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  • #2136486

    Ethernet without internet


    by libindaniel2000 ·

    Scenario: Trying to connect a device to the desktop via ethernet. It works when the hub has uplink (internet) connected. However, without the internet, the connection times out.
    The device (a DSA scanivalve pressure transducer)uses RS 232 as a trigger device that is connected using Hyper Terminal. The device uses ethernet as the output. We want to connect this device to a laptop. However, the issue is that without the internet it does not seem to connect.
    We used a crossover wire (as stated in the device manual) to connect it directly to the laptop. However, it did not connect.
    We used a straight wire through a hub to a desktop. It works when the internet connection is active. However, it fails when the wall internet connection (uplink on the hub) is unplugged.
    Are we trying to achieve something that is not possible i.e. connect a device that uses ethernet as an output to a laptop that has no active internet?

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  • Author
    • #2902388


      by libindaniel2000 ·

      In reply to Ethernet without internet


    • #2902387

      This is possible,

      by Wizard57M-TR ·

      In reply to Ethernet without internet

      there have been a number of products over the years to aid in this sort of connection,
      LapLink, Interserv/interlink (old MSDOS 6) and direct connection using terminal in
      Win3X/9X, as well as HyperTerminal. So, there must be something in the connection
      details you have defined that are attempting to connect over the internet. Double
      check all settings, choose something like “null modem cable”.

    • #2428475

      DHCP issue?

      by gechurch ·

      In reply to Ethernet without internet

      Just so I’m clear, you are trying to use the ethernet to give you standard network (TCP/IP) access to another device, correct? (As opposed to, say, carrying the serial cable over ethernet cabling). And what is the error when trying to connect? Does it say the cable is unplugged? That it can’t pick up an IP address? Or do all the network settings look fine, but you simply can’t connect to the device at the other end?

      What are the IP settings you are using? If you are using DHCP on the PC then unplugging the uplink could well be unplugging the DHCP server too (in small setups the modem is usually acting as the DHCP server). If this is the case you could set a static IP address on the PC and the other network device. They should then be able to communicate with each other through the hub (use the ping command to test). If you want to remove the switch and go direct then the straight-through cable won’t work – you must use a crossover cable in this instance (and must set static IP addresses since you won’t have a DHCP server).

      Most switches these days auto-sense straight-through and crossover cables so you can use either when going through the switch (I’m assuming you have a switch and use the term ‘hub’ interchangeably. If it really is a hub you will need to use a straight-through cable).

    • #2428447

      I tried to make sense of the manual

      by glen.harris ·

      In reply to Ethernet without internet

      But it isn’t fun reading.
      It looks like you have to connect the device via Serial-Serial connection in order to set the unit to talk over ethernet. The laptop model you are using doesn’t have a com port but you could pick up a USB to Serial adapter in order to connect it (I use these pretty much every day to connect legacy equipment and they work pretty well.) It looks like you can then connect to it via Hyperterminal but need to tell it to use the settings 9600, 8, 1, NONE in order to be able to talk to it. You’ve probably already got this far though.
      I realise I’m starting to ramble, but perhaps it’s something simple like Windows Firewall which is getting in the way. The firewall will alter its behaviour depending on the type of network it is connected to. For example more things will be allowed when connected to an office network compared to being connected to a public hotspot. As the laptop will in effect not be on a traditional network, and there therefore isn’t a risk of attack, it may be worth turning it off to see if it makes a difference. You’ll need to use crossover though as when connected through a switch, the switch will ‘switch’ a straight through cable for you by altering which individual wires it sends the signal along. The Laptop itself wont do this.

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