# AB Micrologix Question



## DesignerMan (Jun 13, 2008)

I have a project I will be starting in a couple of weeks that another person started, but is no longer with the company.
Very simple application using 2 micrologix 1000 PLCs, but looking over the programs I see one of the units is set up as a DH-485/ HDSlave (selected from the processor choices).
I've never dealt with this selection before and was wondering if someone could shed some light on what this is used for?
And maybe a little info of how it is used.
I appreciate the help...
Thanks!


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## oliquir (Jan 13, 2011)

maybe it is used as a remote IO


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## emtnut (Mar 1, 2015)

DesignerMan said:


> I have a project I will be starting in a couple of weeks that another person started, but is no longer with the company.
> Very simple application using 2 micrologix 1000 PLCs, but looking over the programs I see one of the units is set up as a DH-485/ HDSlave (selected from the processor choices).
> I've never dealt with this selection before and was wondering if someone could shed some light on what this is used for?
> And maybe a little info of how it is used.
> ...


Been 20 yrs since I used DF1 !
DF1 is a half duplex protocol where one unit is the master, and the other slave. We used it when the communication between the 2 units was an RF RadioModem.


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## JRaef (Mar 23, 2009)

When you have more than one device on a serial network like DH, it can be tricky to have all devices being "masters", so you typically have one become the master and the others slaves.

The difference is this: A "master" can INITIATE communications in either direction. It can accept requests for communication from other devices on the network, such as your PC or some higher level controller, it can send messages to those other devices, as well as send messages to slaves on its same network node. A "slave" can ONLY RESPOND to messages and requests coming from a master, it cannot initiate communications on its own. 

If all the devices on a network are set as masters, then to avoid data "crashes" or communications timeouts, you have to have something that acts like a traffic cop and maintain a list of priority functions that take precedent over other communications packets, telling the lower priority devices to hold on and wait their turn. It's doable, but more complicated and needs more hardware or a more fluid network topology like Ethernet (which by definition, still has that "traffic cop" in it). 

In your case with just these two devices on a single machine, a master/slave setup makes it easier because the master is the only one that can initiate communications and when it needs data from the slave, it fetches it.


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## DesignerMan (Jun 13, 2008)

oliquir said:


> maybe it is used as a remote IO


I believe that is exactly what he was doing.
But it makes no sense since the other mixer is 10 feet away.


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## DesignerMan (Jun 13, 2008)

JRaef said:


> When you have more than one device on a serial network like DH, it can be tricky to have all devices being "masters", so you typically have one become the master and the others slaves.
> 
> The difference is this: A "master" can INITIATE communications in either direction. It can accept requests for communication from other devices on the network, such as your PC or some higher level controller, it can send messages to those other devices, as well as send messages to slaves on its same network node. A "slave" can ONLY RESPOND to messages and requests coming from a master, it cannot initiate communications on its own.
> 
> ...


Thanks for the info...I plan to dig a little deeper when I'm back there in another week. Still don't see the reason to do it this way, unless you are making complicated on purpose.
The 2 mixers along with a delivery system consist of 8 inputs and 10 outputs- very simple.


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## oliquir (Jan 13, 2011)

DesignerMan said:


> I believe that is exactly what he was doing.
> But it makes no sense since the other mixer is 10 feet away.


only advantage is you only have one program to edit for both machines.
is there any common operation for the 2 mixers ?


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## DesignerMan (Jun 13, 2008)

oliquir said:


> only advantage is you only have one program to edit for both machines.
> is there any common operation for the 2 mixers ?


Exactly the same, but he has them as 2 separate programs.


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## emtnut (Mar 1, 2015)

DesignerMan said:


> Exactly the same, but he has them as 2 separate programs.


Sounds like it's a hot standby, so you can work on one without interrupting the process.


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