10. Iforce Protocol¶
Johann Deneux <email@example.com>
by Vojtech Pavlik.
This document describes what I managed to discover about the protocol used to specify force effects to I-Force 2.0 devices. None of this information comes from Immerse. That’s why you should not trust what is written in this document. This document is intended to help understanding the protocol. This is not a reference. Comments and corrections are welcome. To contact me, send an email to: firstname.lastname@example.org
I shall not be held responsible for any damage or harm caused if you try to send data to your I-Force device based on what you read in this document.
10.2. Preliminary Notes¶
All values are hexadecimal with big-endian encoding (msb on the left). Beware, values inside packets are encoded using little-endian. Bytes whose roles are unknown are marked ??? Information that needs deeper inspection is marked (?)
10.2.1. General form of a packet¶
This is how packets look when the device uses the rs232 to communicate.
CS is the checksum. It is equal to the exclusive or of all bytes.
When using USB:
The 2B, LEN and CS fields have disappeared, probably because USB handles frames and data corruption is handled or unsignificant.
First, I describe effects that are sent by the device to the computer
10.3. Device input state¶
This packet is used to indicate the state of each button and the value of each axis:
OP= 01 for a joystick, 03 for a wheel LEN= Varies from device to device 00 X-Axis lsb 01 X-Axis msb 02 Y-Axis lsb, or gas pedal for a wheel 03 Y-Axis msb, or brake pedal for a wheel 04 Throttle 05 Buttons 06 Lower 4 bits: Buttons Upper 4 bits: Hat 07 Rudder
10.4. Device effects states¶
OP= 02 LEN= Varies 00 ? Bit 1 (Value 2) is the value of the deadman switch 01 Bit 8 is set if the effect is playing. Bits 0 to 7 are the effect id. 02 ?? 03 Address of parameter block changed (lsb) 04 Address of parameter block changed (msb) 05 Address of second parameter block changed (lsb) ... depending on the number of parameter blocks updated
10.4.1. Force effect¶
OP= 01 LEN= 0e 00 Channel (when playing several effects at the same time, each must be assigned a channel) 01 Wave form Val 00 Constant Val 20 Square Val 21 Triangle Val 22 Sine Val 23 Sawtooth up Val 24 Sawtooth down Val 40 Spring (Force = f(pos)) Val 41 Friction (Force = f(velocity)) and Inertia (Force = f(acceleration)) 02 Axes affected and trigger Bits 4-7: Val 2 = effect along one axis. Byte 05 indicates direction Val 4 = X axis only. Byte 05 must contain 5a Val 8 = Y axis only. Byte 05 must contain b4 Val c = X and Y axes. Bytes 05 must contain 60 Bits 0-3: Val 0 = No trigger Val x+1 = Button x triggers the effect When the whole byte is 0, cancel the previously set trigger 03-04 Duration of effect (little endian encoding, in ms) 05 Direction of effect, if applicable. Else, see 02 for value to assign. 06-07 Minimum time between triggering. 08-09 Address of periodicity or magnitude parameters 0a-0b Address of attack and fade parameters, or ffff if none. *or* 08-09 Address of interactive parameters for X-axis, or ffff if not applicable 0a-0b Address of interactive parameters for Y-axis, or ffff if not applicable 0c-0d Delay before execution of effect (little endian encoding, in ms)
10.4.2. Time based parameters¶
10.4.2.1. Attack and fade¶
OP= 02 LEN= 08 00-01 Address where to store the parameters 02-03 Duration of attack (little endian encoding, in ms) 04 Level at end of attack. Signed byte. 05-06 Duration of fade. 07 Level at end of fade.
OP= 03 LEN= 03 00-01 Address 02 Level. Signed byte.
