HARDWARE AND
HARDWARE OPTIONS
The location and placement of system hardware options are shown on Sheet 4 of the 800 MHz Dual Format MDX Mobile Interconnect Diagrams.
OPTION CABLE
Option Cable Option PMCD7Z (19C851585P14) is used to bring all option connections from the System Board through the back of the radio to the outside. This cable is required with all external options.
NOISE SUPPRESSION KIT OPTION
Noise Suppression Kit Option PMPD1A (consisting of Filter 19A148539G1 and Installation Manual LBI-31363) is available for installations where excessive alternator or elec- trical noises, present on the power cable, do not permit the radio to operate properly. Refer to the interconnect diagram for the radio and options.
POWER CABLE OPTION
1 8 - f o o t P ow e r C a b l e O p ti o n PM C D 9 A , (19B801358P17), is available for installations requiring more than the standard 9-foot cable.
EXTERNAL SPEAKER OPTION
External Speaker and Cable Option PMZM1T provides the user with a five-inch waterproof speaker in a LEXAN housing. PMCC9M is an 18 inch interconnecting cable for the speaker. The radio’s 10-watt amplifier drives the speaker’s 4-ohm impedance. The speaker leads are con- nected to pins 2 and 9 of Option Cable Option PMCD7Z (19C851585P14), using External Speaker Cable Option PMCC9M (19A149590P8) (18 inches) included in the PMZM5T kit . A 16 -foot cable, Option PMCD1W (19A149590P10) is also available.
EXTERNAL ALARM
E xtern al Ala rm Hor n R elay Option PM SU1C (19A705499P1) can sound the vehicle horn when a call is received. The option connects to pin 13 of Option Cable Option PMCD7Z (19C851585P14) and is controlled by a front panel option switch.
SYSTEM DESCRIPTION
EDACS
The Dual Format MDX mobile radio operates in either EDACS (digital) mode, or in GE-MARC (tone) mode, pro- viding customers another dimension of flexibility in opera- tion. Both modes provide opportunities to increase RF channel utilization through faster channel access and the privacy inherent with selective signalling.
The EDACS system uses 9600 baud, high speed, digital signalling to identify individual units, user groups, fleets, and agencies. Agencies contain multiple fleets and fleets contain multiple user groups (sub-fleets). By using this ad- dressing scheme, large user groups can be accessed simulta- neously all the way down to individual users. The programming to determine transmit encoded groups and decoded received groups is contained in the personality EEPROM of the mobile. This information is individually programmed to suit each users needs via the PC programmer for the radio.
The typical system configuration consists of at least 2 repeater stations (with a maximum number of 25) and the associated mobiles. One repeater always is a control channel dedicated to sending out continuous control data and also to receive channel request data from the mobiles. When a mobile is first turned on, it scans the available list of frequen- cies programmed in the personality EEPROM for a control channel. When a control channel is found, the mobile locks on to the frequency and monitors the data for a channel assignment (incoming call).
When receiving a channel assignment (incoming call), the monitoring mobile immediately switches over to the assigned voice channel and waits for a high speed data confirmation message. Upon receipt of this message, the voice paths are unmuted and the user can hear the call.
While on the voice channel, the mobile also continu- ously monitors the low speed, 150 baud (subaudible) data and carrier noise squelch to ensure consistent operation. Upon loss of subaudible data reception (i.e., deep fade, or out-of-range), the mobile returns to the control channel frequency.
To initiate a call, the user keys the radio (which is locked to the control channel), and a 30 millisecond high speed data slotted channel request is transmitted to the control channel receiver. The control channel processes the request from the mobile and transmits back a voice channel assignment on an unused channel.
When all available voice channels are in use, the control channel places the mobile into a queue, transmits a queue message back to the mobile, and will give a channel assign- ment to the requesting mobile as soon as a voice channel is free. If the system is busy and the station queue is filled to capacity, a system busy message is returned to the requesting mobile and an alert signal is given to the user.
After the initiating mobile receives a channel assignment from the control station, it immediately switches frequency over to the assigned voice channel and sends a burst of 9600 baud dotting. The microphone voice paths are then unmuted and the transmission begins. The transmitting mobile also continuously sends out a subaudible tone (along with voice) for system reliability. If the station loses this signalling, the voice channel is muted and all receiving mobiles are sent back to the control channel.
