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 some- one 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. Other Components Inside The PBX Cabinet You may see a power supply, a fan and a variety of other electronic components within the PBX cabinet. The Voice Mail electronics may also reside within the cabinet. 28 20 The Telecom Handbook CHAPTER 2 It is important to remember that the circuit boards, like the telephones, are specific to a particular manufacturer's PBX. They are not interchangeable with other PBX's boards. They may not even be interchangeable with circuit boards of other PBX mod- els in the same manufacturer's product line. The PBX also incorporates a CPU (Central Processing Unit), the "brains" of the PBX and a power supply. PBX Sizing and Growth When planning the purchase of a PBX, growth capability is a major consideration. Although they may be modular in design, in terms of telephones and outside lines that can be connected to the system, PBXs have a maximum capacity. In most systems, one port represents the ability to connect one telephone or one outside line. A 400 port system can accommodate a total combi- nation of 400 outside lines and telephones. There are no precise ratios in terms of how many outside lines are required for the number of telephones. A conservative ratio would be 25 outside lines (for incoming and outgoing calls) for every 100 telephones. If there are not a lot of calls, 10 lines may be sufficient. If it's very busy, more than 25