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Old 22-05-2013, 02:55   #1
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USB GPS to DSC VHF

Hi there.
Is there a way to connect USB GPS dongle directly to a VHF (RO4700)?
I did an optimistic attempt to connect the USB to 5V and wired the usb data (green and white) to the vhf NMEA wires (green and yellow) without luck.
Is this doable or plain stupid?
Thanks in advance
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Old 22-05-2013, 04:44   #2
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Re: Usb gps to DSC VHF

Quote:
Originally Posted by Hugljuf View Post
Hi there.
Is there a way to connect USB GPS dongle directly to a VHF (RO4700)?
I did an optimistic attempt to connect the USB to 5V and wired the usb data (green and white) to the vhf NMEA wires (green and yellow) without luck.
Is this doable or plain stupid?
Thanks in advance
I'm afraid, the latter.

Your VHF, if it is like 99.9% of DSC sets, needs NMEA0183 data. You will need a GPS receiver which will output data in that format, or you'll need to connect the radio to a network which has GPS data on it, and which can output NMEA0183.
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Old 22-05-2013, 05:14   #3
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Re: Usb gps to DSC VHF

Thanks for quick reply.
Some USB's are said to support NMEA0183 protocol, e.g. BU-353S4 SiRF Star IV USB GPS Receiver. Can they be connected to VHF's or other instruments than PC's?

PS. I ended up posting this to a forum dedicated for other topics, sorry.
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Old 22-05-2013, 06:37   #4
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Re: Usb gps to DSC VHF

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Originally Posted by Hugljuf View Post
Thanks for quick reply.
Some USB's are said to support NMEA0183 protocol, e.g. BU-353S4 SiRF Star IV USB GPS Receiver. Can they be connected to VHF's or other instruments than PC's?

PS. I ended up posting this to a forum dedicated for other topics, sorry.
The USB GPS receivers will probably be talking NMEA 0183 sentences over USB, while the radio will expect NMEA 0183 sentences over RS232. You could probably take a Raspberry Pi (or any other computer - but low power use and low price makes the Raspberry pi a suitable candidate) and pipe the input from the USB port into the RS 232 serial port which might be connected to the radio. It might not be worth it, since, while Raspberry Pi's are cheap, you might be able to get a proper serial GPS for the same money. Also I would worry that too much DIYness might end up being less stable than a purposemade solution.
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Old 22-05-2013, 06:42   #5
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Re: USB GPS to DSC VHF

What he said. You still can't connect a USB cable to your radio. You'll need to either convert the data with a computer, which will be a troublesome way to do it, or better just buy a GPS receiver with a proper serial output, which you can connect directly.
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Old 22-05-2013, 06:57   #6
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Quote:
Originally Posted by Dockhead View Post
What he said. You still can't connect a USB cable to your radio. You'll need to either convert the data with a computer, which will be a troublesome way to do it, or better just buy a GPS receiver with a proper serial output, which you can connect directly.
I highly suspect that the only problem is the voltage levels of the link, but that the data is all the correct NMEA sentences already. I wonder what an opto-isolator input like on most NMEA multiplexers would make of it.

But in this case I agree that a proper GPS unit for a boat should be aboard and wired to the instruments and VHF.
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Old 22-05-2013, 07:05   #7
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Re: USB GPS to DSC VHF

NMEA0183 is based on RS-422. The electrical standards are:

RS-422StandardEIA RS-422Physical MediaTwisted PairNetwork TopologyPoint-to-point, Multi-droppedMaximum Devices10 (1 driver & 10 receivers)Maximum Distance1500 metres (4,900 ft)Mode of OperationDifferentialMaximum Baud Rate100 kbit/s – 10 Mbit/sVoltage Levels−6V to +6V (maximum differential Voltage)Mark (1)Negative VoltagesSpace (0)Positive voltagesAvailable SignalsTx+, Tx-, Rx+, Rx- (Full Duplex)Connector typesNot specified

Typical Baud rate4800Data bits8ParityNoneStop bits1HandshakeNone


USB is a horse of an entirely different color:

