The DIU Clock submenu is:
(1)THRU (2)INT (3)EXT 422 (4)EXT TTL (5)NET (6)EQ (7)TIU (8)DIU >(1)THRU selects passive timing behavior. The CSU uses the clock provided by the connected local DSX-1 Equipment to drive the network signal. THRU is recommended in many cases for Drop and Insert mode and it is the actual mode in which the CSU functions when it is in CSU mode. Even though some selection other than THRU may appear as the current value, if the CSU is in CSU mode it is actually operating with THRU timing.
(2)INT selects the oscillator built in to the CSU card as the timing source. This is often used in Mux mode. INT should only be selected at one end of a point to point T1 and only if the T1 does not have timing applied to it by the service provider (phone company).
(3)EXT 422 selects the balanced pair of input leads that are present in the DIN connector on the CIM behind the CSU. Use of this option requires that the external clock being provided must be at 1.544 MHz in a balanced signal which meets RS422 specifications. EXT 422 might be used in any CSU mode.
(4)EXT TTL selects the unbalanced pair of input leads that are present in the DIN connector on the CIM behind the CSU. Use of this option requires that the external clock being provided must be at 1.544 MHz in a +5V to 0V signal level. EXT TTL might be used in any CSU mode.
(5)NET is the most widely used value for this option. If it is known that the T1 passes through a DACS or if the far end CSU is set for INT clock, then always choose NET. NET is commonly used in Mux mode and may occasionally be used in Drop and Insert mode for situations where the T1 equipment does not provide a reliable clock source. When connecting to a fractional T1 or frame relay T1 and operating in Mux mode always select NET.
(6)EQ is similar to THRU. The clock signal derived from the DSX-1 T1 equipment is used as the transmit clock source for the CSU. This choice is invalid for Mux mode, may be useful in Drop and Insert mode.
(7)TIU is used only if the shelf has a TIU2850 Timing circuit card installed. This is a special circuit card which takes a T1 signal in on one of several ports, recovers a clock signal from that input, then propagates that clock signal to all of the slots in the shelf using bus C. Obviously, this option should not be selected unless a TIU2850 card is actually installed in the shelf.
(8)DIU is used for tail circuit timing. A single port of a single DIU2130 card may be used to provide clock for that DIU and the CSU if necessary. Three options must be set to achieve tail circuit timing. 1) In the DIU2130 the port to be used to provide timing must be set to TT clocking. 2) In the DIU2130 the timing source must equal the port being used (port 1 or port 2) as opposed to the usual value of CSU. 3) DIU may then be selected as the timing source for the CSU. This option is not recommended except when using Mux mode.