EQ6/EQG/Atlas Mod Circuits






You can use the information on this site COMPLETELY AT YOUR OWN RISK.  The modification steps and other information on this site  is provided to you "AS IS" and WITHOUT WARRANTY OF ANY KIND, express, statutory, implied or otherwise, including without limitation any warranty of merchantability or fitness for any particular or intended purpose. In no event the author will  be liable for any direct, indirect, punitive, special, incidental or consequential damages or loss of any kind whether or not the author  has been advised of the possibility of such loss.


Circuit modifications implemented on your setup could invalidate any warranty that you may have. Use this information at your own risk. The modifications involve direct access to the stepper motor controls of your mount. Any "mis-control" or "mis-command"  / "invalid parameter" or "garbage" data sent to the mount could accidentally activate the stepper motors and allow it to rotate "freely" damaging any equipment connected to your mount. It is also possible that any garbage or invalid data sent to the mount could cause its firmware to generate mis-steps pulse sequences to the motors causing it to overheat. Make sure that you perform the modifications and testing while there is no physical "load" or dangling wires on your mount. Be sure to disconnect the power once this event happens or if you notice any unusual sound coming from the motor assembly.

Circuit Modification Objective - SYNC GOTO and Autoguiding with Direct PC Control

NOTE: if you wish to avoid modification of the handcontroller, you can check out Shoesstring's readymade mod found
at the bottom of this page.

The modification allows the user to control the mount's stepper motor board via the PC's serial connector. The mod is implemented on the handcontroller utilizing the existing RJ11 PC serial interface connector and its built-in RS232C to TTL level converter. This will avoid any external circuit required to connect the mount's DB9 TTL level connector to the PC's serial port.

 Any PC application or software can now be coded to control the mount's stepper motors (Figure 1).


Figure 1: Mod Block Diagram


Mode of Operation

The mod circuit utilizes a Quad 2-1 Multipexer 74HC157   (Figure 2) that switches serial signals between the PIC controller of the paddle and the mount and PC. The mode of operation is set through an external SPST toggle switch;

Toggle Switch OFF:   Normal Mode

     PC communicates directly to the paddle's PIC controller, Mount controller is also connected to the

     Paddle's PIC controller

Toggle Switch ON:  Extended Mode

    The PC serial lines is connected directly to the mount's serial lines allowing the user to a control the mount's stepper motors.

    controller (See Figure 2).


Figure 2: Serial Data path during "Normal Mode" and "Extended Mode"


Mod Circuit Diagram

Below is a diagram of the modification (Figure 3). It shows the connections of the individual multiplexer component of the 74HC157 chip to the respective pinouts and tap markings of the handcontroller board. Also, pinout labels of the actual chip with respect to the circuit diagram is also shown. Use this as a reference in connecting the wires to the board.


Figure 3: Mod Circuit diagram


Circuit Mod TAP points


Figure 4: Mod circuit board Tap/Cut points

The tap points image (Figure 4) shows were to solder the wires as labeled in the circuit diagram. It also shows which part of the board patterns need to be cut. The power and ground line is obtained from the the output pin of the 7805 +5V voltage regulator (Tap point K and L).

Figure 4a shows the tap points for an "older version" of the controller. The only thing that is different from the newer version are the location of tap points "A" and "C". The other tap points and cut points are pretty much the same.

Figure 4a: Mod circuit board Tap/Cut points for an "older" version of the controller


Circuit Mod Parts List

1 pc.   74HC157  ( 2-input QUAD Multiplexer)

              * Alternate chip would be : 74LS157 or 74HCT157

1 p.c  SPST Toggle Switch (the smallest you can get)

1 pc.  10Kohm 1/4 resistor

1 set.  wires  (type commonly used for wirewrapping)

1 Soldering Iron and a good quality soldering Lead

1 Screwdriver

1 Knife cutter (used for cutting board patterns)

1 Double Adhesive tape or Glue

1 Long Nose pliers

1 Wire Cutter pliers

1 Circuit tester / VOM Meter

1 Drill for toggle switch hole mounting



1. Remove the four rear screws and the  back cover of the handpaddle to expose the circuit board

2. Look at the board circuity and compare it with the actual images presented above (Figure 4).

   If there are any differences then you may have to determine the actual tap/cut points yourself based on the circuit above (Figure 3). You may need to utilize a circuit continuity tester for this.

 3. TRIM the pins of the 74HC157 (all 16 of them) and leave some length of the pin  for the wires to be soldered on it. Removing the excess part of each pin will allow you to piggyback the 74HC157 chip on top of the paddle's PIC controller (the largest square IC on the paddle). Refer to Figure 5 for the actual chip mounting. Make sure that you dont touch any of the pins of the IC as this is a CMOS chip which is very sensitive to static electricity.


Figure 5: Chip mounting and tap points


4. Using double adhesive tape, mount the 74HC157 chip on top of the PIC chip (refer to figure 5). Take note of the "PIN 1" orientation of the 74HC157  using the chip's "notch" as a reference.

5. Using a sharp knife cutter (or any blunt tool), cut the indicated patterns on the PCB. Make sure that only the indicated patterns are cut and that you dont scratch the board patterns near the area. Figure 5 shows which patterns needed to be cut. The actual lines are actually indicated in the circuit diagram (figure 3).

There are actually 4 patterns to cut. Two of them are near the PIC controller chip pins (figure 6) and the other two are on the MAX232 chip side (figure 7).


