2.1.4. Configuring a Mixed CT Bus/SCbus System

This section provides guidelines for configuring a mixed SCbus/CT Bus (H.100) system. The following subjects are discussed:

• Board Placement
• Cabling
• Clock Master Selection and Location
• Cable Adapter
• Terminations

In this discussion, the following terminology is used to refer to different board types:

• H.100 boards with H.100 connectors (68-pin): H.100/PCI/68-pin, referred to as board type HP68
• SCbus boards with H.100 connectors: referred to as board type SP68
• SCbus boards with SCbus connectors (26-pin): referred to as board type SP26

Note that these board types are not officially recognized but merely created as identifiers for this discussion.

Board Placement

Use the following guidelines when selecting board slot locations within the chassis:

1. Place the SP26 boards (if any) in the PCI slots closest to the chassis ISA slots.

   If there are no ISA slots, then use the PCI slots toward the right side of the chassis, when viewed from the board faceplates, with the PCI/ISA backplane (or motherboard) oriented horizontally.

2. Place the HP68 boards in the next available PCI slots closest to the SP26 boards (to the left of the SP26 boards when viewed from the faceplates).
3. Place the SP68 boards (if any) in the next available PCI slots closest to the HP68 boards (to the left of the HP68 boards when viewed from the faceplates).

In addition, note the following:

• Be sure that your chassis has enough PCI (and ISA) slots for the boards that you intend to use. Some boards (with daughterboards) may consume the space of an adjacent board.
• A mixed H.100 (CT Bus)/SCbus (or H.100 only) system can have any combination of boards, up to 20 total.
• H.100 boards will not work with PEB-only boards or with SCbus/PEB boards operating in PEB mode.

Cabling

While the SCbus uses a 26-lead flat ribbon cable, the H.100 bus uses a 68-lead finer pitch flat ribbon cable. Also, new connectors are used on both the H.100 boards and the H.100 cables. The maximum length of the H.100 cable is 20 inches. (SCbus is 21 inches.)

Be aware of the following guidelines for cabling:

• An H.100 cable can have no more than 20 connectors (drops).

  The H.100 cables currently available come in 4, 8, 12, and 16 drop configurations, each drop 1.1 inches apart.

• The maximum length of a combined H.100 cable and SCbus cable (or H.100 cable alone in an H.100-only system) is 20 inches. In practice, this will be less due to the need for a cable adapter, discussed below.
• Ensure that no more than 7 inches of unconnected cable (5 unused drops on a standard Intel Dialogic H.100 cable) exists between any two boards. Rearrange the boards in your system and/or use a cable with fewer drops to meet this requirement.
• Both ends of an H.100 cable (and SCbus cable) must be connected to a device, for example, a board or a cable adapter.

Clock Master Selection and Location

The system's clock master is one of the boards in a system that is designated to provide reference timing for all boards attached to the H.100 bus (and/or SCbus). This board must derive timing from a digital network (for example, a T-1 or E-1 line), from a secondary reference (for example, the H.100 or H.110 CT_NETREF or SCbus SREF8K), or, as a last alternative, from its own local oscillator (that is, independent) on the board. For detailed information about bus clocking, see Section 2.1.5, Understanding the CT Bus.

Use the following guidelines for clock master selection and location:

• In the current release, any system that contains an SCbus-only board (such as Springware boards and v1.5 DM3 PCI boards) must operate in SCbus mode, even though you may have one or more H.100 compliant boards in your system. Because all boards in your system will therefore operate in SCbus mode, it is possible to designate any of these boards as the SCbus clock master.
• It is highly recommended that selection of the clock master should be based upon a board's access to a digital network interface such as a T-1 or E-1 line.
• H.100 boards require certain signals that can only be provided by another H.100 board and routed over the H.100 cable. Make sure that the board that you intend to use as the system's clock master can do the following:
  • Derive timing from a digital network and/or secondary reference
  • Provide both H.100 core signals and compatibility bus signals.
• Typically, the clock master board is located at either end of the 68-lead H.100 cable. This is much more important for H.100 than it is for SCbus.
• In systems that include at least one board with a 26-pin (SCbus) connector, the HP68 board to the furthest right (when viewed from the faceplate) should be designated as the clock master. If the board currently in this location is not your intended clock master, relocate boards as appropriate.
• In systems that use the H.100 cable only, the location of the clock master is less important.

Cable Adapter

When configuring a mixed H.100 (CT Bus)/SCbus system that uses SCbus boards with the 26-pin SCbus connector, a transition adapter or cable adapter must be used. The CT Bus/SCbus Adapter, available from Intel, is a small printed circuit board that becomes part of the cable assembly. This adapter resides in only one place in the chassis, determined by where the changeover from the SCbus cable to the CT Bus cable is made within the system. All products using the CT Bus are on one side of the adapter, and all products using the SCbus are on the other side.

Use the following guidelines for installing the CT Bus/SCbus Adapter:

• The HP68 board closest to the SP26 boards should be connected to the adapter if SP26 and/or ISA-based SCbus boards with the 26-pin SCbus connector are used in the system.

  Also, this same HP68 board should have termination enabled as discussed below; and it is highly recommended that this HP68 board also be designated as the system's clock master. (Another HP68 board could be the clock master, although it is best to source both SCbus and H.100 clocks from the board that uses the adapter.)

• As with all similar adapters, use of this device will reduce the total number of boards that can be reliably operated in a mixed system to less than 20.
• Use of this adapter may create a physical clearance problem with the side or top of some types of PC chassis.
• When installing the adapter onto a board with H.100 connectors, the proper orientation is such that the SCbus connector is to the side facing the ISA board slots (to the right when viewed from the board faceplates).
• Only one adapter should be used in order to preserve electrical signal integrity on the bus.

Terminations

Unlike SCbus, the H.100 CT Bus specification requires terminations for four signals on the bus. Terminations are enabled on boards located at both ends of the H.100 cable, but never in between the ends. Note that the board at the end of the SCbus cable (if used) opposite the cable adapter is never terminated.

Termination techniques may include physical jumpers and/or using a soft-jumper technique. Soft jumpering eliminates the need for a user to install physical jumpers by using programmable registers (to jumper these signals), controlled through a user interface.

Be aware of the following guidelines for terminations:

• The HP68 board nearest the chassis ISA bus (the furthest right when viewed from the board faceplates) should have termination enabled. Check the documentation supplied with your board to determine the appropriate method to terminate the H.100 bus.
• The last PCI board, which should be an SP68 board, must also be terminated.

  If no termination provisions exist on this board, then rearrange the boards within this group to allow you to do so.

• If no SP68 boards are used in the system, then the HP68 board furthest from the board with the first termination must also be terminated.

Click here to contact Telecom Support Resources