1. Focus

   This page describes the Off Air Decoder MKII operating in Springhill. There are two decoders required to cover the 2 national paging frequencies on the Hutchison Paging Network.

    

2. Overview

   The OAD MKII is a second generation MTEL device which replaced the original 1997 MTEL unit. It is a self contained 2 RU 19inch rack mounted system.

   The unit is a collection of common modules used in other MTEL products. The decoder is a hand built device as there are no generic decoders with this level of reliability demanded of this application. The radio receiver, processor and TCP server are mounted in the rack. Requiring just 240 volts and an antenna connection the unit will operate down to -110dbm. The receiver operates on the Hutchison 860MHz broadcast link frequency.

   The received audio is FSK and requires demodulating into a digital data stream. The digital data stream is then bit and word  synchronized by a 87C51 processor. The output is fed into a ZWorld RCM3200 series processor which buffers the 87C51 data stream and decodes it into ASCII message packets. The RCM3200 also provides the TCP socket which acts as a server.

    

3. Unit Description

   1. Radio Receiver

      The two decoders have radios on the two broadcast Hutchison frequencies commonly referred to F1 and F2. The F2 decoder has a MDS receiver and the F1 has the newer MDS SD9 model. Both radios are DSP products and operate well down to -114dbm. The radios should be set for wideband channel or a minimum 25khz bandwidth.

       

   2. FSK Decoder

      The broadcast link operates in Bell202 analogue mode to simplify simulcasting the remote sites, therefore it must be converted back to a digital data stream so it can be decoded. The received analogue signal is first converted by a PLL into a digital data stream which is then decoded by a dedicated 87C51 processor. These functions are performed on the main STC Controller board which also contains power supply regulation. The decoder output is POCSAG ASCII data stream at 19.2kbits.

      The 87C51 is a very reliable processor running stcdecod.as V1.65 Jan/2007. This version has been successfully running non-stop in MTEL equipment since 2007. It is a digital PLL which synchronises onto the data stream and then strips the 32 bit word POCSAG format into an ASCII data stream with start of transmission, baud rate and end of transmission markers.

       

   3. POCSAG Decoder and TCP server.

      The ASCII data stream is then decoded by the RCM3200 module. The RCM is a high speed processor with flash memory and a TCP socket. The module is mounted on a PC2060 card which provides power and interface logic. The decoder uses C code to decode the POCSAG and convert it into message packets which are accessible through the TCP socket operating as a server.

      The output is formatted and sent to PagerDEC running on a workstation. The picture below shows the two motherboards and the RCM3200 (red board) sandwiched together to provide the overall system.

    

   

    

    

4. Unit Operation

   The OAD MKII is a standalone device and does not require user interaction. The TCP address may require setting up initially however after that the device begins decoding immediately after power up.

   There are lights on the front of the device to indicate its operational state. The descriptions for LEDs from left to right on the front panel are given below;

    

   

    

   Carrier Detect (Green)

   This LED indicates the presence of in-band signal from the receiver. If the level of signal is good this indication is very accurate. In a noisy signal the LED may flash during absence of carrier. In this case the signal should be increased for best operation. Normally this would indicate a poor signal from the antenna or cable.

   CRC Error (Red)

   The data stream contains CRC information for each 32 bits of data. If the data is corrupted the CRC LED will be on for the duration of the corruption. A Piezo will also sound whenever the LED is ON.

   If the CRC indication activates it will usually be a result of lack of signal on the broadcast link or it may indicate interference on the link receiver.

   Baud (Red)

   This LED has three states. When there is no signal it will flash at about 5Hertz, indicating it is searching the data stream for valid synchronisation data.

   ON indicates the processor has locked onto a valid high data rate paging transmission (1200 baud). OFF indicates the processor has locked onto a valid low data rate paging transmission (512).

   The LED will remain in this state until the data stream ceases.

   BUS (Yellow)

   This LED reflects the data stream from the 87C51 decoder to the RCM3200 module. It flashes when there is a valid POCSAG stream. Each flash is 32 bits of data.

   TCP Data (Green)

   The message packets are sent by TCP to PagerDEC. This LED will flash when 80 bytes of data are sent by the TCP server to the TCP client.

   If the socket is not connected this LED will not flash on the F1 unit. On the F2 unit it will flash when data is sent to the WBB modem.

    

5. End of Document

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