Guider relay port circuit question

[rogerg (Roger)]

I'm wondering if anyone out there understands the electronics behind the standard ST4 guider ports we use on cameras and mounts?

Reason I ask, in as brief a summary as possible:

• My Paramount ME has developed a problem where it thinks it is being told to guide in RA and Dec continuously.
• The problem can be averted by plugging in to the mount guider port a unpowered ST camera, causing the mount to think it should be guiding in 4 directions and hence doesn't guide anywhere
• Other functionality of the mount remains normal, including slewing and guiding (which uses Direct Guide - the guider port is redundant for my purposes)
• Its almost as though the unpowered ST camera causes a loop back or such which mitigates the other incorrect instruction to guide.

So... I am wondering if, to save myself US$800 in replacement Paramount ME electronics, I can create a simple dummy plug/cable that simulates what the unpowered ST camera is doing.

I'm also wondering if that would cause any damage. Based on physical observations of my mount (visual and audible) I would say it does not create damage, the mount isn't actually fighting its self in 4 directions, rather it's moving in none.

I can't use the camera, because as I have to turn it on to take photo's

Make sense? probably not but I can explain more.

Regards,
Roger.

[billdan (Bill)]

Hi Roger,

The ST4 port is quite simple you have 4 directional pins plus a ground pin.

These are the pin-outs for a RJ12 plug (front view) on an EQ6.

pin 1 is NC
pin 2 is GND
pin 3 is +RA
pin 4 is +DEC
pin 5 is -DEC
pin 6 is -RA

All you need to do, is short GND to one of the directional pins to drive the motor (usually a narrow milli-sec pulse from the guide cam).

If your ME thinks it is being guided in all directions, that would probably mean that those 4 directional pins are somehow shorted out to GND inside the ME. So a dummy plug would not help.

With the mount switched off you could use an ohmmeter to measure the resistance from those directional pins to GND and verify if they are shorted out. If it is, try and trace back from there on the ME PCB board. Could be a faulty optocoupler.

Cheers
Bill

[AndrewJ]

Agree with Bill, that you simply short the pin to ground to assert.
That said, inside the mount, the unconnected pins should float at 3.3V or 5V depending on the CPU supply voltage.
This can be done using very weak internal or external "pullup" resistors )
As such, testing the voltage of the pins when the mount is powered is another test to show if the ports are all working. If one isnt working, a plug with built in pullups might work, but you would need to know why the proper ones werent working.
Andrew
Andrew

[rogerg (Roger)]

Thanks everyone for your responses, here and private message.

Testing resistance across the guider port pins when the mount is turned off I don't see any reaction, no indication of a connection.

Now understanding the way the guider port works I've made up a small cable which connects the common to each of the NSEW pins. With this connected to the guider port of the paramount behavior of the mount appears correct and normal. I'm not sure though whether using this method to rectify the problem risks further damage to the mount.

I haven't been game enough to test for a voltage with the mount on, me and my fat fingers, don't want to short something else out.

[AndrewJ]

Gday Roger
You wont short anything out.
Normally, there will be a 10 to 100k resistor between the 5V rail and the port. ie
+5V -- 100k resistor -- CPU Port -- AssertPin -- switch --- Ground
When the switch is open, ie the Assert pin is floating, the pullup makes the voltage at CPU Port = +5
When the AssertPin is connected to ground, the CPU Port gets pulled to ground, so there is no way to actually short anything via the ST4 port.

Andrew