Usages for PnPSA

Usages are non-contiguous assigned in a way that rarely-used usages occupy 16-bit (0x0100 upto 0xFFFF) whereas basic usages are 8-bit only (0x00 upto 0xFF). This helps for short HID report descriptors.

Table

Extra data types

• Lower and upper limit for sample rate and data rate
• Error coefficient (linear error part)
• Lower and upper limit frequency of signal
  • guaranteeeing measurement precision
  • 3 dB drop (maybe rise)
  • custom drop (maybe rise)

• 0x3001 1000 (Hz for example)
• 0x03E8 1000 (Hz for example, prefered coding)
• 0x0000 0 (zero value; while the exponent is not important here, it should be 0 for –2046 upto +2846)
• 0xD00A 0.01 (relative uncertainty of measurement value: 1 %)

Extension to the HID standard (proposal and possible workarounds)

• Omitting data to items should be the same as putting a null value.
  • 15 00   // Logical Minimum (0)
  • 14      // Logical Minimum ()
  Should be interpreted as the same. The Windows HID driver rejects such items, the driver won't load successfully.

• Null values for Report Count indicate disabled items.

• Values larger than 1 for Report Count (Report size > 1) may indicate buffered data with constant sample rate.

• Extra values defined for signed integers for encoding NaN, +∞, –∞:
  • 8 bit: 0x7F, 16 bit: 0x7FFF, 32 bit: 0x7FFFFFFF = +∞
  • 8 bit: 0x80, 16 bit: 0x8000, 32 bit: 0x80000000 = NaN (no measured value, empty slot)
  • 8 bit: 0x81, 16 bit: 0x8001, 32 bit: 0x80000001 = –∞
  Same procedure for any other bit size. It's important for the host software to handle these 3 cases while sign-extending, range-compressing, or converting to a float value! Here a procedure for sign-extension of 8 or 16 bits to 32 bits:
  1. Load the value to a 32-bit register
  2. Flip sign bit
  3. Sign-extend
  4. If (unsigned)(value+1) ≥ 3 (Have regular value)
     • Flip bits from sign bit towards MSB, or
     • Get value again and sign-extend it as usual
  5. else (have denormal value)
     • Flip MSB

  long expand(char x) { long t=(char)(x^0x80); if ((unsigned long)(t+1)>=3UL) return x; return t^0x80000000L; }

  long expand(short x) { long t=(short)(x^0x8000); if ((unsigned long)(t+1)>=3UL) return x; return t^0x80000000L; }

  float expand(long x) { switch (x) { case 0x7FFFFFFF: return -INF; case 0x80000000: return NaN; case 0x80000001: return +INF; } return x; }

• Some Set_Feature requests can change the Report Descriptor!
  Note that the same features may be volatile. In this case, all pipes react sending STALL until cleared.
  Example: Multimeter with non-lockable dial for measurement mode and range.
  The client software has to clear the stall condition by re-reading the report descriptor, interpret for changed values and to resume the measurement and recording process with changed units, range, uncertainty etc.
  String descriptors are assumed to be static.

Main items

• 0000 00nn Output over a Bulk/Isochronous endpoint (possibly high-speed) [Data item, generating data on — maybe supplementary — OUT pipe]
• 0001 00nn Input over a Bulk/Isochronous endpoint (possibly high-speed) [Data item, generating data on — maybe supplementary — IN pipe]
• 0010 00nn Structure Builder [Data item, it defines structured data later used in a variable-length array, to be used in a specific collection ONLY]
• 0011 00nn Constant feature (i.e. an “informational” item) [Data item, not generating data]
  • Current workaround: Regular Feature item with “Const” flag set

• Input, Output, and Feature item flags (i.e. the bits for Data items)
  6. volatile flag for Input items: This item can be read only once. Similar to streaming with FIFO size == 1
  9. streaming flag for Input/Output items: This item is FIFO buffered.
     • READ over BULK/INTERRUPT pipe moves the FIFO pointer
     • READ over CONTROL pipe reads the current value (no FIFO involved)
     • WRITE over BULK/INTERRUPT pipe puts data into FIFO
     • WRITE over CONTROL pipe puts data ahead to the output (i.e. out-of-band data)
     Both FIFOs should discard oldest data when full. The minimum FIFO size should be calculated for holding data for 0.2 seconds.
  9. Invalidating Report Descriptor [DF/DV Feature Items only] This Feature changes the Report Descriptor
  10. variable-length array flag for Data items: This bit is only allowed for the last item of a report. This enables variable-length reports and can help preserving USB bandwidth considerably. The maximum length is calculated from “Report Count()” value (or a new LOCAL item??).
      Unluckily, this lacks support for variable-length cluster arrays.
  11. signed flag for Data items: The value should be sign-extended, independently of the (unsigned) values of Logical_Minimum and Logical_Maximum
      Needed for a thermometer that erraneously reads a temperature below 0 K which should be interpreted as an underflow, not an overflow.
  12. Exponent/Mantissa type (for multiples of 16-bit data types only)
  13. IEEE float point, single precision (for multiples of 32-bit data types only)
  14. IEEE float point, double precision (for multiples of 64-bit data types only)
  15. Complex data (additional imaginary part for any data type)
• Collection items
  6. Same sample rate group
  7. Rational sample rate group
  8. Table (one dimension, given by Usage_Minimum and Usage Maximum)
     Multi-dimensional arrays can be constructed by nesting tables.
     Tables are a special case for report generation currently not supported by any HID stack!
  9. Sparse Array (multiple dimensions, to be defined)
  10. Structure definition
  Collection items should not only take UsagePage:Usage values but also String ID and other local items.

