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Model SID-101 Monochromator Controller

CONTENTS

1

SYSTEM OVERVIEW

Introduction

Theory of Operation

System Specifications

Indicators

External Connections

Command Summary

2

INSTALLATION

Included Parts

Setup

Upgrade Installation

3

OPERATION

Controlling the Monochromator in ASCII Format

Motor Stepping, Shutter Control and Signal Measurement

Advanced Control

Calibration

Included Reference Programs

4

SERVICE

Troubleshooting

 

 

 

 

SYSTEM OVERVIEW

Introduction

The SID-101 Monochromator Controller allows you to control your PTI monochromator with commands sent over a serial (RS-232C) interface. In addition, the SID-101 allows you to do photon counting or photocurrent measurements using any PTI PMT housing.

The SID-101 will allow computer control to:

  • set your monochromator to a particular wavelength.
  • move between two wavelengths at a constant scan rate.
  • move between two wavelengths with pauses at preset wavelength steps.
  • adjust the calibration of your monochromator.
  • step the motor drive directly.
  • control multiple monochromators from a single serial interface.
  • measure photocurrent to 12-bits.
  • count photons at up to 4 MHz.
  • automatically control a shutter (if installed)

The structure of the commands is programmable. You may select between a simple character-based (ASCII) command structure or a fast binary command structure.

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Theory of Operation

The grating equation gives the relation between the wavelength of light diffracted by a grating and the angle of diffraction

n *l = 2 * d * cos(j) * sin(q)

where l is the wavelength in nanometers, d is the grating groove period in nanometers, cos(j) is a constant set by the optical design, and n is the order of diffraction (usually 1).

In PTI monochromators, a mechanical sine-bar mechanism generates a sin(q) function from motor rotation. Therefore, the wavelength is linear in motor rotation. This sine-bar mechanism has proved to be more durable and more accurate than "direct drive" mechanisms that use a worm wheel gear reduction in conjunction with electronic calculation of the sin(q</) function. The worm wheel drive suffers from periodic errors that cannot be removed in calibration.

The SID-101 monochromator controller contains a microcontroller that will receive commands from your computer, translate wavelength commands into motor steps, then set the monochromator to the requested wavelength.

The range in wavelength to which your monochromator may be set depends upon the grating that you are using. A table of wavelength limits is given below. While any wavelength within these limits may be selected, the grating diffraction efficiency will determine whether the monochromator has significant transmission efficiency at a given wavelength.

Table 1: Wavelength Limits

Grating
[grooves/mm]
Lower wavelength limit nm Upper wavelength limit nm
50 0 27,600
75 0 18,400
150 0 9,200
300 0 4,600
600 0 2,300
1200 0 1,150
1800 0 766
2400 0 575
3600 0 383
N 0 1200*1150/N

 

PTI Monochromators are driven by stepping motors that rotate in fixed angular steps. Consequently, the monochromators can be preset to wavelengths that are at fixed increments. This wavelength increment is always much less than the resolution limit of the monochromator. The wavelength increments for the PTI monochromators is a function of the grating that is used. These increments are given in Table 2 below for the two types of stepping motors

Table 2: Wavelength Increments

Grating
[grooves/mm]
Vexta Motor Increment
nm
Slo-Syn Motor Increment
nm
50 3.0 6.0
75 2.0 4.0
150 1.0 2.0
300 0.5 1.0
600 0.25 0.5
1200 0.125 0.25
1800 0.0833 0.1667
2400 0.0625 0.125
3600 0.0417 0.0833
N 150/N 300/N

 

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System Specifications

Interface: RS232-C
19,200 baud to 300 baud, 1 stop bit, no parity

Power: External power pack
Input: 90-264 VAC, 47-63 Hz, 30 W
Output: 12 V, 2.5 A
AC Connector: IEC 320
Approvals: CE/UL/CSA/TUV/GS
Size (inches): 4.42 x 2.36 x 1.42

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Indicators

The SID-101 Monochromator Controller has a single indicator labeled STATUS. This LED will be dim when the controller has power and is waiting for commands. It will blink when serial input is being received, and it will brighten when an action is in progress.

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External Connections

The SID-101 Monochromator controller has six external connections.

POWER: connects the monochromator to its external power pack.

FROM COMPUTER: connects the monochromator to the controlling computer.

TO NEXT DEVICE: allows several monochromator controllers to be connected to a single computer serial port.

SHUTTER: allows TTL control of the monochromator shutter.

WARNING: Permanent physical damage will result if standard TTL defined voltages are exceeded!

 

ANALOG: allows measurement of a 0-10 0-5volt analog signal to 12 bit precision.

