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Cluster Controllers

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A cluster controller is an IBM-manufactured or compatible device used to channel-attach 3270 terminals to a host system. A cluster controller may also communicate with a host via an SDLC (Synchronous Data Link Control) link, or a bisynchronous link to a host-attached communication controller. There are channel-attached cluster controllers and link-attached cluster controllers:

  • Channel-attached cluster controller    These controllers are directly attached to the multiplexor channel of the host system. Model numbers for this type of cluster controller end in A (SNA [Systems Network Architecture] controller) or D (non-SNA controller), such as IBM 3274 Model 41D.
  • Link-attached cluster controller    These controllers are connected to communication controllers. The communication controller is then attached to a channel of the mainframe. The link between the cluster controller and the communication controller is either a modem for analog circuits or CSUs/DSUs (channel service units/data service units) for digital circuits. Model numbers for this type of controller end in C, such as IBM 3274 Model 41C.

SNA cluster controllers are called PU (physical unit) Type 2 devices and terminals attached to the cluster controller are called LU (logical unit) Type 2 devices. A printer device attached to a PU Type 2 is either an LU Type 1 or LU Type 3, depending on the printer type.

The 3274 series cluster controllers are actually the older series IBM cluster controllers. New models are the IBM 3174 series. The new models provide advanced features like IBM Token Ring attachment interfaces, management, and monitoring. Also note that SNA gateways such as Microsoft SNA gateway provide network connections to IBM systems and can replace cluster controllers in many cases.

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