Data Highway Plus Protocolsby Stephen Byron Cooper
Data Highway Plus is a networking system created by the Allen-Bradley Company (now called Rockwell Automation). It is a networking protocol suite for manufacturing automation. It is common practice in networking technology to express protocols in layers of functions. The Data Highway Plus suite contains three layers. These are the physical layer, the data link layer and the application layer.
The Data Highway Plus physical layer specifies the hardware used in the Data Highway Plus network, specifically the cable. The Data Highway Plus network is organized as a “bus.” The bus topology is one wire to which many nodes are connected. The cable specified for this network is baseband shielded twin axial cable.
Data Link Layer
Data Highway Plus uses a token passing system called “floating master.” The holder of the token is the master on the network and does not need to check if the cable is available for transmission, because possession of the token gives control of the cable. If a node needs to transmit to another node, it waits until the token is received. If it receives the token, but does not need to transmit any data, it passes the token on to the next node.
When data is sent, the sender waits for a response from the receiver before sending the next segment. This response comes in the form of an “ACK” (which means “got it”) or a “NAK” (which means “I can't take it”). A message that is replied to by a NAK will not be resent. If the first transmission does not receive an ACK or a NAK within a specified period, the message is resent. A message can be sent a maximum of three times.
The Data Highway Plus protocol specifies a limited set of message types. These messages contain a header with the message type name in it. The message may either be control messages, or may carry application data. Messages are ASCII (American Standard Code for Information Interchange) control codes -- these are non-printable characters represented by a number in the ASCII table (they do not represent numbers).
The purpose of the Data Highway Plus protocol is to carry instructions to a PLC (programmable logic controller) that controls a piece of industrial machinery. The protocol specifies a message packet format to carry these commands. The message header contains a byte each to explain the source of the message, the destination of the message and the command to be executed. There is also, optionally, a function byte. The reply to this command will reply in a packet headed by source, destination and command bytes, followed by one or two bytes for status codes. Commands and responses are matched together by a TNS ("transaction") code. The response to a command will contain the TNS of the command to which it is a reply. Both the command message and the response message can optionally contain a data section.
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