WARNING: Permanent physical damage will result if the voltage range is exceeded!

 

DIGITAL: allows counting of TTL pulses from a PTI photon-counting PMT at rates to 4 Mhz.

WARNING: Permanent physical damage will result if standard TTL defined voltages are exceeded!

 

The pinouts of these connectors is shown below.

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Command Summary

Command description syntax

In ASCII format

Letters are as given.
# represents a number character.
<CR> is the carriage return character.
Characters outside the range {A-Z}, {0-9}, and <CR> are ignored.

In Binary format

<Letter> is the binary ASCII code for that letter
<HB> <MB> <LB> are the three bytes of a 24-bit binary number.

Wavelength Commands

Command ASCII format Binary format
Wavelength setting WAVE ###### <CR> <W> <HB> <MB> <LB>
Higher wavelength selection HIGH ###### <CR> <H> <HB> <MB> <LB>
Lower wavelength selection LOWR ######<CR> <L> <HB> <MB > <LB>
Incremental wavelength selection INCR ###### <CR> <I> <HB> <MB> <LB>
Dwell time selection TIME ###### <CR> <T> <HB> <MB> <LB>
Scan wavelength range SCAN ###### <CR> <S> <HB> <MB> <L B>

 

Motor, Shutter, A to D, and Photon Counting Commands

Command ASCII format Binary format
Positive N step movement POSI ###### <CR> <P> <HB> <MB> <LB>
Negative N step movement NEGA ###### <CR> <N> <HB> <MB> <LB>
Shutter control SHUT ###### <CR> <X> <HB> <MB> <LB>
Count photons CNTP ###### <CR>
(returns ######<CR>)
<C><HB><MB><LB>
(returns <HB> <MB> <LB>)
Analog measurement ANAL ###### <CR>
(returns ###### <CR>)
<A> <HB> <MB> <LB>
(returns <HB> <MB> <LB>)

 

Advanced Commands and Parameters
(values marked * are retained when power is removed)

Command ASCII format Binary format
Set baud rate* BAUD ###### <CR> <B> <HB> <MB> <LB>
Set command format* FORM ###### <CR> <F><HB><MB><LB>
Zero set ZERO ###### <CR> <Z> <HB> <MB> <LB>
Grating [g/mm] set* GRAT ###### <CR> <G> <HB> <MB> <LB>
Turret position* TPOS ###### <CR> <Y> <HB> <MB> <LB>
Monochro. calibrate MCAL ###### <CR> <M> <HB> <MB> <LB>
Unit select UNIT ###### <CR> <U> <HB> <MB> <LB>
Reference angle* REFA ###### <CR> <R> <HB> <MB> <LB>

 

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INSTALLATION

NOTE: The SID-101 is normally shipped as part of a monochromator assembly. It is, however, also available as an upgrade for field installation onto an existing monochromator. If your SID-101 was ordered as an upgrade, please refer to the Upgrade Installation instructions at the end of this section.

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Included Parts

The following parts comprise a SID-101 monochromator controller system:

  • an instruction manual
  • the monochromator controller attached to the monochromator
  • a floppy disk with demo programs (the latest version can be downloaded from www.pti-nj.com)
  • a power pack
  • an AC power cord
  • a 9-pin ‘D’ male to female cable

Other parts that you may need include:

  • an adapter to attach the 9-pin male ‘D’ connector to a 25-pin male ‘D’ serial port used on some computers
  • BNC cables for connecting to the analog or photon counting signals from a PTI PMT housing

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Setup

The monochromator should be prepared according to the instructions in this book. The monochromator controller and computer should be connected using the cable provided by PTI, connecting the "FROM COMPUTER" connector on the controller to the com port of the computer. If several monochromators are to be interconnected, the "TO NEXT DEVICE" connector on the first SID-101 is connected to the "FROM COMPUTER" connector on the next SID-101.

The power cable from the power pack should be plugged into the "POWER" connector on the monochromator controller. Plug the power pack into an AC outlet. Turn on the SID-101 power supply, boot the computer, and the system is ready for use. (Skip to the next section, OPERATION.)

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Upgrade Installation

Preparation

Remove the monochromator cover. Note that the original M3 flat head screw used on the monochromator cover will not fit through the cover screw guide. It should be replaced with a socket head cap screw or a pan head screw.

Disconnect the motor from the motor drive shaft, and remove the motor from the motor-mount plate. Remove the motor mount plate from the monochromator.

On the SID-101, remove the cap plug from the cover screw guide. Remove the four side screws holding the SID motor mount plate to the cover assembly. Remove the cover assembly by pulling the SID motor mount plate straight back from the cover assembly.

