Details
These radios are seen as obsolete and there is only one known instance of this system operating in the U.S. today (Grant County, Oklahoma). The general category of this kind of trunked system is called, "Scan-based trunking." In the U.S. and Australia, these systems used analog FM, operated in the 806-869 MHz band, and were primarily used for commercial, non-public-safety trunking. Some earlier systems offered half-duplex, (push-to-talk) telephone interconnect and later versions also offered full duplex telephone interconnect. This feature was popular before the rollout of analog cellular telephones.
Radio models used in these systems included the names, "Classic", "Corona", "Centura", generally referred to as 3C radios. While most radio models were sold with a speaker and microphone, some had a telephone handset and cradle attached to the front of the dash-mount radio housing. There was an earlier trunk-mount model that looked like a MASTR Executive II with the name Mastr. A MPR-series hand-held model was offered for GE Marc V trunked systems. Later models included a Japanese-made radio with model numbers beginning with TMX (mobiles) and TPX (portables). Certain later models of PCS and MDX radios were dual-mode, supporting GE Marc V and EDACS.
A large system operator might own their radio backbone (repeaters). GE Marc V Specialized Mobile Radio systems, where subscribers paid a company to operate the trunked radio system, were present in major U.S. cities. The operators typically charged a monthly fee for each radio plus airtime charges.
Some mobile units could be programmed to work on more than one GE Marc V trunked system. For example, if the SMR operator had a system in Bloomington and another covering Chicago, many radio models offered an "area" switch. This selected the transmit area priority. Different radio models had 29- to 100-channel capacity and there were limitations on the number of areas and the number of channels per area. In its default configuration, the radios function in the same way a talk-back-on-scan option works: regardless of which area was selected, the radio would join a conversation on the system where it was taking place.
Early first generation radios were crystal controlled and had crystal oscillator modules with temperature compensators (ICOMS), and later models were synthesized and had an 82S123 32x8 (not 32Kx8) PROM which defined RF frequencies. Option settings and tones were selected by jumpers and laser-etched hybrids called Versatones. Second generation "3C" radios used an Intel MCS-48 microprocessor and programming was burned into two 82S123 PROMs. In subsequent generation radios, a PC programming interface set the radio's RF channels and options. In U.S. systems, the set of channel frequencies for a system or area were usually unique to each trunked system. The radios could scan multiple systems provided that the total number of channels in the combined systems did not exceed 20. The radios were not capable of roaming. (Roaming in this use means working with an unknown GE Marc V system the user happened to run across in their travels).
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