Technical Bulletin
Date:
February 11, 2016
Subject: Interface Changes of Second Generation 1775/1750/1725
IMUs and DSP-1760 Gyros
This bulletin contains proprietary information of KVH Industries, Inc. and neither this document nor said proprietary information
shall be published, reproduced, copied, disclosed, or used for any purpose without the express written permission of a duly
authorized KVH representative.
Summary
This bulletin describes the interface changes KVH has implemented in the 1775, 1750, and 1725 IMUs
(referred to hereafter as “17XX IMUs”) and the DSP-1760 gyros. It does not apply to the DSP-1750 or any
other legacy KVH FOG products.
Product Identification
All 17XX IMUs and DSP-1760s that have serial number 1510K013 or later will have new electronics and
firmware. Note that the new firmware is not interchangeable between hardware platforms and has different
part numbers.
Reason for Change
This design/firmware change improves the reliability and quality of KVH products, including enhanced power
protection and full differential signaling on the RS-422 interface.
Description of Change
The effective date of this change is 4 November 2015. It consists of the following enhancements:
1. RS-422 Interface: The 17XX IMUs and DSP-1760 gyros now use an industry-standard RS-422
differential interface as detailed in this bulletin. Single-ended RS-422 signaling is supported, subject to
the limitations described in this bulletin.
2. Firmware: The new versions of firmware for the 17XX IMUs and DSP-1760 gyros include bug fixes
and built-in self test improvements as described in this bulletin.
3. Technical Manuals: The following new revisions of the 17XX IMUs and DSP-1760 gyro manuals
expand and clarify technical descriptions such as user settings, commands, and signal processing
details:
•
1775 IMU Technical Manual: 54-0938 Rev. B
•
1750 IMU Technical Manual: 54-0858 Rev. G
•
1725 IMU Technical Manual: 54-1054 Rev. B
•
DSP-1760 Technical Manual: 54-0924 Rev. B
The following pages further describe details of the major changes.
Bulletin Ref #: 16_T_FOG_1
Page 1 of 8
RS-422 Overview
The 17XX IMUs and DSP-1760 gyro units use industry-standard ANSI TIA/EIA-422-B (known commonly as
RS-422) type receivers and drivers to accept and transmit communications and digital I/O. The RS-422
standard is a well-documented signal standard. A variety of application notes that detail the RS-422 standard
can be found on the Internet. A few notable website links are provided below.
Website
Address
Wikipedia topic
https://en.wikipedia.org/wiki/RS-422
Texas Instruments application note:
SLLA070D RS-422 and RS-485
Standards Overview and System
Configurations
http://www.ti.com/lit/an/slla070d/slla070d.pdf
National Instruments white paper
http://www.ni.com/white-paper/11390/en/
Analog Devices application note
http://www.analog.com/media/en/technicaldocumentation/application-notes/AN-960.pdf
Texas Instruments application note:
AN-972 Inter-Operation of Interface
Standards
http://www.ti.com/lit/an/snla041a/snla041a.pdf
Texas Instruments application note:
SCEA035A Selecting the Right
Level-Translation Solution
http://www.ti.com/lit/an/scea035a/scea035a.pdf
Bulletin Ref #: 16_T_FOG_1
Page 2 of 8
RS-422 Differential Signal Inputs
The 17XX IMUs and DSP-1760 gyro units use typical RS-422 receiver devices to convert the differential
inputs to single-ended signals that are then used internally. Different inputs may use physically different
receiver or transceiver devices, but they all operate in a similar way. In the schematic representation below,
the dashed box represents the receiver buffer inside the unit. The Data+/- signals, while not shown as such
below, should use twisted-pair wires for external connections longer than a few inches. Internally the received
data, RO, will be a logic high if the voltage at input A is greater than B by at least 200mV. The received data
will be logic low if the input A is less than B by at least 200mV. Inputs are further protected against electrostatic discharge (ESD) that may occur during physical connection.
RS-422 receiver
RO = 1 for A > (B + 200mV); RO = 0 for A < (B - 200mV)
The receivers being used have internal “fail-safe” logic to allow the input data pairs to be left unconnected or
even shorted without causing false logic transitions. The receiver inputs can tolerate some voltage bias
differences between their signals and the host drivers. This is represented as different grounding symbols on
the schematic above. Typically, the ground potential difference can be as much as +/-28Vdc without causing
problems. It is expected that typical applications will have a ground potential difference within this range.
There is a nominal 100 ohm input termination network (Rt) at the receiver to prevent signal reflections. This
may be constructed of several bias resistors in addition to the main termination resistance. This is to prevent
signal reflections and cable crosstalk and to minimize EMI/RFI.
RS-422 Differential Signal Outputs
The 17XX IMUs and DSP-1760 gyro units use typical RS-422 driver devices to transmit communications and
output digital signals (e.g., TOV). Different outputs may use physically different receiver or transceiver
devices, but they all operate in a similar way. In the schematic representation below, the dashed box
represents the driver output buffer inside the unit. The Data+/- signals, while not shown as such below, should
use twisted-pair wires for external connections longer than a few inches. The receiver buffer should include a
standard termination network that appears as a 100 ohm nominal impedance to the driver. This is to prevent
signal reflections and cable crosstalk and to minimize EMI/RFI.
