The two indicator LEDs blink at the same frequency. If both
SEL1 and SEL2 are OFF, the GCC communication indicator
LEDs 1 to 28 indicate the communication states of GCCs 1
to 28 respectively. If both SEL1 and SEL2 are ON, the GCC
communication indicator LEDs 1 to 28 indicate the
communication states of GCCs 29 to 56 respectively.