"Signaling


"USB allows the following signaling rates. The terms speed and bandwidth are used interchangeably. "high-" is alternatively written as "hi-".
  • A low-speed (USB 1.0) rate of 1.5 Mbit/s is defined by USB 1.0. It is very similar to full-bandwidth operation except each bit takes 8 times as long to transmit. It is intended primarily to save cost in low-bandwidth human interface devices (HID) such as keyboards, mice, and joysticks.
  • The full-speed (USB 1.1) rate of 12 Mbit/s is the basic USB data rate defined by USB 1.0. All USB hubs can operate at this speed.
  • A high-speed (USB 2.0) rate of 480 Mbit/s was introduced in 2001. All hi-speed devices are capable of falling back to full-bandwidth operation if necessary; i.e., they are backward compatible with USB 1.1. Connectors are identical for USB 2.0 and USB 1.x.
  • A SuperSpeed (USB 3.0) rate of 5.0 Gbit/s. The written USB 3.0 specification was released by Intel and partners in August 2008. The first USB 3 controller chips were sampled by NEC May 2009[74] and products using the 3.0 specification arrived beginning in January 2010.[75] USB 3.0 connectors are generally backwards compatible, but include new wiring and full duplex operation.
"USB signals are transmitted on a twisted-pair data cable with 90Ω 15% characteristic impedance,[76] labeled D+ and D−. Prior to USB 3.0, these collectively use half-duplex differential signaling to reduce the effects of electromagnetic noise on longer lines. Transmitted signal levels are 0.0 to 0.3 volts for low and 2.8 to 3.6 volts for high in full-bandwidth and low-bandwidth modes, and −10 to 10 mV for low and 360 to 440 mV for high in hi-bandwidth mode. In FS mode, the cable wires are not terminated, but the HS mode has termination of 45 Ω to ground, or 90 Ω differential to match the data cable impedance, reducing interference due to signal reflections. USB 3.0 introduces two additional pairs of shielded twisted wire and new, mostly interoperable contacts in USB 3.0 cables, for them. They permit the higher data rate, and full duplex operation.

"A USB connection is always between a host or hub at the "A" connector end, and a device or hub's "upstream" port at the other end. Originally, this was a "B" connector, preventing erroneous loop connections, but additional upstream connectors were specified, and some cable vendors designed and sold cables that permitted erroneous connections (and potential damage to circuitry). USB interconnections are not as fool-proof or as simple as originally intended.
The host includes 15 kΩ pull-down resistors on each data line. When no device is connected, this pulls both data lines low into the so-called "single-ended zero" state (SE0 in the USB documentation), and indicates a reset or disconnected connection.

"A USB device pulls one of the data lines high with a 1.5 kΩ resistor. This overpowers one of the pull-down resistors in the host and leaves the data lines in an idle state called "J". For USB 1.x, the choice of data line indicates of what signal rates the device is capable; full-bandwidth devices pull D+ high, while low-bandwidth devices pull D− high.
"Example of a Negative Acknowledge packet transmitted by USB 1.1 Full-speed device when there is no more data to read. It consists of the following fields: clock synchronization byte, type of packet and end of packet. Data packets would have more information between the type of packet and end of packet.


"USB data is transmitted by toggling the data lines between the J state and the opposite K state. USB encodes data using the NRZI line coding; a 0 bit is transmitted by toggling the data lines from J to K or vice-versa, while a 1 bit is transmitted by leaving the data lines as-is. To ensure a minimum density of signal transitions remains in the bitstream, USB uses bit stuffing; an extra 0 bit is inserted into the data stream after any appearance of six consecutive 1 bits. Seven consecutive received 1 bits is always an error. USB 3.0 has introduced additional data transmission encodings.
A USB packet begins with an 8-bit synchronization sequence '00000001'. That is, after the initial idle state J, the data lines toggle KJKJKJKK. The final 1 bit (repeated K state) marks the end of the sync pattern and the beginning of the USB frame. For high bandwidth USB, the packet begins with a 32-bit synchronization sequence.

"A USB packet's end, called EOP (end-of-packet), is indicated by the transmitter driving 2 bit times of SE0 (D+ and D− both below max) and 1 bit time of J state. After this, the transmitter ceases to drive the D+/D− lines and the aforementioned pull up resistors hold it in the J (idle) state. Sometimes skew due to hubs can add as much as one bit time before the SE0 of the end of packet. This extra bit can also result in a "bit stuff violation" if the six bits before it in the CRC are '1's. This bit should be ignored by receiver.
A USB bus is reset using a prolonged (10 to 20 milliseconds) SE0 signal.
USB 2.0 devices use a special protocol during reset, called "chirping", to negotiate the high bandwidth mode with the host/hub. A device that is HS capable first connects as an FS device (D+ pulled high), but upon receiving a USB RESET (both D+ and D− driven LOW by host for 10 to 20 ms) it pulls the D− line high, known as chirp K. This indicates to the host that the device is high bandwidth. If the host/hub is also HS capable, it chirps (returns alternating J and K states on D− and D+ lines) letting the device know that the hub operates at high bandwidth. The device has to receive at least three sets of KJ chirps before it changes to high bandwidth terminations and begins high bandwidth signaling. Because USB 3.0 uses wiring separate and additional to that used by USB 2.0 and USB 1.x, such bandwidth negotiation is not required."
Universal Serial Bus - Wikipedia, the free encyclopedia


No way can an RS422 device understand a USB device.
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