One point that should be mentioned is that just because you are using the Omron protocol doesn't mean you have to be using an Omron PLC.On the contrary... it is an open protocol so many vendors actually support it. This means, even more, that so we should know about it!
So, how do we use it? Let's see. First we need to figure out our setup. If we are using a single PLC connected to a single computer we will use what's called a 'single-link' system. This is the most common setup. We are using the computer to log data or just make simple changes to the PLC.
If we are using a single computer but are connected to multiple PLCs we will use what's called a 'multiple-link' system. Gee... the naming makes sense, doesn't it?? And by the way, as we noted, we can also connect multiple computers and multiple PLCs together to really have some good communications going on... but this is far less common.
So, how do we connect everything together so they can freely talk with each other? Well, we use the same 'stuff' as we would when using RS-232 or RS-422. Actually, we commonly talk over those communications
lines.
If we are using RS-232 then we are going to be limited to a maximum distance of ~45ft (15m) between the computer and the PLC... plenty of distance for most applications.
If we are using RS-422 then we have a much longer cable allowance...
we can use up to ~1500ft (500m). If you need to be farther than that distance from the PLC then a different technology is probably better for your application.
Why use RS-232 or RS-422? Simple, if talking with more than one PLC at the same time then use RS-422. If you're only talking with one PLC then use RS-232. The only exception is when you need to talk with one PLC but must be more than ~45ft (15m) away from the PLC.In that case RS-422 is also the best choice for your application.
So, what can we do with Host Link? Answer, what do you want to do? We can read memory out of the
PLC. We can write data to the memory of the PLC. We can force some inputs/outputs to be on/off.
Probably the most common use of Host Link protocol though is in using an HMI (Human Machine Interface). In case you don't know, an HMI is basically the operator interface panel on a machine that gives/takes
instructions form the operator and translates those human understandable instructions into something the PLC can interpret. It is sometimes also called an MMI... as in Man Machine Interface.
When using an HMI, we need to often get data from the PLC and display it. Perhaps we want to see a product count or a temperature/pressure or maybe we just want to see an alarm condition. Instead of just seeing 'error 64' or whatever, we can program the HMI to display 'over temperature alarm on cookie oven', for example.
The HMI would communicate with the PLC to get this type of data. The language that it will talk (in this example) will be Host Link.remember, however, that the PLC and the HMI must both have built-in
Host Link capability... otherwise they won't be able to understand each other.
Well, that's a really brief overview of what Host Link is and what it can do. Next time we'll see the actual language itself and follow a conversation between a PLC and a computer to really get an understanding of the process. You'll see things on a byte level and realize that it isn't all 'black magic'... we can understand it
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