Board_002: Start README.md with scope, steps and usb information

Electrical/Board_002/README.md

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+
+# Overview
+
+This represents an alernative PCB for interfacing the HAN M-bus connector
+on AMS meters.
+
+The biggest difference between this board and [the original board](../Board_001)
+is that this board supports communication over a physical wire.
+
+So you can for instance use this board together with an arduino to make
+a robust watt-meter display that will never fail because of connection
+issues (unlike wifi).
+
+This solution uses the industry standard chip for M-bus communication,
+[TSS721A](http://www.ti.com/lit/ds/symlink/tss721a.pdf) with full
+galvanic isolation to the bus, e.g. connecting a cable from this PCB
+to a mains connected PC for instance will not result in a [ground
+loop](https://en.wikipedia.org/wiki/Ground_loop_%28electricity%29).
+
+The schematic and drawings are made with
+[KiCad](http://www.kicad-pcb.org/).
+
+
+# Scope
+
+## Software
+
+Reuse existing [code](../../Code).
+
+## Hardware
+
+Only support receiving transmitted serial data from the bus since the
+Kamstrup AMS meters have no physical support for receiving anything.
+
+The primary use case is to be a shield on an Arduino board (e.g. Uno or
+Leonardo). A standalone ATtiny version would be nice to have as well.
+
+Support daisy-chaining multiple units on the bus.
+
+
+# Status
+
+This is unfinished work, currently just started.
+
+# Future
+
+First phase is only supporting converting M-Bus data to serial I/O.
+Support for ethernet or USB will be implicit via Arduino/shields. Later
+standalone versions with ATTiny will be most useful by including
+ethernet/usb support directly.
+
+## Step 1
+
+Create a simple M-Bus master simulator (transmit only) that takes
+serial data and modulates onto the bus. Write arduino code to send a
+test pattern and verify on oscilloscope.
+
+https://electronics.stackexchange.com/questions/99388/designing-a-m-bus-master-up-to-10-slaves/99390#99390
+https://electronics.stackexchange.com/a/214477/568
+
+## Step 2
+
+Connect the TSS721A chip with all required components with a optocoupler
+between it and Arduino (e.g. M-Bus to TTL conversion). Connect with the
+simulator from step 1 and verify that received data equals the data sendt.
+
+Possibly also test with off-the-shelf M-Bus to TTL conversion hardware
+for comparison.
+
+## Step 3
+
+Finish PCB design to be a usable Arduino shield. Integrate the
+AmsToMqttBridge software. Display received data on LCD.
+
+## Step 4
+
+Put code onto ATTiny on a standalone board with support for either USB
+or ethernet.
+
+The MCP2221 is an inexpensive<sup>1</sup> USB to serial chip that people
+[seems](https://www.element14.com/community/groups/open-source-hardware/blog/2016/02/01/implementing-non-ftdi-usb-to-uart-serial-interfaces)
+[very](http://www.eevblog.com/forum/reviews/alternatives-to-ftdi-usb-to-uart-converter/)
+[satisfied](https://hackaday.io/project/18845-usb-serial-uart)
+[with](https://hackaday.com/2016/03/04/dual-uarti2c-breakout-goes-both-ways/).
+It does [not](http://blog.zakkemble.co.uk/mcp2221-hid-library/)
+[achieve](https://hackaday.com/2017/05/31/counterfeit-hardware-may-lead-to-malware-and-failure/#comment-3636318)
+the maximum speeds is theoretically supports, but that is irrelevant for
+this project. This board should then be powered by USB, both in that that
+is the most natural and convinient choice and that if not then apparently
+MCP2221 needs some additional electronics in order to [work properly as
+an USB device](https://electronics.stackexchange.com/a/323551/568).
+
+
+-------------------
+
+<sup>1</sup> Less than 20 NOK at [Elfa
+Distrelec](https://www.elfadistrelec.no/en/interface-ic-uart-usb-so-14-microchip-mcp2221-sl/p/11087556),
+cheaper elsewhere.
+