In normal operation, the transmitting mobile sends a high speed data burst to indicate that the user has unkeyed, causing all listening mobiles to switch back to the control channel.
CONVENTIONAL MODE
In conventional mode (not trunked) the radio can operate either with tone Channel Guard, digital Channel Guard, or carrier squelch, depending on personality programming. Tone Channel Guard range is 67.0 to 210.7 Hz. Squelch Tail Elimi- nation (STE) is used with Channel Guard to eliminate squelch tails at the receiving radio by phase shifting the transmitted Channel Guard tone when the PTT is released.
Direct mode works identically to conventional mode ex- cept that the transmit frequency band is changed to 851 to 870 MHz to permit direct mobile-to-mobile communications.
GE-MARC
The GE-MARCtrunked mobile radio system consists of a repeater for each channel and the users’ mobile radio units. The system uses tone signalling with each mobile being as- signed two and/or four tone group tone sequences. Groups of mobiles are assigned the same tones so that any unit can talk to all other units in the same group. A block diagram of the GE-MARC MDX is shown in Figure 1.
When originating a call, the mobile identifies an idle repeater channel and interrogates it with a single burst of "busy" tone; the repeater keys its transmitter and sends a burst of "acquisition" tone back to the mobile unit. When the interrogating mobile detects the acquisition tone, it then trans- mits its collect and group tones which the repeater regenerates for all idle mobile units in the system.
The idle mobiles, which continually scan all channels, will stop on the active channel if any of the programmed collect tones are detected and wait for group tone(s).
If the correct tone sequence is detected, the mobiles will alert the operator of an incoming call and open their audio circuits. If the correct sequence is not detected, the idle mobiles will resume scanning the channels. Once the mobile is "locked" on a channel, it will remain there until the repeater times out or the operator terminates the call.
Operational Modes
The radio will always be in one of three operational modes: Idle, Wait, or Ready. The three operational modes and the
conditions that cause the radio to switch from one mode to another are shown in Figure 2.
The radio enters the Idle mode when power is turned on and begins scanning channels for incoming calls. The Wait mode is entered when the user places a call. The radio remains in the Wait mode until a channel is acquired, or if no channel is available. The Ready or Conversation mode is indicated by an alert tone and the mode indicator on the control panel. A signal timing diagram is shown in Figure 3.
Idle Mode
When the radio is in the Idle mode, the audio is muted and all channels programmed for call decode are sequentially scanned for an incoming call. An incoming call is identified by detecting one of the collect tones programmed in the area. Upon receipt of a collect tone, the mobile looks for a short interval for the group or individual tones providing that their collect tones are the same. When no valid tone is found, the mobile will resume scanning the channels for an incoming call.
If a group (or individual decode) tone is detected, the mobile then looks for busy tone for a 90 millisecond period. If four tones are properly decoded, the mobile will then look for busy tone for 270 milliseconds.
When no valid tones are found, the mobile will resume scanning for a call with the next channel. When a busy tone is found, the mobile will enter the Ready mode. If busy tone is not detected, the mobile remains in the Idle mode and continues scanning channels looking for an incoming call.
Removing the handset from the hanger, pressing the PTT switch or pressing the SEND key on the handset will cause the radio to enter the Wait mode.
Wait Mode
When the user enters the Wait mode, the display group is checked to make sure it is a valid call-originate group. If it is not valid, a low-frequency tone is heard for one second. If valid, the radio will scan the call-originate frequencies for brief intervals until it finds one with no busy tone on it. If no channel is free, the radio, if programmed for this option, will activate the Call Retry state and display "RETRYING" in the display. Retrying will cause the radio to revert to the Idle mode and scan for an incoming call while trying to acquire a free repeater approximately every 5 seconds for a 2 minute period. If the Retry option is not enabled, the mobile will sound the low-fre- quency tone and then return to the Idle mode and display
"BUSY".
If a channel with no busy tone is found, the mobile trans- mits a burst of busy tone to acquire the repeater. The repeater then responds with a burst of acquisition tone. Upon receipt of the acquisition tone, the mobile proceeds to transmit the group tones (either two or four tones). If a four-tone sequence is sent, the mobile must detect all four tones and busy tone before entering the Ready mode. If a two-tone sequence is sent, the busy tone must be present within 90 milliseconds of the last