RS-422 driver
Bulletin Ref #: 16_T_FOG_1
Page 3 of 8
Electrical Interface Differences between 1st and 2nd Generation Products
Generation
st
Serial Numbers
1 Generation
1510K012 and earlier
2nd Generation
1510K013 and later
Note: The first four digits of the serial number indicate the year and month (YYMM) the product was built.
For customers who have only the 2nd generation product
KVH recommends using the fully differential RS-422 voltage level signaling as described in the associated
product manual and ICD, and as described in industry-standard application notes. Any of the typical RS-422
driver and receiver devices from a variety of manufacturers should provide reliable control and
communications with the unit. The interface cable should use twisted pairs for communications, digital I/O,
and power to minimize possibility of electro-magnetic radiation being broadcast or corrupting the unit’s
operation. In some cases, the digital I/O can be driven or received as single-ended signals if care is taken to
minimize signal crosstalk within the cabling. KVH recommends using newer devices that have ESD
protection, “fail-safe” logic, and appropriate bandwidth limiting.
For customers who have the 1st generation product
One of the desirable characteristics of RS-422 signal levels is that they are compatible with voltages in the
range of 0 to 5V that are typical of standard interface logic devices. This allows some backward compatibility
from 2nd generation to 1st generation. The differences are discussed below.
Serial Communications (RX and TX)
The 1st generation units use the same RS-422 signal levels for the main serial communications interface.
These are referred to in the product manuals as RS-422 RX+/- and TX+/-. Therefore, no changes to these
signals are required.
Digital I/O (TOV, MSYNC, CFG-RST-IN, EXT-RST-IN)
With the 1st generation units’ input digital I/O structure, unit inputs typically were received by optical-isolators
that were designed to turn on using TTL-based voltage levels. The external input drives the opto-isolator
through a current-limiting resistor. Referring to the figure below, the impedance of the resistor is such that an
external voltage of greater than 2V will turn on the opto-device, thus turning on the switch and bringing the
internal signal to ground. The internal logic (not shown) takes care of any signal inversion of the isolator. One
benefit is that the opto-isolator device is slow and forgiving and, as such, provides noise immunity to fast
transients and cross-talk in the cabling. Another benefit of this interface is that higher voltage levels can be
supported by adding an external series resistor, since the opto-diode is a current-driven device. The diagram
below shows the basic input structure.
1st generation digital input structure
Bulletin Ref #: 16_T_FOG_1
Page 4 of 8
The 1st generation output structure uses the process in reverse. The internal signal going logic low turns on
the opto-diode, thus turning on the switch causing it to sink current to the external output which goes logic
low.
1st generation digital output structure
The newer 2nd generation units use the RS-422 receiver and driver type circuitry for digital I/O. Thus, the
inputs and outputs use all differential signaling pairs. This allows a wider range of ground potential difference
that is much more immune to signal interference from nearby equipment and from signal cross-talk when
using the differential pairs. This also allows use of longer cable runs from the unit to the user’s system.
However, the primary reason for changing the interface devices was to support higher serial baud rates
needed in the newer generation IMUs.
The figure below shows the typical RS-422 output signal driven into an RS-422 receiver with a 100 ohm
termination. In this case, it is the TOV+/- signal output of an IMU. The unit was configured for an output data
rate of 1 KHz.
TOV output when driving a 100 ohm terminated RS-422 receiver
Note that the unit puts out two signals that swing in opposing polarities about a center common mode level;
these are shown as the blue and red (dashed) traces. Also shown is an LVTTL (+3.3V logic) signal (green
dotted trace) that results at the output of an RS-422 receiver (e.g., Linear Technologies LTC2864). Note,
however, that the differential output voltage when low has very little margin with respect to a valid TTL or
Bulletin Ref #: 16_T_FOG_1
Page 5 of 8
LVTTL low voltage input VIL. In the TOV signal above we see the low levels are driven to about 0.6V. Typical
5.0V (TTL) or 3.3V (LVTTL) logic VIL signal level is about 0.8V. This leaves only about 200mV of margin. For
a positive output swing, 5.0V and 3.3V logic VIH is about 2.0V. The TOV signals are driven at about 3.0V for
logic high; this gives about 1.0V of noise margin, which is acceptable.
The figure below shows the same TOV+ output without the 100 ohm termination to its differential pair. In this
case, it would be driving a single-ended load, such as a typical high-impedance logic gate, with perhaps an
RC termination of 100 ohm and 100 pF nominal to ground instead of 100 ohm to its differential pair. We see
that the logic low level is much closer to 0V and provides sufficient noise margin to be used in a single-ended
application.
TOV+ output without 100 ohm termination
KVH recommends that existing users should convert to use the differential signaling to get its full benefits.
