PBX Circuit Boards

Each shelf in the PBX is designed to accept a certain number of printed circuit boards. These are also known as circuit cards, or just boards, cards or circuit packs. Just as each PBX manufacturer puts its system together somewhat differently, the names of system components may also differ, so it is important to clarify what the terminology really means.

The devices used and the manner in which they interact differ from one PBX to another, but the basic building block of the PBX is the printed circuit board.

There are different types of circuit boards in the PBX:

Both-Way Trunk Circuit Boards

Both-way trunks (also called combination trunks) are outside lines that may be used to receive incoming calls or to place outgoing calls. There are ports (electronic places) on each circuit board, one port per trunk. Some systems have 32 trunks per circuit board. Others have sixteen, eight, four or two. A few older systems use two ports per trunk instead of one.

Every time you add outside lines, determine whether or not you have sufficient ports on the circuit board to handle them. If you do not, you need to buy another board. Trunk circuit boards can run from $1,000 to $3,000.

Sometimes both-way trunks are used for outgoing calls only and may be referred to as DOD (Direct Outward Dial) trunks.

DID Trunk Circuit Boards

DID (Direct Inward Dial) trunks are a special type of trunk used for incoming calls only. They enable the direct dialing of each individual desktop telephone in the PBX. You may have ten DID trunks for 100 separate DID telephone numbers. DID trunk circuit boards handle thirty-two, sixteen or eight trunks in most systems. Note: DID trunks delivered on a PRI ISDN circuit may also be used for outgoing calls. This has created the oxymoron two-way DID.

Universal Trunk Circuit Boards

A universal trunk circuit board enables you to mix both-way trunks and DID trunks on the same board. This creates efficiencies in the use of the ports and therefore can lower the cost in terms of the total number of circuit boards to be purchased. It can also save space (sometimes called real estate) within the PBX cabinet.


The trunks that use the above types of circuit boards are sometimes collectively referred to as copper trunks or analog trunks to distinguish them from the outside lines that are delivered on the T-1s, which is common in PBXs today. Copper trunks are still used for back up. The cost of T-1s has dropped dramatically in recent years, so T-1 has become the more prevalent method of delivering the outside lines to the PBX.

T-1 Circuit Boards

A T-1 is a high-capacity circuit using two pairs of wires, enabling the transmission of up to 24 voice conversations at one time. In order for it to do this, a piece of hardware called a multiplexer is required at each end.

The T-1 circuit board is a multiplexer that fits right into the PBX shelf. In some older PBXs the T-1 circuit card must be placed on a shelf specifically designed to handle it. If you are going to use a T-1, you may need to buy a separate shelf for it. Each PBX has limits, so find out how many separate T-1s your PBX can hold. You may hear the term channel bank which is a multiplexer external to the PBX. The channel bank can be used if your PBX does not have T-1 circuit board capability.

PRI/ISDN Circuit Boards

Most PBXs can accept ISDN (Integrated Services Digital Network) lines. The circuit board may be for a PRI (Primary Rate Interface) ISDN line which has 23 B channels plus one D channel. A few PBXs can also accept BRI (Basic Rate Interface) ISDN lines with 2 B channels plus one D channel although the BRI lines are seldom found in a PBX. The ISDN lines can be used for voice conversations and data transmissions.

T-1/PRI Circuit Boards

On newer PBXs, the same circuit board is used for both T-1 and PRI.

Tie-Line Circuit Boards

Tie-lines are point-to-point lines connecting two PBXs so that the users of both systems may communicate without dialing an outside call. Some systems have separate circuit boards for these tie-lines. Most systems use a circuit board which handles two, four or eight tie-lines.

Digital Telephone Circuit Boards

Most telephone systems now use digital telephones. This means that the analog voice signal converts to a digital form right in the telephone and travels back to the PBX cabinet in a digital form (combinations of ones and zeroes). For every 8, 16 or 32 of these telephones in the system it is necessary to have a digital telephone circuit board. This may also be called a digital station board. Each port on the board corresponds to a specific digital desktop telephone in the system. Every telephone has an associated numeric location in the PBX cabinet indicating the port, the circuit card and the shelf.

Analog Telephone Circuit Boards

Most telephone systems are installed with at least one analog circuit board with ports for either 8 or 16 analog telephones. Many single-line telephones are analog. The voice signal is sent from the telephone to the PBX circuit board in an analog form.

The analog ports provide analog extensions required for using fax machines and computer modems through the PBX.

There are differences of opinion as to whether or not faxes and modems are best run through the PBX or through separate outside lines. It is believed that some PBXs may slow down the data transmissions.

Most PBX manufacturers will not guarantee throughput of data beyond a certain speed. If you expect to have computers and fax machines go through the PBX, it is important to find out what speeds can be expected. You may decide to bypass the PBX.

Some systems also use analog ports for interfacing with a Voice Mail or Automated Attendant system although smoother integration is accomplished with a digital interface.

DTMF Circuit Boards

DTMF (standing for Dual-Tone Multi-Frequency, also known as Touch-tone) signaling requires a separate circuit board within the PBX with DTMF receivers. If you have to wait too long for PBX dial tone after you lift your telephone handset, you may need more DTMF receivers. The receiver is engaged when someone within the system is dialing a telephone number and is freed up after the dialing is completed.

Common Control Circuit Boards

These circuit boards house the central processing capability of the PBX. Most PBXs now have common control circuit boards on every shelf, which is a distributed type of processing. Many of the pre-1980's PBXs had the common control on a single shelf only. If it failed, the entire system failed.

1 comment:

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