Figure 6: Cut points on the PIC Controller side


Figure 7: Cut Points (MAX232 side)


6. At this point you can now solder the wires on the 74HC157 and the respective tap points on the controller. One thing to watch for is the soldering of the 4 wires on Tap Points B, H, E, J (figure 4 and figure 8) on the PIC controller of the paddle as this may be difficult specially if the soldering iron tip is not that sharp. A good approach is to "thin" (from solder thinning) the end of the wires first with a small amount of solder. Then place the tip of the wire on one of the PIC pins and tap the tip of the wire with the soldering iron. Eventually the wire will stick on the pin. Avoid using the soldering lead wire directly on the PIC pins as the melting solder will eventually fuse the adjoining pins of the PIC. (If this happens, you need to clean the connection using the soldering iron and a good amount of soldering paste).

Continue soldering the rest of the wires while referring to the circuit diagram (figures 4,8,9).

It would be better if you use color-coded wires;

  RED : (+5V Vcc taps)

  BLACK :  GND Taps

  BLUE : Signal Lines


Figure 8: Tapping on the 4 pins of the PIC controller


Figure 9: Tapping on the 2 pins of the MAX232 chip (tap point D and G) and the two chip resistors (tap point A and C).


7. Solder the 10Kohm pull-up resistor on pins 1 (mux sel pin)  and pin 16 (VCC +5V pin) of the 74HC157. Refer to figure 10.

   Make sure the height of the resistor does not go over the height of the 74HC157 chip or you may not be able to put back the rear cover of the handcontroller.


Figure 10.  10kohm Pull-up resitor mounting.

8. Locating for the position of the switch is quite difficult as you need to look for the right space inside the paddle. Eventually i found one at top of the LCD board (figure 11). It might be different on some installations depending on the size and type of switch you obtained. Drill a hole on the backplate of the paddle (drill size depending on the purchased switch). Mount the SPST toggle switch as shown (figure 12) and use enough wire to connect the switch on the mux chip (pins 1 and 16 of the 74HC157);


Figure 11: Looking for a place for the switch on the paddle


Figure 12. Toggle Switch mounting


Also, TAKE NOTE of the switch's ON and OFF position before you put back the cover.  You could put a label at the back of the cover.




Checking for any "Shorted Connections"

Checking is basically done by looking at the solder points "visually" and checking the connections using a circuit tester (better if you can use a tester that emits an audible beep indicating a good connection or a "short" ).


Wrapping up the Construction Process

At this point you can now put the rear cover back on the paddle and make sure the switch body fits snuggly on the alloted space (figure 12).

Here are more of the  construction images ;


Figure 13: Completed mod of the paddle


Figure 14:  "wide field" view of the mod


Figure 15: Paddle with the rear cover mounted showing a portion of the toggle switch.



1. Boot-up test

One quick test is connect your paddle to the mount using the usual DB9 coiled cable and apply the usual 12V power to your setup. The switch should be at the "OFF" (NORMAL MODE) position. If everything is OK, the paddle should boot-up normally and it will display the usual firmware version. You should also test the paddle's functionality (slew, goto, etc.) and should be working in the same state and condition prior to the implementation of the modification. Any problem on the circuit modification would result to a "COMM Error" message on the paddle.

2. RS232-C Communication Test

Connect the RJ11 port of your paddle to your PC or laptop computer and  put the the paddle in RS232C mode (not needed for version 3 and up).  With the toggle switch still at the "OFF" position (NORMAL Mode), connect you usual planetarium program and try do some telescope slewing tests.

3. Extended Mode Communication Test

If you reached this part, then we could say that half of the mod's intention is already working. The other half is putting the toggle switch in the "ON" position and testing the "EXTENDED" mode portion of the mod. This is basically sending and receiving stepper motor control commands  directly to the mount using the same RJ11 connector. On this part, you need to use a serial terminal software (Hyperterm , etc.) for this test and the paddle should be connected to the PC using the usual RJ11 connector.

  - After the usual boot-up sequence, toggle the switch to the ON position

  - Activate Hyperterm and select the actual COM Port (not the modem name)  where the RJ11 port is connected to your PC

  - Select the correct serial port settings ; 9600 Baud, 8Bit data, 1 bit parity, Flow control OFF

    Type in the mount command :


    Then press enter. (That is a byte sequence in hexadecimal   0x3a 0x61 0x31 0x0d  of the command).

    The mount should reply back with a 32BIT hexadecimal number;


    If you get the 32bit number reply then Congratulations ! You have just completed successfully your circuit mod project.

SAMPLE MODs made by other users

Here is one version from Andrej Mraz using a different mount point for the three components. The toggle switch is actually facing at the front side of the paddle.



It is also possible to connect your PC to the mount without the handcontroller mod in between. This option is provided for people who would want to avoid modifying the handpaddle.

The  page that decribes this option can be found here.

A Readymade Mod Circuit for the EQMOD driver

Shoestring Astronomy has a ready made commercial  version of EQDIRECT called EQDIR.  This was made possible by Mr. Doug Anderson of Shoestring Astronomy.

You may contact him at sales@shoestringastronomy.com

Below is an image of Shoestring Astronomy's EQDIR:


Portions of the information on this site should be attributed to Mr. John Archbold from his initial observations and analysis of the interface circuits and of the ASCII data stream between the Hand Controller (HC) and the Go To Controller.


   Email your questions at circuitmod@yahoo.com


You can use the information on this site COMPLETELY AT YOUR OWN RISK.  The modification steps and other information on this site  is provided to you "AS IS" and WITHOUT WARRANTY OF ANY KIND, express, statutory, implied or otherwise, including without limitation any warranty of merchantability or fitness for any particular or intended purpose. In no event the author will  be liable for any direct, indirect, punitive, special, incidental or consequential damages or loss of any kind whether or not the author  has been advised of the possibility of such loss.

VISITS since Dec , 2006