docs/templates/pages/sega/software/jvs.html

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<h1>JAMMA Video Standard</h1>
<a href="{{ROOT}}/static/JVST_VER3.pdf">A copy of the JVS standard can be found here</a>. The astute reader may observe
it is in Japanese. A rather excelent translation can be <a href="http://daifukkat.su/files/jvs_wip.pdf">found here</a>.

<p>JVS is a communication standard designed for I/O devices in arcade macines. It typically operates over RS485 serial,
    however the framing format can and sometimes is used outside of this context.</p>
<p>This page documents the canonical JVS structure. Some devices, notably the {{ part("838-14971")|safe }}, use a slight
    variant on the format. <a href="./rs232c.html">The differences between the two formats are document here</a>.
</p>

<h2>Escaping</h2>
<p>Every packet sent begins with the <mark>SYNC</mark> byte, <code>0xe0</code>. Read buffers should be continually
    flushed until this
    byte is observed.</p>
<p>As such, <code>0xe0</code> is reserved, and is escaped. A byte is escaped using <mark>MARK</mark>, <code>0xd0</code>,
    followed by
    the byte's value subtract one. While any byte can be escaped, the only two that require escaping are
    <mark>SYNC</mark> and <mark>MARK</mark>, escaped as <code>D0 DF</code> and <code>D0 CF</code> respectively. All
    operations on packets, such as checksums and length calculation, are performed on the <b>unescaped</b> packet.
</p>

<h2 id="frame">Packet framing</h2>
<figure>
    <figcaption>Request packet (master->slave)</figcaption>
    <table>
        <tr>
            <td>E0<sub>h</sub></td>
            <td>Dest</td>
            <td>n</td>
            <td><mark>D</mark><sub>0</sub></td>
            <td><mark>D</mark><sub>1</sub></td>
            <td><mark>D</mark><sub>2</sub></td>
            <td>...</td>
            <td><mark>D</mark><sub>n-2</sub></td>
            <td><mark>SUM</mark></td>
        </tr>
    </table>
</figure>
<figure>
    <figcaption>Response packet (slave->master)</figcaption>
    <table>
        <tr>
            <td>E0<sub>h</sub></td>
            <td>Dest</td>
            <td><mark>n</mark></td>
            <td>Status</td>
            <td><mark>D</mark><sub>0</sub></td>
            <td><mark>D</mark><sub>1</sub></td>
            <td><mark>D</mark><sub>2</sub></td>
            <td>...</td>
            <td><mark>D</mark><sub>n-3</sub></td>
            <td><mark>SUM</mark></td>
        </tr>
    </table>
</figure>

<p><mark>SUM</mark> is calculated by summing all bytes in the packet, excluding <mark>SYNC</mark> and <mark>SUM</mark>
    itself. <code>n</code> indicates the number of bytes succeeding itself. Due to the existance of the checksum,
    <code>n=0</code> is nonsensical.
</p>

<h2 id="body">Commands and responses</h2>
<p>A request packet may contain multiple commands within it. <code>D<sub>0</sub></code> always contains the first
    command. It is followed by <code>m</code> bytes of data, typically a constant number per command but not always (see
    the command reference), at which point either we have consumed <code>n-1</code> bytes and the next byte is
    <mark>SUM</mark>, or the next byte is another command and we repeat.
</p>
<table>
    <tr>
        <td><mark>CMD</mark><sub>0</sub></td>
        <td><mark>D</mark><sub>0,0</sub></td>
        <td><mark>D</mark><sub>0,1</sub></td>
        <td>...</td>
        <td><mark>CMD</mark><sub>1</sub></td>
        <td><mark>D</mark><sub>1,0</sub></td>
        <td><mark>D</mark><sub>1,1</sub></td>
        <td>...</td>
        <td><mark>CMD</mark><sub>2</sub></td>
        <td><mark>D</mark><sub>2,0</sub></td>
        <td><mark>D</mark><sub>2,1</sub></td>
        <td>...</td>
        <td><mark>SUM</mark></td>
    </tr>
</table>
<p>Response packets are structured similarly, however the <mark>CMD</mark> byte is not present, instead relying on the
    response to be ordered exactly the same as the request. The data of each command is preceeded by a report code,
    indicating that individual command's status. The below diagram also includes the entire-packet status byte too for
    clarity.</p>
<table>
    <tr>
        <td>Status</td>
        <td>Report<sub>0</sub></td>
        <td><mark>D</mark><sub>0,0</sub></td>
        <td><mark>D</mark><sub>0,1</sub></td>
        <td>...</td>
        <td>Report<sub>1</sub></td>
        <td><mark>D</mark><sub>1,0</sub></td>
        <td><mark>D</mark><sub>1,1</sub></td>
        <td>...</td>
        <td>Report<sub>2</sub></td>
        <td><mark>D</mark><sub>2,0</sub></td>
        <td><mark>D</mark><sub>2,1</sub></td>
        <td>...</td>
        <td><mark>SUM</mark></td>
    </tr>
</table>

<h2 id="commands">Individual commands</h2>
<p>I may document these at some point in the future, but honestly the <a
        href="http://daifukkat.su/files/jvs_wip.pdf">translated reference</a> linked at the top of this page does a
    great job.</p>
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