Author Topic: Two PLC'S on one router  (Read 1141 times)

Lorne Van Dusen

  • Jr. Member
  • Posts: 78
  • I'm a old guy
    • View Profile
Two PLC'S on one router
« on: April 05, 2018, 06:52:05 PM »
I am having an issue when connecting two PLC's to the same router
I am guessing that I need to change one of the settings in the router.
In my setup one PLC has an ID of 01 the other PLC has an ID of 02
If I power each PLC up separately there is no issue they each get a different IP address
such as 1 gets 192.168.0.130 the other one gets 192.168.0.161
However if both PLC's power up at the same time then the both show the same IP address of example 192.168.0.130
The router shows two devices with the same IP but different mac address

support

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 3102
    • View Profile
    • Internet Programmable PLCs
Re:Two PLC'S on one router
« Reply #1 on: April 07, 2018, 10:44:49 AM »
What is the make and model number of the router?

For some reasons it seems that either the DHCP server is sending out the same IP address to two PLCs that are requesting IP address at the same time, or that both PLCs are picking the first assigned IP address from the DHCP server.

Is this repeatable?

We will try this on a different router to see if we get a different results.

Why is your router DHCP assignment jump from 130 to 161 when two devices are requesting DHCP simulataneously? Is it because there are other devices already been assigned 131 to 160?
Email: support@triplc.com
Tel: 1-877-TRI-PLCS

Lorne Van Dusen

  • Jr. Member
  • Posts: 78
  • I'm a old guy
    • View Profile
Re:Two PLC'S on one router
« Reply #2 on: April 20, 2018, 05:54:00 PM »
OK I have tested this scenario many times and found the problem is in the Router
I spoke to someone that is well versed in the subject and he stated that it is an issue with the routers and that yes they have been know to issue the same IP to two different devices if they request an IP at the exact same time.
The only solution was to put a delay of 1 1/2 seconds in the startup of one of the PLC's
This now solves the issue as now both PLC's are no longer requesting an IP address at the same time.

I now have a new question
When you monitor a router it will always show the MAC Address, the IP Address as well as a name
However when I connect 1 or more PLC'S the name always shows as (unknown)
I downloaded a node name into each PLC but that did not change anything.
Can anyone tell me if there is a way to show a name of a PLC in the router?

support

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 3102
    • View Profile
    • Internet Programmable PLCs
Re:Two PLC'S on one router
« Reply #3 on: April 23, 2018, 06:35:20 PM »
Unfortunately the PLC node name defined in i-TRiLOGI is not recognized by the DHCP server and thus it is not displayed. So only the MACID should be used to identify the PLC.

In PLC application it is often better to use a static IP address. The DHCP IP address can change when the router loses power for some time. So if you want to use DHCP you should do an IP address reservation in the router so that the router will always assign the reserved IP address to the PLC. The reservation table inside the router often allows you to define a name for the device that you reserve the IP address. This is where you can put in a meaningful name.

The following picture is captured from a Cisco router that shows you where you can define the name of the device.
« Last Edit: April 23, 2018, 06:36:07 PM by support »
Email: support@triplc.com
Tel: 1-877-TRI-PLCS

Lorne Van Dusen

  • Jr. Member
  • Posts: 78
  • I'm a old guy
    • View Profile
Re:Two PLC'S on one router
« Reply #4 on: April 23, 2018, 08:14:05 PM »
Thanks for the information I asked for my
My DLink 7850 does not have a place to change the unknown to a name.

However when I was doing my initial testing I did use a static IP address
I then switch to a DHCP as we will have no control over the router that the PLC's get connected to.
The PLC's will be shipped all over the world and could be any make or model.
However whenever the PLC boots up I am showing the assigned IP address on the LCD screen
Plus we supply a hand held text display that the tech can use to find the signed IP address.