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<h3>Relay computer "trainer"</h3>

<center><a href="relaylarge.jpg" align=center><img src="relaysmall1.jpg"><p>Click for high resolution</p></a>
</center>

<p>The idea behind this project is to make a low relay count, single board
computer similar to the single-board trainers of the early microcomputer era
which can be mass produced for a reasonable price.</p>

<p>Relays are expensive and large, so primary importance is given to
keeping the relay count low.  The basic strategy is implement just the CPU
and some basic I/O using relays, but implement the memory and front panel
user interface with semiconductors.  The CPU is designed using low cost DPDT
DIP relays. The front panel and memory are implemented using Microchip PIC
microcontrollers.</p>

<p>Architectural and circuit design tricks keep the relay count for the CPU
low.  The Relay Trainer uses edge-triggered D flip-flops implemented with
just 1.5 DPDT relays / bit.  Multi-port memory reduces the number of
required register bits.  Self-modified code (borrowed from very early
computers) keep the number of registers and logic gates needed to an
absolute minimum while still maintaining an interesting instruction set.</p>

<p>Here are some of the basic characteristics of the Relay Trainer:</p>

<ul>
<li>Relay count: 83</li>
<li>Datapath width: 8 bits</li>
<li>Instruction width: 32 bits</li>
<li>Instruction type: two-address (each instruction specifies two memory locations and the result is written back to one of them)</li>
<li>Memory size: 256 32-bit words (1 KB)</li>
<li>Registers: 13 bits total including 8-bit Program Counter, 1-bit carry flag, 4-bit output register</li>
<li>Performance: one cycle per instruction, up to ~12 Hz for ~12 instructions per second</li>
<li>I/O: 4 switch closure inputs and 4 relay contact outputs brought to
screw terminals.  Programs can also read from keypad and serial console and
write to serial console.</li>
<li>Supported concepts:</li>
  <ul>
  <li>Subroutines (supported with self-modified code: the return address is inserted into a jump instruction)</li>
  <li>Indexing (supported with self-modified code: the index is inserted into a memory reference instruction)</li>
  <li>Multi-precision arithmetic</li>
  <li>Multiply and Divide implemented as subroutines</li>
  <li>Complete set of conditional jumps (altough there are no Zero or Negative flags, a memory word can be tested for zero and negative)</li>
  <li>Single-instruction loops with "incjne" (increment and jump if not equal to zero) instruction</li>
  </ul>
</ul>

<p>Other basic features:</p>

<ul>
<li>Keypad and LED display allows you to:</li>
  <ul>
    <li>Display and modify memory</li>
    <li>Halt / run and single step</li>
    <li>Vary clock speed (15 pre-programmed speeds or use knob)</li>
  </ul>
<li>Discrete LEDs</li>
  <ul>
    <li>Show current 32-bit instruction given to CPU</li>
    <li>Show two 8-bit operands fetched from memory</li>
    <li>Show current program counter value</li>
    <li>Show 8-bit data being written back to memory</li>
    <li>Show carry flag and condition logic result</li>
    <li>Show input and output port bits</li>
    <li>Show the clock</li>
  </ul>
<li>Uses hard drive molex connector for 12V and 5V power.  Small switching power supplies
  for hard drives for IDE/SATA to USB converter kits are widely available at
  low cost.</li>
<li>512 bytes of Non-volatile memory is provided to save and restore the RAM</li>
<li>Serial port provided for 9600 serial console</li>
  <ul>
    <li>Built-in assembler, disassembler and instruction trace display</li>
    <li>Load or save programs by cutting and pasting into terminal emulator window</li>
    <li>Provides 6-pin Arduino logic-level RS-232 header for use of widely
    available USB to TTL RS-232 converter cables</li>
  </ul>
<li>The relay clock is stopped when the CPU halts: this saves the life of
  the relays.  The clock resumes if you press the step or run buttons.</li>
</ul>

<p>Yes, there are videos!  Check out the <a href="example-code.html">example programs
page</a>.</p>

<p>I'm selling some of my prototypes on eBay: search for "Relay Computer
Trainer"</p>

<br>
<br>
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<center>
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<center><h2>Homebuilt CPUs WebRing</h2>
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<center>JavaScript by <a href="mailto:qirien@earthling.net">Qirien
Dhaela</a></center>
<p/>
<h3>Join the ring?</h3>
To join the Homebuilt CPUs ring, drop me <a
href="mailto:daveb@iinet.net.au">a line</a>,
mentioning your page's URL. I'll then add it to the list.
<br/>
You will need to copy this code fragment into your page. <br/>
<i>Note</i>: The ring is chartered for projects that include a home-built
CPU. It can emulate a commercial part, that&prime;s OK. But actually using
that
commercial CPU doesn&prime;t rate. Likewise, the project must have been
at least partially built: pure paper designs don&prime;t rate either. It can
be built using any technology
you like, from relays to FPGAs.
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