However, it is possible to use the unit as single-ended digital I/O with some consideration of the signal level
conversion. Also, the use of short interface cables helps minimize signal crosstalk. KVH recommends that
users review existing application notes such as Texas Instruments AN-972 “Inter-Operation of Interface
Standards” for guidance on connecting other types of logic to the unit’s RS-422 type digital I/O. To use singleended outputs (e.g., TOV), the user can wire to the positive output signal. For backward compatibility reasons,
the positive signals of the differential pairs are located on the same interface pins as the formerly singleended signals.
Signal Ground
The 1st generation units included a separate signal ground wire (pin 15 in the Micro-D interface). This was
used to minimize the ground potential between the unit and the user’s system. As the newer 2nd generation
units use differential signaling, this connection is no longer needed and should no longer be used. Eliminating
the ground wire will prevent system ground loops and possible electrical noise susceptibility and radiation.
Instead, the unit’s power return (ground) should be used to keep the ground potential differences between the
unit and the user’s system to within a few volts positive or negative. If using the digital I/O as single-ended,
Bulletin Ref #: 16_T_FOG_1
Page 6 of 8
then the ground potential differences should be kept well under 1 volt (a few 100ths of a millivolt is
recommended).
Cabling Considerations
Existing 1st generation unit users may experience problems when using the newer 2nd generation units related
to the cabling to their systems. As mentioned before, the older units’ use of opto-isolators was forgiving of fast
transients and cable cross-talk. The newer units’ RS-422 type transceivers are much faster and will respond
to signal reflections and particularly to cross-talk from adjacent wires. For example, if a user’s cable has wires
that connect to the MSYNC- input at the unit but the wire is not connected at the user’s end, then it may
couple with (i.e., pick-up) signal transitions from neighboring wires. This may be seen as triggering an
unwanted MSYNC signal inside the unit. In particular, the MSYNC+ wire falling edge may be coupled into the
partially connected MSYNC- wire and this may be interpreted by the unit as another MSYNC signal. The 2nd
generation units have some added filtering and debounce logic on its digital inputs, but it may not be sufficient
to prevent false triggering. Further, the newer 2nd generation units have implemented a 90 usec hold-off time
from a valid rising edge of the MSYNC signal to the next MSYNC signal. Users should consider implementing
their MSYNC signal duty cycle such that the logic high time is less than about 50 usecs to take advantage of
the added hold-off time.
DSP-1760 Connector Types
The circular Mighty-Mouse connector has 13 pins, two connections fewer than the Micro-D type connector. To
reduce the pin count by two, the negative polarity differential signals of the EXT-RST-IN and CFG-RST-IN
signals are not brought out to the user interface. If the external reset and configuration reset functions are
needed, the user can drive the positive polarity signals as single-ended, low-voltage TTL inputs. The negative
polarity signals, while not at the connector, are internally tied to the positive signals with nominal 100 ohm
termination resistors. This, plus their internal bias resistors, will allow the inputs to work as single-ended if
driven by the correct voltages.
To reset the unit, drive the EXT-RST-IN+ signal above +2.7Vdc and no more than +5Vdc. To prevent reset,
leave the EXT-RST-IN+ signal unconnected or apply a signal below +0.6Vdc to no less than 0.0Vdc.
To reset the configuration, drive the CFG-RST-IN+ signal above +2.7Vdc and no more than +5Vdc while
applying power or using the external reset signal. Once the unit has begun output of data, the CFG-RST-IN+
signal may be disconnected or driven below +0.6Vdc to no less than 0.0Vdc.
Bulletin Ref #: 16_T_FOG_1
Page 7 of 8
Firmware Differences between 1st and 2nd Generation Products
Generation
Serial Numbers
Current Firmware
1 Generation
1510K012 and earlier
Rev. G
2nd Generation
1510K013 and later
Rev. D
st
As mentioned earlier, all 17XX IMUs and DSP-1760s starting with the serial number 1510K013 will have new
electronics and firmware. The firmware is not interchangeable between hardware platforms. The new
firmware has different part numbers.
The electronics and firmware changes accommodate a common platform for new features and functions
across the 17XX IMU and DSP-1760 product lines. The major difference between the legacy Rev. G firmware
and the newer Rev. D firmware is primarily in the command interface. However, the new firmware is
backwards compatible with the legacy 17XX IMUs and DSP-1760s. No modifications are necessary to
existing systems.
While keeping the legacy commands the same, the new Rev. D firmware supports some new commands,
listed below.
Normal Mode Commands
1750 Legacy Command
New Command
Comment
N/A
?bit,2
Legacy 1750 has ?bit but not
?bit,2 (specifies extended BIT
test results)
1750 Legacy Command
New Command
Comment
=TempUnits,<C|F>
=tempunits,<C|F|C_100|F_100>
Temperature output (adds
option for finer precision to
100ths of degrees)
N/A
?logs
Retrieves the commanded
BITor BIT2 results that were
logged
Configuration Mode Commands
If you have any questions about the differences between the two generations of IMUs/gyros, please contact
KVH Technical Support at fogsupport@kvh.com.
Bulletin Ref #: 16_T_FOG_1
Page 8 of 8