Rockwell Automation Publication 1766-UM001O-EN-P - September 2021 179
Appendix E Connect to Networks via RS-232/RS-485 Interface
DH-485 Communication
Protocol
The DH-485 protocol defines the communication between multiple devices
that coexist on a single pair of wires. DH-485 protocol uses RS-485 Half-Duplex
as its physical interface – RS-485 is a definition of electrical characteristics; it is
not a protocol. RS-485 uses devices that are capable of co-existing on a
common data circuit, thus allowing data to be easily shared between devices.
The DH-485 network offers:
• interconnection of 32 devices
• multi-master (peer-to-peer) capability
• token passing access control
• the ability to add or remove nodes without disrupting the network
• maximum network segment of 1,219 m (4,000 ft.)
The DH-485 protocol supports two classes of devices: initiators and
responders. All initiators on the network get a chance to initiate message
transfers. To determine which initiator has the right to transmit, a token
passing algorithm is used.
Control of message transfers on the DH-485 network is performed by rotating
the token along the nodes on the network. A node holding the token can send a
message onto the network. Each node is allowed a fixed number of
transmissions (based on the Token Hold Factor) each time it receives the
token. After a node sends a message, it passes the token to the next device.
The allowable range of node addresses is 1…31. There must be at least one
initiator on the network, such as a MicroLogix controller, or an SLC 5/02 or
later processor.
DH-485 Configuration Parameters
When MicroLogix communications are configured for DH-485, the following
parameters can be changed:
See Software Considerations
on page 181 for tips on setting the parameters
listed above.
Devices that use the DH-485 Network
In addition to the MicroLogix controllers, the devices shown in Table 21 also
support the DH-485 network.
DF1 Full-Duplex Configuration Parameters
Parameter Options
Baud Rate 9600, 19.2K
Node Address 1…31 decimal
Token Hold Factor 1…4
Table 21 - Devices that Support DH-45 Network
Catalog Number Description Installation Function Publication
Bulletin 1762 MicroLogix 1200 Series A or later These controllers support DH-485 communications. 1762-UM001
Bulletin 1763 MicroLogix 1100 Series A or later These controllers support DH-485 communications. 1763-UM001
Bulletin 1747 Processors SLC 500 Processors SLC™ Chassis These processors support a variety of I/O requirements and functionality. 1747-UM011
1746-BAS BASIC Module SLC Chassis
Provides an interface for SLC 500 devices to foreign devices. Program in BASIC to
interface the 3 channels (2 RS-232 and 1 DH-485) to printers, modems, or the
DH-485 network for data collection.
1746-UM004
1746-PM001
1746-RM001
1784-PKTX, 1784-PKTXD PC DH-485 IM PCI Computer us Provides DH-485 using RSLinx. 1784-UM522
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