Hardware Comparison: SLC 500 Controllers vs. CompactLogix L5370/80

The Allen-Bradley SLC-500 product line was introduced nearly thirty years ago. The product line has reached Active Mature status, with many modules at End of life or discontinued. Active Mature products are still fully supported; however, you can gain value by migrating to a new product of family of products. In this post, we compare the SLC-500 family to the preferred migration product family, the CompactLogix processor.

When Allen-Bradley released the CompactLogix 5380 controller it boasted significant performance improvements (+20% capacity and 5-20x faster scan times) over previous CompactLogix models. This performance is orders of magnitudes faster and more capable than the SLC-500 platform. For an in-depth comparison, we will compare system architecture, memory, scan time, I/O capability and power connections.

Architecture:

Memory:

For memory usage comparison, a good “rule of thumb” is to allow for approximately 10-12x program size when converting from SLC-500 to Logix based programs. For example, a full 32 KB SLC program converted to a Logix program will consume approximately 360 KB in the Logix controller. For migration purposes, 32K or less SLC programs will fit in any Logix controller. For 64 KB SLC programs, select a Logix controller with at least 1Mb of memory.

Scan Time:

The SLC-500’s 16-bit architecture is based on executing logic contained in program files. Scan time can be roughly calculated based on logic program size. CompactLogix controllers use 32-bit operations with logic execution based on based on tasks, programs and routines. In the Logix controller, each task is assigned a priority level; higher priority tasks interrupt lower priority tasks. The continuous task has the lowest priority. Scan time of Logix processors is based on the configuration of Tasks and priorities. Overall, expect a decrease of somewhere from 50% to 80% in overall program scan time when using a Logix controller as compared to a SLC controller.

I/O Capability:

The SLC-500 system has a maximum I/O module capacity of 30 I/O cards and 4,096 total I/O points.

In the CompactLogix 5380 controller, Compact 5000 I/O modules are installed to the right of the controller as local I/O modules. A total of 31 modules can be installed in the local system.

The CompactLogix controller can also use the following I/O modules over an EtherNet I/P network:

• Chassis-based I/O – Compact 5000, 1756 I/O, 1769 Compact I/O, or 1746 SLC I/O modules
• In-cabinet I/O – 1734 POINT I/O or 1794 FLEX I/O
• On-Machine I/O – 1732E ArmorBlock I/O

Calculating an exact maximum of I/O points for the CompactLogix system depends on the controller’s maximum number of EtherNet I/P nodes, the type of I/O adapter selected, and the density of the I/O module selected.

For Example 5380 L350 controller with Point I/O modules on EtherNet I/P:

• 5380 L350 can support a maximum of 120 EtherNet I/P nodes.
• 1734-AENTR Point I/O adapter can support a maximum of 63 modules with One (1) I/P address.

Assuming 4 points per module, this would be 120 x 63 x 4 = 30,240 I/O Points !!!

The above limit is somewhat theoretical because any real application would have EtherNet I/P nodes dedicated to other devices such as HMI’s, Variable Frequency Drives, etc. It should be clear, however, that even the smallest CompactLogix controller has more I/O capacity than the largest SLC-500 system.

Power Considerations

SLC 500

The SLC 500 requires a power supply module attached to the leftmost of the chassis. This power supply provides power to al modules in the chassis via the backplane. The SLC 500 does not have a common Sensor / Actuator (SA) bus and requires an additional, external power supply for I/O modules and sensors such as analog modules and field devices. This requires “Field Power” to be connected to each module individually.

CompactLogix 5380

The Logix5380 controller requires an external 24VDC power supply connected to the MOD power terminals to supply power to all the connected modules. Unlike the SLC, the Logix 5380 controller does not require a chassis but rather has backplane connections at the side of each module.

Also unlike the SLC, the Logix 5380 has a common Sensor/Actuator (SA) bus that distributes sensor/actuator power to all modules connected to the backplane. This simplifies the wiring, especially if all I/O devices utilize same voltage. The SA power bus can be separated with a Field Potential Distributor Module If the I/O devices use two different voltages (i.e. 24V DC, 120V AC.) This allows two separate power supplies to be connected to banks of modules through isolated Sensor/Actuator buses.

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