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Chris Elston

Slc 5/05 dhcp?

8 posts in this topic

Does the 5/05 support DHCP?

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Nope, just BootP.   A-B has a pretty good BootP utility (about 6 months old) that I use. I am told that DCHP servers (like the ones included with WinNT/2000 Server and with Linux) can also perform BootP, but I get funny looks from network admins when I tell them I want to put a controller on their network that doesn't have a Host Name.

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As noted previously, the SLC does not support DHCP only BootP.  However.......most DHCP servers will perform BootP functions. In other words, I have a 5/05 plugged into our office network and the DHCP server found it and assigned it an IP address.  The problem with DHCP and a PLC though is that when you power down should your DHCP lease expire, when you power up again you may be assigned a different IP address.  Whereas BootP will always assign you the same IP address. So, long story short...DHCP will work, but it isn't a good long-term (permanent) solution.

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Hey wow....great explaination. I liked it! And it made good sense. Why not sign up as a registered user and stick around a bit, we could use some more engineers to help.  

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The plc does not have a name but it has a MAC address.  This address is a universal id, which is unique among all Ethernet devices.  The MAC address is printed on the SLC with the following label: Ethernet address. As painful as it may be, if your network has a DHCP server, you will want to work with you network administrator. As stated before if you let the DHCP server assign the address you may not always get the same address.  This is bad if you have HMI connected to the PLC or you are using PLC to PLC messaging (the both use IP addresses to communicate). If you enter a static address into the SLC but don't tell the network administrator, the DCHP server may try to use this address for something else.  Then you will have two devices with the same IP and the devices tend to get confused. I think the best option is to have your network administrator create a reservation in the DHCP server.  If you tell the administrator what the MAC address is and that you always want that MAC to get the same IP, there shouldn't be too many funny looks. With the reservation the DHCP will always give out the reserved IP to that MAC address when requested by the BootP query. Once you have that done, I would consider entering the IP into the SLC as a Static IP.  This is redundant, but the reason for doing it is this:  If you loose power to you facility and computers and PLCs are powered down, when power is restored, the SLCs will boot fast  (before the DHCP server).  I believe the SLC will wait for the DHCP server but may timeout after some time (I have not tested this).  To be safe, I suggest using the Static address in the PLC. An alterative would be to have you network administrator give you a small block of addresses.  The DHCP would need to be setup so it doesn't assign those addresses.  You would then be responsible for deciding what is assigned what address in that block. This is a little wordy; if you have any questions let me know. Preston Vander Plaats Interstates Control System preston.vanderplaats@interstates.com

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I found one DHCP server that won't support BOOTP requests from a PLC:   A Linksys BEFSR41 Cable/DSL Router with 4-port Switch. This device is the neatest thing since sliced bread for a DSL connection you want to share;  the 4-port switch works fine for computers you plug in whilst configured to get their configurations via DCHP.   It's an excellent firewall for home use;  I got a couple of probe attacks an hour until I got this, now none. But it ignores BOOTP requests.   Happily, once I set the IP address manually I was able to set the router for Port Forwarding to let RSLinx traffic through the firewall, so for a quick and dirty demo it let me control a SLC over the Internet.   It's not probable that I'd install such a thing in a real production environment.

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That’s a good point. I made it sound like I was talking about DHCP in general, in fact all of my experience is with Microsoft Server's support of DHCP and Bootp. Preston Vander Plaats Interstates Control System preston.vanderplaats@interstates.com

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Although I cannot participate in the more technical aspects of this thread, I thought I'd let y'all know that the SLC 5/05 Ser. C processor support DNS.  You will need to be using RSLogix 500 r5.20 or later to access the DNS functionality.

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