lora-irrigation/RS485HID.md

USB Keyboard over RS-485
========================

The project is to have a device that appears to a computer as a USB
keyboard, but it receives it's key presses from another computer via an
RS-485 multidrop bus.  It will use the Syote library for communications,
and as there can be multiple devices on the bus, a single gateway
controller can service multiple such keyboards, making it a nice option
for an IPMI like solution.  It is also expected that GPIOs of the device
can be used to control the power switches, along with observing various
LEDs.  As most microcontrollers have an ADC, it could theoretically
support beeps from the PC speaker.

Devices
-------

These are the device that are under conideration for the project:

### STM32F103C8

64KB flash, but the devices I have seem to have 128KB flash.  They
appear to be BluePill clones.  Green LED is GPIOC Pin 13.

Currently looking at USART3, B0 for DE, B1 for RE.

TIM2_CH4 can be mapped to PB11 (UART3 RX).  This is to do break detection.
Table 45 says TIM2_REMAP, either full or partial (value 1x).

Table 5 pin definitions of stm32f103c8.pdf
(all can be remapped)

USART has idle line detction

USART1_TX PA9 5v tollerant
USART1_RX PA10 5v tollerant
USART2_TX PA2 NOT 5v tollerant
USART2_RX PA3 NOT 5v tollerant
USART3_TX PB10 5v tollerant
USART3_RX PB11 5v tollerant

flash: 0/x
boot loader (an2606) pattern 1: 1/0
embedded sram: 1/1
top jumper BOOT0/44
bottom jumper BOOT1/PB2/20

### MAX485

This requires a 5V VCC.  There is a 3.3V part called the MAX3485, but
as often micros can be 5V tollerant, and the MAX485 is fine w/ logic
levels down to 2V, this should be fine.

D - Driver
	DE Driver enable (high enable)
	DI Driver input
R - Receiver
	RO Receiver output
	/RE Receiver enable (low enable)

Note: Some pre-assembled devices have pull up resistors on the DE/RE
lines.  This is a BAD design as it means that it will be driving the
bus by default.  This can often be fixed by removing the pull-up
resistors.


Programming
-----------

Command to program the HID device:
```
sudo openocd -f interface/ftdi/digilent-hs1.cfg -f interface/ftdi/swd-resistor-hack.cfg -f target/stm32f1x.cfg -c "init" -c "reset init" -c "program build/rs485hid.elf verify reset exit"
```

Keying
------

There are two keys involved, the initiator key, which is made w/ the
Makefile, by issuing the command:
```
bmake hid_priv_key
```

Which creates the private key in the file `.hid_priv_key`.  This is
needed for the build process.

The other key is the device key, which is generated by the USB HID
device at first start.  The device will "type" the hex encoded public
key when the A7 pin is grounded.  This public key should be saved to
a file, which can then be passed to the `lora.py` initiator program
via the `-p` argument.

Low level info
--------------

Definitions:
Mark:	logic 1, B > A
Space:	logic 0, B < A

Idle state: Mark

Async comms: Space | 8 bits LSB, 0 through 7 | Mark

As the start of the sequence begins w/ a space, it requires that the
line be "idle" (aka mark) before things start, so any non-mark state
before TX starts should consider the line as busy, and not be ready
to transmit.  Additional info on this is in the Wiring section.

Wiring
------

For long runs, it is recommend to have terminating resistors w/ a value
of 120 Ω, the impedence of twisted pair, to prevent reflections.

Wikipedia recommends to add biasing resistors to help w/ noise
immunity, BUT, care must be done when using them.  When the bus is
idle, make sure that the receivers are outputing a hi value (aka
mark), that is 5V if you're using a MAX485 converter.  If it is 0V,
then the receiver will not work.  I have seen in some cases where
grounding A w/ a 2.2k Ω resistor makes things work.