IBM Token Ring MAU 8228

IBM Token Ring MAU 8228 ... external power supply; all its power requirements are provided by the connecting token ring adapters themselves which cause relays inside the 8228 to operate when a connection is made. There are no indicator lights on the 8228, so there is no way to know whether or not any particular port is in use or not.

The 8228 also has ring-in and ring-out ports, which can be used to link to other 8228s to form a complete ring. Two 8228s can be connected using a single cable between one unit's RO port and the other unit's RI port; greater resilience will be provided for networks of two or more 8228s if all RI/RO ports are attached, with the RO port of the last 8228 being connected back to the RI port of the first 8228. Doing this will allow the ring to remain active and complete even if any one of the RI/RO cables should fail or be disconnected. If the 8228 is to be used on its own then it is not necessary to connect the ring-out port to the ring-in port to complete the ring.

In a ring formed solely of 8228s, the speed at which the ring operates (4 Mbps or 16 Mbps) is determined by the first device to connect to the ring. This sometimes causes problems when the first device is set to auto-sense the speed of the ring: because there is not yet any ring activity it is unable to determine which speed to use and is unable to connect. This sometimes requires pre-configuration of ring speed in one or more devices. Watch out, however, because if the ring is operating at one speed (say 16 Mbps) and a new device is pre-configured with the other speed (4 Mbps in this case), attaching the new device to the 8228 will cause the ring to break. No recovery is possible until the offending device is disconnected from the 8228. The only totally foolproof mode of operation is one in which all connecting devices are pre-configured with the same ring speed.