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Protocol

Device is operated in HID mode by sending a request and reading a response.

Requests and responses are basically messages in a form of byte arrays. Device specifies a fixed length for such arrays, and thus remaining bytes in messages shorter than maximum length will be ignored.

Request are prefixed by a request type (1 byte). Response is prefixed with the same byte.

(All examples are in hex)

Request types

  • 1: Get capabilities
  • 2: Get input values
  • 3: Set output values

Get capabilities (request type 1)

The idea of providing this capability information to host is to allow making of generic control program. It tells how many indicators for inputs and how many buttons (or the like) for outputs should be displayed and what selections for outputs are available.

There are things like high/low voltage levels, which pins/pad is which input/output number etc, that are not relevant to software. Each implementation of the device should have that information available for the person connecting the peripherals.

Request:

Pos Value Description
0 1 Fixed request type

Example: 01

Response:

Pos Value Description
0 1 Fixed request type
1 0-255 Amount of inputs
2 0-255 Amount of outputs (assumed 0 if msg len < 3)
3 0-3 All outputs are this type (assumed 0 if msg len < 4)
4 Output type is specified for each output starting from pos 4
4 - (n+3) 0-255 Output types, 4 types per byte, first type in MSBs

n = (amount of outputs+3)/4, where "/" denotes integer division

Bit designations for output types:

Bits Dec Supported modes
00 0 H + L (low impedance only)
01 1 H + L + Z (tristate support)
10 2 L + Z (⎐ open collector output to low/gnd )
11 3 H + Z (⎏ open collector output to high/vcc )

H = High, L = Low, Z = Floating / High impedance

Example: 01 01 02 00 => This device has 1 input and 2 outputs, outputs support H + L + Z

Example: 01 01 => This device has 1 input and 0 outputs

Get input values (request type 2)

Request:

Pos Value Description
0 2 Fixed request type

Example: 02

Response:

Pos Value Description
0 2 Fixed request type
1-n 0-255 Input values, 8 inputs per byte, first input in MSB

n = (amount of inputs+7)/8, where "/" denotes integer division

Example: 02 80 => Input 1 is high (rest of the inputs are low)

Example: 02 00 => All inputs are low

Example: 02 28 => Inputs 3 and 5 are high (rest are low)

Set output values (request type 3)

Each output value is given with two bits:

Bits Meaning
00 No change. Output state should remain
01 Output is set to high impedance state
10 Output is set to low
11 Output is set to high

Mode interpretation

Requested mode should be interpreted as follows per output type

Output type Description 01 10 11
0 H + L (low impedance only) No change L H
1 H + L + Z (tristate support) Z L H
2 L + Z (⎐ open collector output to low/gnd ) Z L Z
3 H + Z (⎏ open collector output to high/vcc ) Z Z H

H = High, L = Low, Z = Floating / High impedance

Request:

Pos Value Description
0 3 Fixed request type
1-n 0-255 Output values, 4 inputs per byte, first input in MSBs

n = (amount of outputs+3)/4, where "/" denotes integer division

Example: 03 36 => (0x36 = 00 11 01 10 ) No change on 1st output, 2nd output high, 3rd output to high imp., 4th output low

Response:

Pos Value Description
0 3 Fixed request type

Example: 03 => Request was processed