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Thread: Motorola radios capable of interperting POCSAG vhf paging.

  1. #1
    meadowcst No Longer Registered

    Default Motorola radios capable of interperting POCSAG vhf paging.

    Evening you Guru's of everything radio.. My Work has went P25 so we lost the Minitor Pagers and are now on this ( Grrr ) remote acess POCSAG vhf system and carry Vhf Motorola Alphanumeric Pagers for callouts and such. As well we have a terminal that local pages can be sent from.

    Is there (Any) Radio that whilst on the channel could read this and after sounding an alert display it on the Front Panel. As in i wouldent have to carry a dispatch pager and a local radio. I assume our prov radio techs would have set it up, if this was possible but i can see these pagers are cheeper than an radio anyways ( motorola PAdvisor)

    Thanks Gents !!!

    About POCSAG PAGING (F/D) The modulation used is FSK with a 4.5 kHz shift on the carrier.
    The high frequency represents a 0 and the low frequency a 1.
    Often single transmission channels contain blocks of data at more than one of the rates.
    Data is transmitted in 32bit 'Pre-amble' code words that are used to synchronise the data transmissions. Each code word can be either an address or data, which is indicated by the first bit transmitted, bit 31. An address code word then contains 18 bits of address (bit 30 through to 13), and 2 function bits (12 & 11).
    A data codeword has 20 bits of data (bit 30 through to 11). Both code words then have 10 bits of ECC that is a BCH code normally referred to as BCH(31,21) and has the ability to correct 2 bit errors in each codeword.
    The BCH ECC code used provides a 6 bit hamming distance between all valid codewords in the possible set (that is every valid 32 bit codeword differs from ever other one in at least 6 bits). This makes one or two bit error correction of codewords possible, and provides a robust error detection capability.
    Code words are transmitted in batches that consist of a sync codeword, defined in the standard as 0x7CD215D8, followed by 16 others containing the data. Any unused code words are filled with the idle value of 0x7A89C197. In practice other values are sometimes used to indicate sync and idle.
    Although the address (also referred to as a RIC - Radio Identity Code) is transmitted as 18 bits the actual length is 21 bits the remaining three bits are derived from which of the 8 pairs of code words in the batch the address is sent in. This strategy allows the receiver to turn off for a considerable percentage of the time as it only needs to listen to the pair that applies to it, thus saving a significant amount of battery power.
    Before a burst of data there will always be a preamble of at least 576 bits of data containing alternating 1s and 0s, allowing the receiver to synchronize itself to the signal, and is another mechanism that enables the receiver to be turned off for a large percentage of the time.
    A message will start with an address codeword followed by a number of data code words and will continue until another address, a sync, or an idle codeword is sent. When the data bits are extracted they will be in one of two formats.
    [edit] Message format

    There are two message coding formats defined for the information content part of messages. Numeric messages are sent as 4 bit BCD values, and alphanumeric messages are sent as 7 bit ASCII. The 7 bit ASCII is commonly referred to as 'alpha-paging', and 4-bit BCD is commonly referred to as 'numeric-paging'.
    [edit] Numeric paging

    BCD encoding packs 4 bit BCD symbols 5 to a codeword into bits 30-11.
    The most significant nibble (bits 30,29,28,27) is the leftmost (or most significant) of a BCD coded numeric datum.
    Values beyond 9 in each nibble (i.e. 0xA through 0xF) are encoded as follows:

    • 0xA Reserved (possibly used for address extension)
    • 0xB Character U (urgency)
    • 0xC " ", Space (blank)
    • 0xD "-", Hyphen (or dash)
    • 0xE ")", Left bracket
    • 0xF "(", Right bracket

    BCD messages are space padded with trailing 0xC's to fill the codeword. There is no POCSAG specified restriction on message length, but particular pagers of course have a fixed number of characters in their display.
    [edit] Text paging

    Alphanumeric messages are encoded in 7 bit ASCII characters packed into the 20 bit data area of a message codeword (bits 30-11). Since three seven bit characters are 21 rather than 20 bits and the designers of the standard did not want to waste transmission time, they chose to pack the first 20 bits of an ASCII message into the first code word, the next 20 bits of a message into the next codeword and so forth.
    What this means that a 7 bit ASCII character of a message that falls on a ******** can and will be split between two code words, and that the alignment of character boundaries in a particular alpha message code word depends on which code word it is of a message. The side benefit of this is a slightly increased ECC reliability for messages that span more than one POCSAG packet.
    Within a codeword 7 bit characters are packed from left to right (MSB to LSB). The LSB of an ASCII character is sent first (is the MSB in the codeword) as per standard ASCII transmission conventions, so viewed as bits inside a codeword the characters are bit reversed.
    ASCII messages are terminated with ETX or EOT to distinguish them from numeric messages.

  2. #2
    cyrus's Avatar
    cyrus is offline Trailer Park Superintendent
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    No, you will need to carry your pager still.

    Bubbles: I'd like to see that Red Blue Green c***sucker put one of those together, duct-tapin' it.