Z-Index configuration
This page shows how to assemble and wire the LC485-IDX board.  The board is used to convert TTL level index pulses from an incremental encoder to RS485 level signals to be transmitted to the CT-2 controller.

Below you will find the following sections:

System connection diagram
Kit Parts
Theory of operation


System Connection Diagram



The LC485-IDX board is mounted to and above the LC485-1 board.  See the notes below for mounting and wiring details.


Here is the schematic for the LC485-IDX board. 
Click on the schematic for a larger view

LC485-IDX Board and Parts


Mounting the LC485-IDX

The photos above show a suggested mounting for the LC485-IDX board.  Use two 4-40 by 1 inch spacers (not supplied in kit) to mount the LC485-IDX just above the lower portion of the board LC485-1 board. This combination of boards is used at the encoder (antenna) end of the data lines.  The encoders connect to the J4 and J5 connections on the LC485-1  board, except for the index pulse wires which connect to the pads on the right of the IDX board.  Two lines carrying RS485 signals are run to the CT-2 controller board.  The RJ45 connector on the LC485-1 board outputs the differential position signals while the RJ11 connector on the LC485-IDX board outputs the differential index pulses.

At the the CT-2 end of the lines, the RJ11 connector plugs directly into the jack on the CT-2, while the RJ45 line plugs into a second RS485 board that is configured as a line receiver.  The output of that board is also via an RJ45 connector and a short CAT-5 jumper is used to connect to the CT-2 board.


LC485-IDX to LC485-1 Wiring

This picture shows the connections between the LC485-IDX (left) and the LC485-1 boards.  The red and black wires are +5 volts and ground (supplying the IDX board).  The connections from the encoders for position data connect to the pads labeled J4 and J5 on the LC485-1 board as detailed in the RJ45 page.  The index pulse leads from the encoders go to the pads on the LC485-IDX board as shown above.

Note the +12v and ground (red and black) wires connecting to the LC485-1 board (at left above).  When the LC485-1 board is used at the encoder end of the line it must be supplied with a local source of +12 to +14 volts.  If this power supply is remote from the board, be sure to use heavy enough wire so that the voltage at the LC485-1 is at least +12 volts.  The regulator on the LC485-1 board has enough current capacity to supply the +5 volts to the IDX board.

Wiring for J1, the RJ11 Jack
Pin 1 Ground Pair #1
Pin 2 Elevation index Pair #2
Pin 3 Elevation index Pair #2
Pin 4 No connection Not necessary to connect at either end
Pin 5 Azimuth index Pair #3
Pin 6 Azimuth index Pair #3

Use cable with 3 twisted pairs between the LC485-IDX and the CT-2 boards. The table at the left shows the usage of each pair of wires, and the pins to connect.  You can order this cable pre-made from US Digital.  Specify part number CA-MD6-SH-MD6-XX, where XX is the length desired up to 500 feet.  This is shielded cable.


LC485-IDX Assembly details
First install all the surface mount capacitors and resistors.  Refer to the assembly instructions for the CT-2 board for information about how to work with these parts.  Note that the parts C1 to C5, the 100 Pf capacitors, are not included with the kit.  They can be added for noise pickup problems, but with the three pair twisted cable being used, noise should not be a problem.

Next mount the RJ11 connector and the two IC sockets.

Mount the transistors, resistors, capacitors, and LED at the locations shown on the board. Note the printed outline on the board, showing the position that the transistors must be mounted in.

Use a small piece of bare wire to make a jumper between pads labeled J2 and J2+.

Install the two MAX-481 integrated circuits in the sockets, paying close attention to assure that the 'dots' at pin 1 are positioned towards the bottom of the board.


Index Pulse Operation
Incremental encoders that provide an index (or zero) pulse output a short data pulse each time the encoder shaft passes the index position, once per revolution.  This pulse is normally a TTL level voltage, zero volts when not at index, +5 volts at index position.  It is necessary to provide isolation and immunity from noise, RF pickup, and potential damage to the encoder and the CT-2 board electronics.  Also TTL level signals are difficult to detect reliably over long lines.  So it is necessary to convert the TTL data from the encoder to RS-485 level signals for transmission to the CT-2 board.  The LC485-IDX board provides this level conversion, noise immunity, and isolation.  Here is how it works:

The index pulse from the encoder is connected to either the elevation or the azimuth channel on the IDX board.  Both channels work the same way, so only azimuth will be described here.  The encoder index pulse is wired to the Z5 pad on the IDX board, the azimuth channel.  The pulse goes to pin 4 of U1, a MAX481 line driver chip. The output of U1 is on pins 6 and 7.  These are complimentary outputs, pin 7 being the inverse of pin 6.

The differential signal from pins 6 and 7 are fed to pins 5 and 6 of J1, the RJ11 connector where they are transmitted down the twisted pair line to the CT-2 board's RJ11 connector.

At the CT-2 board the differential index pulses are connected to pins 6 and 7 of  IC4, another MAX481 chip.  The MAX481 converts this signal back to a single TTL level pulse that is then delivered to the CPU at pin 6 (RA4).  This TTL signal is the same in timing and sense as the original index pulse that was generated by the encoder.