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This page describes the various kinds of encoders that the CT-2 controller is designed to work with, and provides detail on the usage and other considerations for each.


Potentiometers are the easiest encoders to use because the CT-2 has on-board Analog to Digital converters.  By connecting your 'pots' directly to the CT-2, and calibrating, the antenna position can be read without any further hardware.  The potentiometers are connected via the DIN-8 jack on the board.  The connection information is shown here: Potentiometer Readouts

Absolute position encoders by US-Digital are the easiest to connect and use, after potentiometers.  The wiring is simple and available pre-made from US-Digital.  Absolute encoders are normally 'geared' one to one with the rotation of your antenna, and provide an absolute position value.  In this respect they are like potentiometers, but much more accurate.  The following links will take you to the various locations in this web where information on hardware and software for use with Absolute encoders can be found.
Hardware and wiring
Detailed Configuration

The CT-2 controller can work with several makes of pulse type encoders that produce what is called a "quadrature" data output stream.  While designed for the US-Digital incremental encoders, there are several other manufacturers of incremental encoders whose products can be used. The usage of incremental encoders for the CT-2 has been a work in progress and several hardware and software changes have been made over the years as we gained experience with these versatile encoders.  The greatest improvement has been the addition of the capability to use the 'index pulse' that is provided with some incremental encoder models.  This provides a means to reliably reset the pulse count to the exact value required by your antenna calibration, automatically in many cases, or manually when necessary.

This capability is implemented in CT-2 controllers with version 2.1 or later firmware.  This pulse is called an index pulse, or Z-pulse (for Zero Pulse), or sometimes a Z-index.  Since the board was not originally designed for this, it is accomplished by using the RJ11 input jack that is normally used for absolute encoders.  Thus one cannot use absolute and z-index incremental encoders together.  An add-on board is available to handle converting the index pulse from TTL to RS485 levels at the encoder.  This is designated on our Products page as the LC485-IDX.  For assembly, wiring, and connection details see the links below.

  Our initial effort to provide index pulse detection did work for many encoders, but we soon discovered that there could be problems with some models, or when the encoders were mounted such as to provide reverse rotation from normal.  A change in software with a corresponding board modification cured that problem, see below.

The CT-2 has been used with the following manufacturer's incremental encoders
US-Digital S5 and T5 Series - Note 1
BEI H25D Series - Note 2
Heidenhain Note 1
Dynapar Note 2


Note 1: The US Digital and Heidenhain encoders all produce a very short pulse each time the encoder shaft passes the zero point.  To work with these encoders, the jumpers at Pad1-Pad2 (for Azimuth), or at Pad3-Pad4 (for Elevation) must be removed.  Also see notes in the Reverse Gearing section, below.

Note2: The BEI and Dynapar encoders and some others produce a long pulse each time the encoder passes the zero point.  For these encoders the jumpers at Pad1-Pad2 and at Pad3-Pad4 must be connected.  Also see notes in the Reverse Gearing section, below.

The following links will take you to the various locations in this web where information on hardware and software for use with incremental encoders can be found.  For the hardware aspects of Z-Pulse usage, see the paragraphs below.

CT-2 Modifications for use with Index Pulse
Hardware, Wiring, and usage of the LC485-1 level converter.
Index Pulse Conversion Board (LC485-IDX)
Detailed Configuration

Reverse Gearing
Incremental encoders are meant to be geared to the antenna so that the encoder shaft turns clockwise (as viewed from the top) when the antenna turns clockwise.  If your mounting arrangement requires that the encoder shaft turn counter-clockwise when the antenna turns clockwise then some special wiring is required.  Additionally it makes a difference whether you are using the index pulse or not.

If you are not using the index pulse, or if you are using US Digital or Heidenhain encoders, simply reversing the A and B leads from the encoder will allow the controller to count normally (positive for increasing azimuth or elevation).  This means reversing pins 3 and 5 on the wires from the encoder to the RS-485 board at the encoder. (See the LC485-1 board schematic and the connection table below the schematic.)

However for BEI and Dynapar encoders, simply reversing A and B will not work if you are also using the index pulse.  In that case do not reverse A and B as described above.  Instead it is necessary to change the connections in the RJ45 plug where it plugs into the RS-485 board at the controller.  The A channel connections must be reversed for azimuth or elevation (or both).  To accomplish this, for azimuth, reverse the wires to pins 3 and 6 at the RJ45 plug.  For elevation, reverse the wires to pins 7 and 8.  (See the LC485-1 board schematic.)


CT-2 Modifications for Z-Pulse Operation
In order to support the Z-Pulse available with some incremental encoders, a modification to the CT-2 circuit board is required. All new CT-2 controllers incorporate the required board modifications, but if you have an older CT-2, or are upgrading your CT-1 to a CT-2. the modification is presented here.  Note that this modification is required if you have firmware version 2.10 or greater and you are using incremental encoders. (See the download page for information on finding the firmware version.)  The modification effectively swaps the function of the two microprocessor I/O lines called RB1 and RB4 (microprocessor pins 34 and 37).  The two existing traces to those pins must be cut and jumper wires to 'criss-cross' the connections must be soldered on.  The photo below shows the modification.















This photo shows the upper left corner of the bottom of the CT-2 board. The two traces shown must be cut, and the wires soldered on as shown.  The pins of the PIC microprocessor are shown for reference.