8.2.0.1 - Data Highway

Allen-Bradley has developed the Data Highway II (DH+) network for passing data and programs between PLCs and to computers. This bus network allows up to 64 PLCs to be connected with a single twisted pair in a shielded cable. Token passing is used to control traffic on the network. Computers can also be connected to the DH+ network, with a network card to download programs and monitor the PLC. The network will support data rates of 57.6Kbps and 230 Kbps

The DH+ basic data frame is shown in Figure 22.22. The frame is byte oriented. The first byte is the ’DLE’ or delimiter byte, which is always $10. When this byte is received the PLC will interpret the next byte as a command. The ’SOH’ identifies the message as a DH+ message. The next byte indicates the destination station - each node one the network must have a unique number. This is followed by the ’DLE’ and ’STX’ bytes that identify the start of the data. The data follows, and its’ length is determined by the command type - this will be discussed later. This is then followed by a ’DLE’ and ’ETX’ pair that mark the end of the message. The last byte transmitted is a checksum to determine the correctness of the message.