The 5/04 had even more memory and a faster communications platform called DH+ (“Data Highway Plus”). The 5/02 and 5/03 platforms had DH485 (“Data Highway”) communications, the 5/03 was the first processor where logic could be edited online in real time without having to place the processor in program mode and download. Successive processors had more memory, faster speeds and enhanced communication capabilities. The rack allowed I/O cards to be mixed, having digital, analog and special purpose I/O such as thermocouples and High-Speed Counters. This processor was used in a rack which also had a built in power supply. The first rack-based SLC was the SLC5/01. It only had 1K of memory and was intended for use in simple applications. The first SLC, Known simply as a SLC500, was an integrated platform containing the CPU, power supply and IO all in one package. Indeed, SLC is an abbreviation for “Small Logic Controller”. This platform was physically smaller than the PLC5, with an abbreviated instruction set. It is a large rack system, with about twice the horizontal dimension of the SLC500. PLC3s were released in the 1980s also, followed by the PLC5, which is still used today, though not widely. The earliest of this family that I have ever seen was the PLC2, which was made in the 1980s and is now obsolete. The first A-B PLCs were of the PLC5 family. I have chosen to start with Allen-Bradley since it is the platform I am most familiar with and the most widely used in the US. At the same time, it is sometimes necessary to describe specific brands and platforms to provide information to my readers this post is the first in a series of articles I am writing on PLCs, or Programmable Logic Controllers. So far I have refused to place advertisements for any manufacturer’s on this site, though I have been asked several times. Most readers of this blog know that I go out of my way to be non-brand-specific and unbiased in my presentation of automation topics.
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