worlds, is the PHY. The most important tasks of the PHY include the adaptation of the Ethernet signals to the fieldbus line and the detection of collisions on the twisted pair cable.

´Trick´ With The Preamble

A significant element of the CSMA/CD access procedure (Carrier Sense, Multiple Access, Collision Detection), used with Ethernet, consists of collision detection. A collision happens when two devices access and transmit the network simultaneously and transmit. The devices involved must detect collisions of this type to discontinue the transmission procedure in a defined way and repeat it later.

An Ethernet data packet complying with IEEE 802.3 is set up that the data from two stations only differ reliably at the earliest in the source address. By this time 20 bytes may already have been transmitted before the collision is detected. The time elapsing in this way is above the time limit defined in IEEE 802.3 for detecting collisions on the transmission medium.

Does that mean that this is a possible disqualification criteria for the above mentioned method of solution? Not at all: In 10Base-I, the PHY replaces the first bytes of the preamble, which consists of a 10 sequence as standard, with a variable bit pattern. The PHY selects the bit pattern in such a way that it is different for all the devices involved in the transmission of data. The PHY restores the 10 sequence of the preamble by receiving data packets for reasons of compatibility. This means that from the point of view of the MAC, the preamble corresponds to the IEEE 802.3 standard.

Dipl.-Ing. (FH) Harald Wessels is Strategic Marketing Manager