IBM 8260 manual Backplane Signalling for 16 Mbps Operation

Figure 83. 8260 Backplane Signalling for 16 Mbps Operation

Note that the 8260 backplane interface is completely digital, whereas the signals sent on the transmission media (lobe cables and the cabling between two hubs) is said to be analog. In this context an analog signal is one where there is no separate clock signal. A digital signal is one where there are separate data and clock signals.

The use of digital signals on the 8260 backplane allows the signals to be switched to the appropriate location by using digital switches which can be easily implemented on the 8260 media modules. Per-port switching modules, where different ports on the same module may be assigned to different networks on the 8260 backplane, are made economically possible because of the digital signalling on the 8260 backplane.

Because of the use of digital signalling on the 8260 backplane, each 8260 media module must convert the analog signal it has received from the attached stations and convert it to a digital signal and then present the data and clock to the backplane. This is done within both the active and passive modules. In the opposite direction, the modules must receive the data and clock from the backplane and convert them to an analog signal before sending them on the transmission media for transmission to the stations.

The Phase Lock Loop (PLL) function which is implemented on each 8260 module, (DPLL is in 8.3, ªDual Phase Lock Loopº on page 138) is used to perform the conversion between the analog and digital signals. Also, in the 16 Mbps operation, the PLL will double the frequency of the received digital signals (clocked at 16 MHz) to form analog signals (clocked at 32 MHz) for transmission on the lobes, and does the opposite for the signals sent to the backplane from the lobe. In the 4 Mbps mode, the PLL does not change the frequency of the

Chapter 8. 8260 Token-Ring Support 137