Allen Bradley is one of the large PLC providers for North America, and as such is considered one of the default standardizations.
Most everything with AB is proprietary, but their MicroLogix software solution is free from their website if you are already registered.
This is their most basic of simple PLC offerings, with many limitations compared to their others.
Even so, if all you need is a simple solution for a simple problem and insist on staying with the AB solutions offered, this may fit your needs.
Develop ladder logic for the Micrologix family using three software tools. These are:
RSLinx handles communications between RSLogixMicro and the PLC.
RSlogixMicro is a development environment for the MicroLogix 1000 and MicroLogix 1100 processors.
RSEmulate emulates PLCs MicroLogix and SLC families.
To download the software:
1. Register on Rockwell automation
Must be registered, registration can do so at this link and is free to download, but must access your account.