Replace a Fixed SLC-500 (1747-Lxxx) with a MicroLogix

Fixed SLC-500 in serviceRecently a client called asking what I thought would be the best PLC to replace a Fixed SLC-500. Specifically he was looking to replace a 1747-L40A.

To pick a replacement we first needed to understand what this part number represents, and we covered this in detail in our last article here.

We found that the 1747-L40A consisted of (24) 120VAC inputs, (16) AC/DC relay outputs, and was powered by 120VAC or 240VAC. With the these details in hand we can now go about choosing a modern PLC replacement for the 1747-L40A.

When replacing a Fixed SLC-500, I typically recommend a MicroLogix as the replacement for these two reasons:

  1. The MicroLogix is based on the SLC-500, and therefore the majority of code written for the Fixed SLC-500 will import directly into the MicroLogix.
  2. The programming software for the SLC-500 is the same programming software used for the MicroLogix, however an updated version may be needed if the client’s existing RSLogix500 is older than the chosen MicroLogix replacement .

MIcroLogix 1200 FrontIf the client’s RSLogix500 is too old to program the selected MicroLogix replacement, we’ll need to know if the client has a Rockwell support contract in place. If he does, he’s entitled to free updates and can download the latest version of RSLogix 500 that supports the latest SLC-500’s and MicroLogixs.

However, if the client doesn’t have a support contract the least expensive option would be for him to purchase a new copy of the RSLogix Micro Starter programming software, which is roughly $165.00. RSLogix Micro Starter looks and feels identical to RSLogix 500, but it limited to the MicroLogix family.

Note: If the client were to choose a MicroLogix 1000 or 1100, he could use the free RSLogix Micro Starter Lite software – more about that here.

With programming software and conversion discussed, it’s now time to pick an actual hardware replacement with (24) 120VAC inputs, (16) AC/DC relay outputs. As we have already identified the MicroLogix as the most likely candidate, we’ll reference the Allen-Bradley (A-B) MicroLogix Selection guide here.MIcroLogix 1200 Selection Guide

Micrologix 1200 BottomOn page 58 of this selection guide we find the MicroLogix 1762-L40AWA also has (24) 120VAC inputs, (16) relay outputs, and is powered by 120VAC or 240VAC. This is identicle to the L40A we need to replace, but we need to be sure the relay’s have the same rating. On page 61 of the selection guide we find the relay rating chart, and it shows the relay outputs of the 17620L40AWA do have the exact same rating as the relay outputs on the 1747-L40A (found on page 1-8 of it’s user manual here.) So from an I/O and power standpoint, this model is an exact fit.

As far the size of the MicroLogix 1200, it’s 90mm (height) x 160mm (width) x 87mm (depth) dimensions is substantially smaller than the 1747-L40A’s 140mm (height) x 260mm (width) x 145mm (depth.)

The last item to consider is the communications port. The 1747-L40A came equipped with an RJ-45 jack and communicated using the DH-485 protocol over RS-485. The MicroLogix 1200 we’ve selected comes with a mini-din (round) RS-232 port which natively communicates via DF1, but can also be set to communicate with DH-485. Because of this difference, at a minimum the client will need a new programming cable as the 1747-PIC used with the Fixed SLC-500 can’t directly connect to the MicroLogix 1200. The Allen-Bradley serial cable for the MicroLogix 1200 is the 1761-CBL-PM02. But if you don’t have a serial port on your PC you’ll also need an Allen-Bradley 9300-USBS.

USB to Micro from AmazonThe cost of the PM02 ($77) and USBS ($ 144) is substantial. However, if you’re willing to use a third party cable you can get a “USB direct to MicroLogix Mini-Din” cable for under $50 here (pictured on the right.)

Aside from programming cables, if the Fixed SLC-500 is connected to an HMI deivce, like a 1747-DTAM or 2711 PanelView, or if it’s connected to a DH-485 network, you’ll need to add your MicroLogix replacement to that DH-485 network as well, and we’ll cover the hardware needed to do that tomorrow.

I hope this article about replacing a Fixed SLC-500 with a MicroLogix was helpful. If you have any comments or questions please don’t hesitate to use the “leave a reply” form at the bottom of the page to share your comments with me and our readers.


Shawn Tierney
Automation Consultant
Insights In Automation

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