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A  USB/RS232 automatic switcher for Coolpix 995


Many software applications are available for controling Coolpix cameras from a PC. Up to model 990, it was possible to link the camera by USB and perform picture downloads as well as sending commands for exposure settings and shutter release.

Among these fine softwares, photopc.exe, TheForce, its script version TheDarkSide, and Cpix.exe.


However, since the introduction of Coolpix 995, Nikon changed the USB protocol so the camera is now handled as a removable disk by Windows 98, 2000 and XP. This new protocol greatly eases the manipulation of picture files, but it also means the end of sending commands to the camera by USB connection, at least with the old protocol. 


Most control software still work, but in a RS232 serial mode only. In this mode, picture downloads are quite slow.


Moreover,  USB and RS232 share the same camera connector, and two different cables must be used:  UC-E1 for USB and SC-EW3  (SC-EM3 for Macintosh) for RS232.


D.Holmes explains how to build a dual cable that carries both USB and RS232. One could think that connecting such a cable to both RS232 COM port and USB port of a PC would mean getting a USB  removable disk for the picture files and a RS232 communication for sending commands by the use of one of the available softwares, but this is not the case.


When connected to both RS232 and USB at the same time, the Coolpix 995 ignores the RS232 connection and behaves only as a removable disk.


To get around this, a mean should be found in order to unswitch USB, allowing for the RS232 communication, and switch back to USB after the command is executed, in order to resume removable disk access.


Aristarco kindly helped me and proposed to use the RTS signal available in the RS232 connector. But availability of this signal varies greatly depending on software  and Windows version. A closer look at the signals available in the RS232 connector shows that the DTR +12V signal is present during the execution of commands sent by photopc.exe (Win XP) and  TheDarkSide (Win98 and WinXP), which makes it a very good choice for controling the USB connection.


The idea is simply to use the DTR +12V signal in order to energize a relay that will switch off the +5V USB supply. Doing this will stop the USB communication and set the camera in RS232 mode. After command  execution, DTR falls to -12V and the USB relay could be de-energized simply by the means of a blocking diode. This is illustrated below.




In this drawing, the value of resistance R is choosen in order to match the voltage of the relay's coil with the RS232 +12V DTR signal. The blocking diode N1 is used to prevent -12V signal from DTR to activate the relay when DTR is low.

The suppressing diode N2 prevents a negative voltage surge from the relay coil. The micro-relay NC (normaly closed) output is connected to the USB +5v supply, camera side. This is a most simple setup.



A second approach is to use a IC switch instead of a mechanical relay. In this setup the DTR signal is sent to an opto-coupler 4N37. This device is essentially a photo-diode coupled with a photo-transistor. The +12V DTR signal lights up the diode, and this light  activates the photo-transistor. This device provides a complete insulation between the +/-12V  DTR signal and the +5V USB signal. The phototransistor is used to toggle the input of a IC CMOS switch 4066. This switch controls the USB +5V feed.



In this drawing , resistance R1 is choosen to limit the current through the opto-coupler L.E.D.. Diode N1 is used to block reverse -12V voltage from DTR. When the opto-coupler transistor is activated, R3 is set to ground and C1 capacitor discharges until a low-voltage state is reached at IC switch control input. Values or R2 and R3 are chosen accordingly. When control input is low, the IC switch S1 opens and USB +5V feed to camera is interrupted. When DTR is back to -12V, opto-coupler diode stops, transistor is no more conductive and R3 is isolated from ground. C1 charges up to +5V through R2, and control input reaches high state. This resumes +5V USB feed to camera. R2, R3 and C1 prevent short DTR signal surges and spikes from affecting USB +5V signal.  


I choose this second approach and packaged it in the small casing of the RS232 jumper box I previously used for testing. Pictures below show the unit and a wiring diagram is also provided. Please note that if the jumper box is used, metal body of both connectors must be linked by a ground wire (not shown in the drawing).


The unit has a USB A / USB B cable that links the camera dual cable to the PC.


The unit also has a L.E.D. for easy monitoring of the DTR signal. This L.E.D. glows when the DTR signal is high, (command beeing sent to camera).


The drawing shows resistors, diodes and capacitors values I used . All the 4 switches available in the 4066 IC chip are used in parallel in order to reduce internal resistance effect on the +5V USB feed.


This configuration works fine, but can certainly be improved by more qualified people than me. At this time it has been tested with photopc.exe (WinXP), and I am quite sure it will also work with TheForce and TheDarkSide (Win98 and WinXP).


Please note that this information is given freely and I will take no responsability for any lost or dammage of any kind resulting from its application. Use at your own risk!


I will be pleased to add any constructive comment, suggestion or link related to this project.


A close view at the CoolSwitcher, showing the L.E.D. , the RS232 25 pin connector and the USB cable:



A view of the complete unit:



An inside view af the modified jumper box showing the 4N37 and

 4066 IC's and the connection to the USB cable:





A 995 setup showing dual cable and switcher:




Close look at dual cable (camera end) based on D.Holmes':


I merged two genuine Nikon cables, one SC-EW3 and one UC-E1.

I noticed that SC-EW3 has only 3 wires and the pin 5 of the camera

connector is not connected at all. This is the only difference with D.Holmes'.



Wiring diagram:




Last news

2003-04-01 Aristarco has made his own version of a similar switcher and a Coolpix 995 controller software called Krinnicam able to activate shutter and download through USB. See Aristarco excellent website for detailed explanations.

This site is written by Jean Beaumier


last update: 2005-09-06

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