For a DMC engineer it is not atypical to be faced with outdated, legacy technology. Maybe a customer has come to us to upgrade a system that an intern did in the 90's; maybe a company's engineers have built up a system over the past decade and need help configuring or adding more pieces to it; or maybe a factory has used a controller that was made when you were in middle school and all of a sudden that controller breaks down and needs to be replicated and replaced. Regardless of the situation, DMC engineers need to be able to find the tools to connect to, read from, write to, and add on to whatever equipment someone may have.
Finding information on old industrial hardware, unfortunately, isn't as easy as looking up technical support on your new Dell computer. Chances are that if you are looking at something really old, it will fall into 1 of the 4 oldest manufacturers (in order with oldest first): Modicon, Allen Bradley, General Electric, and Omron. All of these manufacturers are still around today, in addition to many others, and make a wide array of products from basic relays to PLC's and HMI's to other various controllers. It is important to remember though that product lines can and have shifted owners many times through acquisitions and takeovers. This means even if something is labeled as a "Modicon" you might actually have to go somewhere completely different to find the support you need.
A recent example from a project I was involved with: I needed to replace a Modicon branded HMI. Modicon is currently owned by Schneider Electric (which might also be referred to as Telemecanique or Square D, which they acquired in '88 and '91). However, this HMI was actually manufactured by Cutler Hammer and simply labeled as a Modicon. Cutler Hammer was acquired by Eaton in 1978 and reorganized in 2003.
You can see how quickly this web gets unbearably complicated, but here are some tips that I have learned that will help your search:
If at all possible, get the exact part number of what equipment you are working with. Often a Google search on that alone will turn up forums (Control.com and PLCtalk.net are both good ones) where others have done the legwork of finding documentation, manuals, and software for you.
Ask anyone who may have been involved with the project originally if you can. It could be that there are documents, programming software, and connection cables sitting around somewhere, maybe stashed in someone's desk or storage. It never hurts to ask.
Contact the original manufacturer. It might take a few phone calls to get the right technical support line, but just be clear that you are looking for information on a legacy system. These companies pay engineers to answer questions, and often times, even if they are working for a different corporate master, these engineers are the same ones who built the stuff originally.
Contact the current manufacturer. This applies if a product line was sold off or if the unit was branded by a different manufacturer. A lot of times the engineers responsible for the product, or at least ones who worked with it when it was being made, will follow the sale. Again, these engineers are paid to help you, and if you ask around for the right person, they usually can direct you to someone who is an expert on your hardware.
Don't be afraid to ask for software, programming cables, etc. Yes the license might have cost $10,000 when it was released in 1985, but often the technical support engineer can at the very least get you a demo version or let you borrow some connecting hardware. Sometimes you get lucky enough that all of the software you need is old enough that they can give you a copy for free.
Talk to sales engineers at the company that sells the newest version of the product. Often times they have had similar customers asking similar questions so they can tell you where to look, and, again, you could be lucky enough that they sold the original product years ago and they know how to use it. Some sales engineers in the automation world just want to make sales, but a lot more of them are very, very helpful and want you to succeed.
This strategy has worked well for me over time and I think that experience in finding answers for legacy systems is a strength of DMC engineers. The most important piece to remember is to explain your predicament clearly and ask questions until you find the right person.
Here is a walkthrough on connecting to an AB PLC-5.