Global items

• Uncerntainty of measurement value
• Data rate

Current implementation situation

• Windows: No workaround necessary, Windows lists unknown global items using HidP_GetExtendedAttributes().
• Linux: unknown

Local items

• String ID for measurement location (String Descriptor editable by user)
• String ID for symbol (String Descriptor editable by user)
• String ID for custom unit (String Descriptor editable by user)

Current implementation situation

Long item format

Current implementation situation

• Windows: The API doesn't seem to support long items.
  
• Linux: unknown

Other extensions

Sometimes, Graphs are more useful. Graphs can be represented by trees with links.

Moreover, Tables (at least two-dimensional) are needed. There is no efficient tree representation for tables, therefore, an extra collection type seems to be useful, and changed processing for the items herein are necessary. Sparse tables (with some or mire empty fields) should be supported.

More descriptors

• 0x21 Report Descriptor (one)
• 0x22 Physical Descriptors (multiple)

• 0x23 Conditioner Descriptors (multiple, count can be variable)
• 0x24 Uncertainty Descriptors (multiple, count can be variable, for more complex uncertainty description)
• 0x25 Factory Certification Descriptor (zero or one)
• 0x26 User Certification Descriptor (zero or one)

Bridge technologies

• least as possible overhead
• Same or mostly the same descriptors
• Same or almost the same transportation
• Synchronization measures as good as it can

PnPSA over TCP

• Inter-process communication between PnPSA client and a PnPSA server. The PnPSA server's purpose is converting a non-PnPSA device to be PnPSA compliant. So it's merely a driver. This will ensure evolution towards PnPSA.
• True networking between computers, for remote sensing
• A pseudo-driver for converting USB to TCP should be available for all major host systems (is a task for me)

This bridge has the following capabilities:

• A TCP socket based bridge has one connection transferring SETUP, OUT and IN data the same way as done in USB. However, the preamble and acknowledge is omitted for simplicity. SET_ADDRESS is not supported. FIFO sizes can be assumed to be quite large, at least 4096 bytes.
• The “USB” configuration descriptor's endpoint descriptor is extended to contain a socket number instead of the USB endpoint number.
• STALL is emulated by closing the data pipe by device
• USB RESET = host closes and re-opens control TCP connection
• SET_ADDRESS unsupported (not needed)
• No remote wakeup feature
• Surprise removal = device closes control TCP connection

PnPSA over serial line

• Sophisticated devices that need (or don't need):
  • low data transfer rate and no synchronization (i.e. per-sample polling)
  • longer cables
  • isolation
• Migration for measurement devices with changeable firmware

This bridge has the following capabilities:

• PnP negotiation, as documented in Serial PnP documentation, starting with 1200,7,n,1 (possibly not possible for RS485 buses)
• Baud rate negotiation: To be defined!
• The data rate should support 115200 Baud, to use the maximum possible rate for most PC equipment
• All other serial settings are fixed to
  • 8 data bits, no parity, 1 stop bit, true binary mode
  • no handshake (it's done within the USB-alike layer)
  • idle-timeout should be 30 bit times, for detecting missing answers
  • end-of-transmission timeout should be 10 bit times, for detecting short packets
  • DTR at +12V, RTS at –12V, to feed low-power equipment like mice or other HID
• Single-Device Mode (RS232, RS422) and Multiple-Device Mode (RS485) via SET_ADDRESS
• Full-Duplex operation support is not intended for now
• Functionality compatible to USB Low-Speed devices:
  • FIFO size: 8 bytes (plus header, 1 byte, plus checksum, 2 byte)
  • Only control endpoint EP0, interrupt endpoints, no bulk or isochronous endpoints
  • Optional: KeepAlive simulated by short BREAK (10 .. 20 bit times)
• USB RESET = long BREAK (50 ms)
• SET_ADDRESS supported (needed for RS485)
• No remote wakeup feature
• No true support for surprise removal

Other bridges

While drivers for TCP and Serial Line has to be written by the Standard maker, other drivers are intended to be vendor specific.