Branching an M841 Radar Signal Cable

[Anonymous]

We have a vintage 1996 Furuno 841 4kw radar which was installed in our J42 at the chart table when we bought her last year. The unit works perfectly with the scanner mounted on a Questus backstay mount.

As the radar is not a MkII I was told Furuno did/does not offer a second remote display option. When sailing shorthanded downeast we would very much like to have the option to mount the display in a second trunnion custom designed to fit in the small starboard aft cabin deck hatch under the cockpit dodger, allowing the cabling to tuck neatly down thru the opened hatch. In this position we would be able to see the display from the helm or the cockpit. We would also like to be able to use the display in its current location at the nav table in heavy weather when the hatches need to be dogged.

Would there be a performance problem if I purchased a second DJ-1 24 pin connector or even a 008-462-060 complete spare signal cable, cut the primary signal cable where it transits the aft cabin, and spliced the new branch cable into a 24 terminal strip mounted in a shielded aluminum box connected to the pin 23 cable shield ground? The junction box would have strain 3 strain reliefs tied to the cable shielding. This would provide two DJ-1 plugs with all wires branching from the respective wire going to the scanner. Only one signal cable plug would be connected at any time, depending on where the display is mounted.

I build PLC controlled manufacturing equipment and would follow all appropriate codes.

 

[Johnny Electron]

Would there be a performance problem if I purchased a second DJ-1 24 pin connector or even a 008-462-060 complete spare signal cable, cut the primary signal cable where it transits the aft cabin, and spliced the new branch cable into a 24 terminal strip mounted in a shielded aluminum box connected to the pin 23 cable shield ground?

Yes, there would be a problem. You would load down the radar video and it most likely wouldn't work at all. (In addition to your control wires and power shorting out)

As the radar is not a MkII I was told Furuno did/does not offer a second remote display option.

The M841 manual and drawing reflects support for a remote display option. I think whom you spoke with might be mistaken. The drawings show connection J58 or remote display connection. If yours has J58, then you could connect one of the existing FMD units being sold.
(FMD1920C/NT, FMD1835, FMD8062)

 

[Anonymous]

I would love to be able to buy a second display which will work seamlessly in our system.

However, I have the display unit on my desk. It is an RDP-113, S/N 3320-0220. It has connectors DJ-1, GND, 10.2-31.2 VDC, HDG and NMEA. There is also a round plastic cover plugging a hole labeled RP, and finally a 1.25" square plastic plate covering an area labeled Option. If there is J58 capability on the circuit board internally, it has not been provided with a connector on the case.

Perhaps you colleague was not mistaken?

I am not a video engineer. But it is not clear to me how a ~ 4' branch line would add much load to the video if nothing is connected to it. Please educate me.

Similarly, the only way the power leads could be shorted is if I allow the positive to touch negative or ground and create a short. I had envisioned using either a heavy euro style or screw ring terminal barrier strip with all connections properly crimped and insulated per ABYC standards. The metal case would provide RF noise shielding equivalent to the cable sheath. The strain reliefs and proper cable tie straps would certainly prevent accidental contact between any of the wires.

Once again, I bought this boat by making a living building electrical equipment. The proposal to extend the utility and life of a great piece of your kit does not seem like rocket science.

 

[Johnny Electron]

Please look at the interconnection diagram of the manual (page S-2)
The connector is internal, and the wiring will go thru the OPTION port knockout at the back of the display. If you wish to investigate this option please check the board for J58.

It is your unit. I say it shouldn't be done; but do what you wish. If it was as easy as just splitting the video wires, Furuno wouldn't have gone to all the effort to build buffering and amplification at J58, onto the board, for this exact purpose.

 

[Anonymous]

Thanks much for your help. I have found a couple similar vintage FMD-811 displays (RDP-114 model) in working order on ebay. Could I follow your suggestion to install one of these as a slave remote display thru the J58 terminal on the board of our existing RDP-113?

If so, I do not see a J58 on the schematic for the FMD-811 in your manual on the website. What cable or terminals would I use to make the connection between the existing RDP-113 master and the proposed new FMD-811 remote display?

Thanks again.

 

[Anonymous]

Hi Again,

I see from the 811 S1 drawing that the connections needed from the FMD-811 to the RDP-113 J58 come from the DJ-1 pins of the 811. Can you sell me a cable to connect the FMD-811 or even better tell me the manufacturers and part numbers of the 24 male pin plug that goes into the DJ1 and the inline plug that goes into the J58?
It should be straightforward to make up the interface cable from that info for the 811 remote display.

Best,

Bill

 

[Johnny Electron]

The FMD811 isn't sold anymore but if you find a used one, it should work fine for you except as noted the FMD811 will not have any gain or clutter control when used with the M841 radar. (See the features tab of the FMD811 on our website www.Furunousa.com)

The 20 meter FMD cable for the FMD811 is still available for order from your Furuno USA dealer. P/N 000-136-812 ($200 list)

If you only want a 24 pin connector kit for the back of the FMD811 display, it is part# AL1600CONN ($65 list)

 

[Anonymous]

Thanks Johnny for your detailed followup.

I have purchased an FMD-811 with interface cable and power cable on ebay for $150. When I plug it into the power cable for the 841 it powers up fine and seems OK. Unfortunately I discovered today that the 8 contact white plug at the end of the interface cable intended to go into the J58 socket on the 841 motherboard does have the same 2.5mm pitch but it does not fit into the J58 socket. It has tabs on each end wider than the 0.81" J58 socket, and it is thicker than the 0.17" height of the socket. Clearly a different style of connector than the J58 socket on my 841 radar.

So I need to buy a plug and contacts to attach to the interface cable so that it will mate to my J58 socket. Can you please tell me the manufacturer and part number of the plug to mate to my 841 J58 socket?

 

[Anonymous]

The J58 horizontal socket on the motherboard has a molded-in designation "8 JST" and it has a horizontal tab slot at the top of each end and a vertical latch slot directly over pin 1 and pin 8. So the correct plug would need mating features.

Thanks.

 

[Johnny Electron]

The FMD811 manual indicates on page 28; when connecting to a M841 you must replace the NH connector with the XH8P connector assembly which was supplied with the unit. If the previous owner didn’t provide this with the unit you can purchase a XH8P connector pigtail assembly from your local authorized dealer. The XH8P connector assembly is part# 000-137-333.