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Disclaimer: This document is written in the hope that you can utilize for your own education to gain knowledge of PLC systems (should you decide to utilize this document). Although I believe the information in this document to be accurate, it is YOUR responsibility to verify this information before implementing it in any way, especially when damage to personnel or equipment could result. By continuing to read this document, you agree to hold no one who writes, modifies, or distributes this document liable in any way (even negligence). Due to the wide variety of plant applications, some of the examples in this document may be prohibited at your location, or could cause damage to equipment, or harm personnel.

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Remote I/O Considerations for the SLC

When running remote I/O on the SLC, there are a few considerations to think about.

The SLC processor can only see I/O in it's local chassis, or an extended local chassis. Therefore all Remote I/O appears to be in the slot number the 1747-SN module is located in.
The 1747-SN module converts the old PLC-5 style addressing into an address the SLC can understand.

The next few pages will explain how this takes place.

Review of PLC-5 Style addressing:

Let's understand some terminology from the old PLC-5 style addressing scheme.

Bit: The smallest unit of information in the PLC. The only allowed states are On or Off.
Word: 16 Bits
Group: One word of input AND one word of output (Input/Output pair)
Rack: A memory location consisting of 8 Groups

Conversion to SLC Addressing:

Using the Remote I/O Configuration worksheet, you can see that each scanner module supports up to 4 Racks (0 to 3). Since each of these Racks contain 8 groups, the scanner contains 32 words of input and 32 words of output. These 32 words of input and output appear to the SLC in the slot number the scanner resides in.

For example, if a switch is in Rack 3, Group 7, and bit 15... The address in the SLC would be I:1.31/15 in the SLC. (Refer to the Remote I/O Configuration worksheet in appendix C of the 1747-SN manual)