• Supported Platforms
• T640 Routers

• Related Topics
• T640 SONET Clock Generators (SCGs) Description
• T640 SONET Clock Generators (SCGs) LEDs
• Maintaining the T640 SCGs

Replacing a T640 SCG

The SCGs are hot-pluggable. If two SCGs are installed and both are functioning normally, SCG0 is the master and SCG1 is the backup. Removing the backup SCG does not affect the functioning of the router. Taking the master SCG offline might result in a brief loss of SONET clock lock while the backup SCG becomes the master.

You can determine which SCG is functioning as the master in one of two ways:

• Check the blue MASTER LED on the SCG faceplate. If this LED is on steadily, the SCG is functioning as the master.
• Use the following CLI command to display which SCG is functioning as the master:
  
  

  user@host> show chassis clocks

To replace an SCG, perform the following procedures:

1. Removing a T640 SCG
2. Installing a T640 SCG

Removing a T640 SCG

The router can have one or two SCGs installed. The SCGs are located in the upper rear of the chassis, above the T-CBs and Routing Engines. Each SCG weighs approximately 1.9 lb (0.9 kg).

To remove an SCG (see Figure 1):

1. Place an electrostatic bag or antistatic mat on a flat, stable surface.
2. Attach an electrostatic discharge (ESD) grounding strap to your bare wrist, and connect the strap to one of the ESD points on the chassis. For more information about ESD, see T640 Preventing Electrostatic Discharge Damage.
3. Press the online/offline button on the SCG faceplate and hold it down until the OK LED goes out (about 5 seconds).
4. Loosen the captive screws on the edges of the SCG faceplate.
5. Grasp the SCG by the handle on the faceplate, and slide it out of the chassis.
6. Place the SCG on the antistatic mat.

Installing a T640 SCG

To install a replacement SCG (see Figure 2):

1. Attach an electrostatic discharge (ESD) grounding strap to your bare wrist, and connect the strap to one of the ESD points on the chassis. For more information about ESD, see T640 Preventing Electrostatic Discharge Damage.
2. Carefully align the sides of the SCG with the guides in the SCG slot.
3. Grasp the SCG by its handle, and slide it straight into the chassis until it contacts the midplane.
4. Tighten the captive screws on the corners of the SCG faceplate.
5. To bring the SCG online, press the online/offline button until the green OK LED lights.
6. To verify that the SCG is installed correctly and is functioning normally, check the LEDs on the SCG faceplate. The green OK LED should light steadily. If the SCG is master, the blue MASTER LED should also light steadily.

   To check the status of the SCGs, use the following CLI command:

   
   

   user@host> show chassis environment scg

   For more information about using the CLI, see the Junos OS manuals.