If the monochromator lacks both auto-calibration and a shutter, remove the option cable from the SID-101. The option cable can be identified by its hexagonal connector. If an unused option cable is not removed, it may cause a short circuit in the assembled unit.

Assembly

Orient the SID-101 motor mounting plate such that the machined notch faces up and toward the monochromator. Attach the motor to the motor mounting plate with the motor power connector facing to the left, as viewed from the back of the motor, using four 10-32 x 1/4-inch screws. Refer to the figure on the following page.

Mount the SID-101 motor mounting plate/motor assembly to the monochromator using the original plate mounting screws.

Connect the motor power cable to the motor, and connect the option cable to the option connector, if applicable.

Carefully slip the SID-101 cover assembly over the motor and motor mounting plate. The cover screw guide tube must pass between the two circuit boards. Avoid crimping the cables within the SID cover assembly.

SID-101 ASSEMBLY

Secure the SID-101 cover assembly with the button head cap screws provided. Replace the cap on the monochromator cover screw guide.

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OPERATION

Controlling the Monochromator in ASCII Format

In the following description it will be presumed that the monochromator is being controlled from a terminal emulation program such as Microsoft Hyperterm.

Startup

The SID-101 controller is shipped preset for ASCII FORMAT operation and for communication at 9600 BAUD, 1 STOP BIT, NO PARITY. If the controller and the monochromator were purchased as a single unit, then the controller has been programmed with calibration constants for that monochromator.

When power is first connected to the Monochromator controller, it will perform an auto-calibration sequence. The auto-calibration sequence will set the monochromator to its preset auto-calibration position. When the auto-calibration sequence is complete, the controller will transmit the ASCII phrase, "OK."

The monochromator retains its calibration information when it is not powered. However, all other scan related information is lost.

General rules

Wavelengths in commands are specified by up to six digits. For gratings with groove densities greater than or equal to 150 g/mm, the units of wavelength are 0.01 nm. For gratings with groove densities of less than 150 g/mm, the units of wavelength are 0.1 nm.

When the monochromator controller receives a command that it understands, it will reply with a "Y"<CR>. If the command is not understood, it will reply with a "N"<CR>. When the action is complete the controller will signify the end with a "D"<CR>. If no action is required, if a parameter is being set, no "D"<CR> will be returned. The External A/D Conversion command will return a value before returning "D".

In ASCII FORMAT operation, the monochromator will ignore characters outside of the set {A-Z, 0-9, <CR>}. For example, the command to set the wavelength to 633 nm could be written as

WAVE63300<CR>
or as
WAVE = 633.00<CR>

Sending a second command before the first command has been completed will halt execution of the first command. This is a useful feature for stopping commands that have long execution times.

Wavelength Setting

The command to set a monochromator to a given wavelength is:

WAVE ###### <CR>

The units of wavelength in the command value, ######, are described in General Rules above.

For example, setting the monochromator to 547 nm could involve the following sequence.

WAVE 547.00 (sent to controller)

Y (reply by controller

(monochromator is set to 547 nm)

D (sent by controller)

Scanning a wavelength range

In a scan, the monochromator scans between two wavelengths in a series of steps of preset wavelength size. At each step, the monochromator stops and dwells for a preset time interval. The monochromator will scan from low to high wavelength, then repeat.

Four parameters must be set to describe the scanning between two wavelengths:

  • the lower wavelength
  • the upper wavelength
  • the wavelength increment, and
  • the dwell time

Lower Wavelength Selection

The lower wavelength of a scan is set using the command:

LOWR ###### <CR>

where the command value, ######, specifies the wavelength.

Higher Wavelength Selection

The higher wavelength of a scan is set using the command

HIGH ###### <CR>

where the command value, ######, specifies the wavelength.

Incremental Wavelength Selection

The wavelength increment of a scan is set using the command

INCR ###### <CR>

where the command value, ######, specifies the wavelength increment. If the wavelength increment is non-zero then an interval scan is selected. If the wavelength increment is zero, then a continuous scan is selected.

Dwell Time Selection

The dwell time serves four functions:

  1. When Analog Measurement and Photon Count are both zero, the dwell time will specify how long the monochromator pauses after each wavelength increment.

  2. When Photon Count is non-zero, the dwell time is the integration time for photon counting. After the dwell time, the controller will report back the photons counted during the dwell time.

  3. When Analog Measurement is non-zero, the controller will take a single analog measurement at the end of the dwell time. The controller will report this analog measurement, then repeat this for the number of analog measurements selected.

  4. If Analog Measurement and Photon Count are both non-zero, the controller will report back an analog measurement and a photon count at the end of the dwell time, and then repeat this for the number of analog measurements selected.

