Purchase now: Allen Bradley 1747-SDN DeviceNet Scanner
Purchase now: Atlantechdrives provides you the 1747-SDN, a high-performance DeviceNet Scanner Module developed by Allen Bradley for the SLC 500 automation system. As manufacturing lines expanded in the late 90s and early 2000s, running individual wires from every field sensor back to the main PLC cabinet became unmanageable. The 1747-SDN solves this by acting as a network master (Scanner), allowing the SLC processor to communicate with up to 63 remote slave devices—such as distributed I/O blocks (POINT I/O), variable frequency drives (PowerFlex), motor starters, and smart sensors—over a single, robust 5-wire DeviceNet trunk line. This decentralization dramatically reduces cabling costs and simplifies machine troubleshooting.
Technical Specifications of the 1747-SDN
The 1747-SDN operates at the Data Link layer of the CAN (Controller Area Network) protocol. It constantly polls the network, gathering input data from field devices and pushing output commands from the SLC CPU. The module requires configuration via RSNetWorx for DeviceNet to establish the network «Scanlist.»
- Part Number: 1747-SDN
- Module Type: DeviceNet Scanner (Master) Module
- Compatible System: SLC 500 (1746 Chassis)
- Supported Baud Rates: 125 Kbps, 250 Kbps, 500 Kbps
- Maximum Network Nodes: 64 (Scanner uses Node 00, leaving 63 for slaves)
- Backplane Current Draw: 500 mA at 5V DC
- Network Power Requirement: Requires external 24V DC DeviceNet power
- Data Mapping: M0 and M1 memory files (up to 395 words)
Expert Tips: Mastering M0 and M1 File Transfers
The biggest hurdle engineers face with the 1747-SDN is moving the data into the processor. Expert Tip: Standard SLC discrete modules map directly to the I: and O: data files. However, a heavily loaded DeviceNet network brings in too much data for the standard I/O tables. The 1747-SDN places the network data into specialized memory buffers called M0 (Output data) and M1 (Input data) files. Because the SLC processor takes a significant performance hit if it addresses M-files directly in every ladder rung, you must write a subroutine using the COP (Copy File) instruction. This instruction should bulk-copy the data from the M1 file into standard N (Integer) files at the start of the scan, and copy N files out to the M0 file at the end of the scan. This technique drastically improves processor scan time and prevents communication timeouts on the DeviceNet trunk.
Maintenance and Terminating Resistors
DeviceNet is a linear trunk-and-drop topology based on high-frequency CAN signals. To prevent signal reflection (which causes packets to bounce back and collide with new data), the network must have exactly two 121-ohm, 1/4 Watt terminating resistors installed—one at the extreme physical beginning of the trunk line, and one at the extreme end. If a machine experiences intermittent «Bus Off» faults on the 1747-SDN (indicated by a solid red network LED), use a multimeter to measure the resistance across the blue (CAN Low) and white (CAN High) wires while the power is off. It should read exactly 60 ohms (two 120-ohm resistors in parallel). If it reads 120 ohms, a resistor is missing or a cable is broken.
Frequently Asked Questions (FAQs)
Q: If the 1747-SDN module breaks, can I just plug a new one in?
A: No. The Scanlist (the map of all 63 slave devices and their data sizes) is stored inside the module’s non-volatile memory, not in the SLC processor. If you replace the hardware, you must connect to the network using a 1784-U2DN adapter and RSNetWorx for DeviceNet software to download the saved Scanlist into the new module.
Q: Does the 1747-SDN power the network?
A: No, the module itself only draws power from the SLC backplane. You must provide a dedicated, isolated 24V DC power supply wired to the red (V+) and black (V-) wires of the DeviceNet cable to power the transceivers of the slave nodes.
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Integrate DeviceNet field devices into your SLC architecture by installing this scanner in a 1746-A7 7-slot chassis.
