With Ethernet, the rate of communication (the speed) is much faster, but the time span (the determinism) in which a response is expected is unpredictable. Modbus, being half-duplex, is highly deterministic but, being serial, it is quite slow, with data transmission rates as rates as low as 300 baud (and typically 2.4Kbaud).
#Ethernet status speed differnt serial#
One of the first, if not the first, industrial data network was Modbus, a half-duplex serial protocol devised by Hung Yu in 1979 for Modicon PLCs (hence the name MODiconBUS). The timing was very predictable (hence deterministic) but it was very slow. There was a defined amount of time to wait for any response after sending any message from the master. Originally, data transmission on the plant floor was done by proprietary twisted pair serial communications protocols (like RS-232 and RS-485), which were deterministic by nature since they were half-duplex. The time it takes for each packet to arrive at its destination should be determined…that is, the process must be deterministic. Because the time it takes for any given packet to arrive at its destination on an Ethernet LAN is not determined, it is difficult to guarantee real-time control functions over Ethernet. The industrial environment requires “real-time” information transfer. If this is a control variable for a high-speed CNC mill, packet loss and speed loss can be disastrous. With Ethernet, data can take variable paths and therefore variable times to travel from the sending node to the receiving node. (Managed switches help by directing the data packet to only the designated receiving node or nodes, but they cannot eliminate collisions.) They can cause both bandwidth reduction and data loss. Since Ethernet is a broadcast network, all of the packets go to all of the nodes, and collisions between data packets are a serious problem. Nearly all generations of Ethernet use the same frame formats and can be readily interconnected using bridges.īut here’s the problem. Adapters come programmed with a globally unique address. Data in the packet causes the node for which the packet is intended to wake up and grab the data. All packets are broadcast that is, they are sent to all nodes. As with other IEEE 802 local area networks (LANs), each Ethernet node is given a media access control (MAC) address used to specify both the destination and the source of each data packet. Switches, especially smart managed switches, have made it possible to have Ethernet networks consisting of a nearly infinite number of nodes.Įthernet nodes send each other data packets. Physical Ethernet networks consist of nodes, switches, routers and repeaters. So it is very important to understand what Ethernet is, how it works, and what it can and cannot do in the industrial environment.Įthernet is a family of frame-based, or data-packet-based, networking technologies that are part of the IEEE 802.3 standard. As long as it is the protocol of choice in IT, and the cost of even industrially hardened Ethernet switches and I/O are low, Ethernet will remain the communication protocol of choice for companies.
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