Line-Time Clock


One of the less understood features of the PDP-11 seems to be the Line-Time Clock, or LTC. This is a signal that is normally generated by the DEC power supply. The power supply samples the A.C. power line, at a voltage the system can handle, and then converts it to a square wave for use by the processor. In the Americas, the signal's frequency will be 60 Hz, while in Europe and Asia, 50 Hz is more common.

The LTC signal is injected onto the BEVNT line on the backplane, where it generates a high priority interrupt in the processor. This interrupt is used for updating the system clock/calendar, among other things. Some operating systems, such as Unix, will not run without it.

Systems that do not have a power supply capable of generating an LTC signal will need to obtain the signal from another source. The following are the DEC modules capable of providing an LTC signal for BEVNT.

The M8047 supplies a 60 Hz LTC signal from a crystal oscillator. Since the clock is crystal-controlled, it will be more stable than a signal derived from the power line. This module is somewhat common and provides an elegant way of supplying the LTC to a system.

The M7195, M8063 and M7676 derive their LTC signals from a DLART chip, and have the option of providing a 50 Hz, 60 Hz, or 800 Hz clock. These modules are fairly common and also provide an elegant way of supplying the LTC to a system.

The M8016, on the other hand, requires an external 24V, center-tapped power transformer to provide the frequency source. This module is less common and more kludgy to use. Since the clock source is derived from the power line, it will likely be more jittery than from the other two modules.

Line-Time Clock Register

The Line-Time Clock Register is used to provide software enable/disable capablility of the LTC. This is a one bit register that is always located at 7775468. Setting bit 6 to 0 will disable the Line-Time Clock (regardless of where it is generated), by clamping the signal to ground. Setting bit 6 to 1 will re-enable the clock.

Certain operating systems rely on this register for proper operation. All of the J11 processor modules include this register, as does the M8189. The remaining processors have to rely on a separate module to provide this functonality. DEC modules containing an LTC register include the following:

The M8012 can be paired with an M8047 module to provide similar functionality to the M7195.


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