OP= 04 LEN= 07 00-01 Address 02 Magnitude. Signed byte. 03 Offset. Signed byte. 04 Phase. Val 00 = 0 deg, Val 40 = 90 degs. 05-06 Period (little endian encoding, in ms)
10.4.3. Interactive parameters¶
OP= 05 LEN= 0a 00-01 Address 02 Positive Coeff 03 Negative Coeff 04+05 Offset (center) 06+07 Dead band (Val 01F4 = 5000 (decimal)) 08 Positive saturation (Val 0a = 1000 (decimal) Val 64 = 10000 (decimal)) 09 Negative saturation
The encoding is a bit funny here: For coeffs, these are signed values. The maximum value is 64 (100 decimal), the min is 9c. For the offset, the minimum value is FE0C, the maximum value is 01F4. For the deadband, the minimum value is 0, the max is 03E8.
OP= 41 LEN= 03 00 Channel 01 Start/Stop Val 00: Stop Val 01: Start and play once. Val 41: Start and play n times (See byte 02 below) 02 Number of iterations n.
10.4.5.1. Querying features¶
OP= ff Query command. Length varies according to the query type. The general format of this packet is: ff 01 QUERY [INDEX] CHECKSUM responses are of the same form: FF LEN QUERY VALUE_QUERIED CHECKSUM2 where LEN = 1 + length(VALUE_QUERIED)
10.4.5.1.1. Query ram size¶
QUERY = 42 ('B'uffer size)
The device should reply with the same packet plus two additional bytes containing the size of the memory: ff 03 42 03 e8 CS would mean that the device has 1000 bytes of ram available.
10.4.5.1.2. Query number of effects¶
QUERY = 4e ('N'umber of effects)
The device should respond by sending the number of effects that can be played at the same time (one byte) ff 02 4e 14 CS would stand for 20 effects.
10.4.5.1.3. Vendor’s id¶
QUERY = 4d ('M'anufacturer)
Query the vendors’id (2 bytes)
10.4.5.1.4. Product id¶
QUERY = 50 ('P'roduct)
Query the product id (2 bytes)
10.4.5.1.5. Open device¶
QUERY = 4f ('O'pen)
No data returned.
10.4.5.1.6. Close device¶
QUERY = 43 ('C')lose
No data returned.
10.4.5.1.7. Query effect¶
QUERY = 45 ('E')
Send effect type. Returns nonzero if supported (2 bytes)
10.4.5.1.8. Firmware Version¶
QUERY = 56 ('V'ersion)
Sends back 3 bytes - major, minor, subminor
10.4.5.2. Initialisation of the device¶
10.4.5.2.1. Set Control¶
Device dependent, can be different on different models!
OP= 40 <idx> <val> [<val>] LEN= 2 or 3 00 Idx Idx 00 Set dead zone (0..2048) Idx 01 Ignore Deadman sensor (0..1) Idx 02 Enable comm watchdog (0..1) Idx 03 Set the strength of the spring (0..100) Idx 04 Enable or disable the spring (0/1) Idx 05 Set axis saturation threshold (0..2048)
10.4.5.2.2. Set Effect State¶
OP= 42 <val> LEN= 1 00 State Bit 3 Pause force feedback Bit 2 Enable force feedback Bit 0 Stop all effects
10.4.5.2.3. Set overall¶
OP= 43 <val> LEN= 1 00 Gain Val 00 = 0% Val 40 = 50% Val 80 = 100%
10.4.6. Parameter memory¶
Each device has a certain amount of memory to store parameters of effects. The amount of RAM may vary, I encountered values from 200 to 1000 bytes. Below is the amount of memory apparently needed for every set of parameters:
period : 0c
magnitude : 02
attack and fade : 0e
interactive : 08
10.5. Appendix: How to study the protocol?¶
1. Generate effects using the force editor provided with the DirectX SDK, or use Immersion Studio (freely available at their web site in the developer section: www.immersion.com) 2. Start a soft spying RS232 or USB (depending on where you connected your joystick/wheel). I used ComPortSpy from fCoder (alpha version!) 3. Play the effect, and watch what happens on the spy screen.
A few words about ComPortSpy: At first glance, this software seems, hum, well… buggy. In fact, data appear with a few seconds latency. Personally, I restart it every time I play an effect. Remember it’s free (as in free beer) and alpha!