Note: BITS input
is a T1/E1 line interface unit (LIU), with AMI (alternate mark inversion)
encoding enabled and B8ZS/HDB3 (Zero Suppression) disabled. The equalization
is set for a 0-133 feet cable. An internal 100 ohm termination is
present, although it might need to be externally augmented based on
the type of cabling used. Any valid AMI signal works properly and
it is not restricted to only the "all 1" BITS signal but the ones
density must be sufficient to prevent LOS (according to the ITU G.775
recommendation). The TTL input has a slice point of 3.3V/2 = 1.65V
relative to chassis ground (GND). Therefore, any signal on the coaxial
center conductor that transitions through that voltage registers
a transition. There are many signals, besides TTL, that satisfy this
criteria. An external termination must be provided that matches the
impedance of the cable that goes to the BNC connector.
If you can configure the rate in CTP menu, then the TTL supports
a frequency of 2048 KHz for the TTL clock input, provided the signal
is good and noise-free (terminated properly). TTL is rate-agile,
while BITS is restricted to T1/E1 frequencies.
The TTL input is high-impedance (no on-board termination provided)
because a variety of cable types might exist that you can use to drive
signal to this connector, such as RG-58 coax (50 ohm), RG-59 coax
(75 ohm), or twisted pair (100-120 ohm). Instead of applying a particular
impedance termination on the board and have it incorrectly done,
we recommend that you configure the impedance termination based on
your network environment. For example, a 50 ohm termination is needed
if you are using RG-58/U coax cable, which has 50 ohm impedance.