The dwell time is set using the command

TIME ###### <CR>

where the command value, ######, specifies the dwell time in units of 10 milliseconds

Scan Command

The Scan command begins scanning. The command is

SCAN ###### <CR>

where the command value, ######, specifies the number of times that the scan will be repeated.

The following is an example of the sequence of events in a scan. In this case, a scan between 400 and 600 nm in steps of 2 nm with a 0.25 second dwell time is programmed and run twice.

LOWR 400.00 <CR> (From computer. Set lower wavelength)
Y <CR> (From controller. Value in range)
HIGH 600.00 <CR> (From computer. Set upper wavelength)
Y <CR> (From controller. Value in range)
INCR 2.00<CR> (From computer. Set increment)
Y <CR> (From controller. Value accepted)
TIME 25 <CR> (From computer. Set dwell time)
Y <CR> (From computer. Value accepted)
SCAN 2 <CR> (From computer. Repeat twice)
Y <CR> (From controller. Command accepted)
(controller performs scan)
D <CR> (From controller. Command done)

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Motor Stepping, Shutter Control and Signal Measurement

Positively stepping the drive motor

WARNING: The rotation of the grating by stepping is not limited by software, but by mechanical stops. Mechanical damage may result if the mechanism is quickly driven to its mechanical stops!

 

The stepping motor on the wavelength drive is stepped by N steps in the positive (higher wavelength) direction using the command

POSI ###### <CR>

where the command value, ######, is the number of steps that the motor will make.

Negatively stepping the drive motor

WARNING: The rotation of the grating by stepping is not limited by software, but by mechanical stops. Mechanical damage may result if the mechanism is quickly driven to its mechanical stops!

 

The stepping motor on the wavelength drive is stepped by N steps in the negative (lower wavelength) direction using the command

NEGA ###### <CR>

where the command value, ######, is the number of steps that the motor will make.

Shutter control

The shutter is controlled using the command

SHUT ###### <CR>

where the command value, ######, is

0 to close the shutter
1 to open the shutter
2 if external control (high = open)
3 if external control (low = open)

Analog measurement

Measurement of an analog signal is controlled using the command

ANAL ###### <CR>

where the command value, ######, is the number of measurements that will be made. The maximum value is 65535.

When this command is issued, the controller will delay for the length of the dwell time, then digitize and report the analog signal. This digital value will be between 4095 for a 12.3 volt signal and 0 for a zero volt signal. If a scan is made when analog measurement is non-zero, analog measurements will be made at the end of each dwell time.

The numbers reported by the controller are ####<CR> or <HB><LB> depending upon the format of data reporting.

Photon Counting

Photons are counted using the command

CNTP ###### <CR>

where the command value, ######, is the number of measurements that will be made. The maximum value is 65535.

When this command is issued, the controller will count photons for the length of the dwell time, then report the count. If a scan is made when photon count is non-zero, the photon count will be done for each dwell time.

The numbers reported by the controller are ######<CR> or <HB><MB><LB> depending upon the format of data reporting. If the photon count exceeds the value allowed by the format, zero will be reported.

The following is an example of opening the shutter, moving the motor one step in a positive direction, taking an analog measurement and closing the shutter.

SHUT 0 <CR> (From computer. Open the shutter)
Y <CR> (From controller. Value in range)
(shutter opens)
DWEL <CR> (From controller. Command done)
POSI 1 <CR> (From computer. Step one positive step)
Y <CR> (From controller. Value in range)
(step done)
D<CR> (From controller. Command complete)
ANAL 1 <CR> (From computer. Do analog conversion)
Y <CR> (From controller. Value in range)
(analog measurement done)
###### <CR> (From controller. Digital value)
D <CR> (From controller. Command done)
SHUT 1 <CR> (From computer. Close the shutter)
Y <CR> (From controller. Value in range)
(shutter closes)
D <CR> (From controller. Command done)

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Advanced Control

Calibration commands

The Zero command, the Grating command, the Angle Reference command, and the Monochromator commands are all used in calibrating the monochromator. They are described in the section on calibration.

Turret position specification

The Turret command tells the monochromator controller which grating turret position has been selected. This command must be used after the grating turret position is changed so that the controller will use the proper calibration constants.

The turret parameter is set using the command

TPOS ###### <CR>

where the command value, ######, is 0 (zero) for turret position one, and 1 (one) for turret position two.

Baud rate specification

The Baud command tells the monochromator controller what communication rate to use. Caution should be used in communication rate because communications will be lost if the computer and controller are using different rates. When the communications rate is changed, the controller will reply with Y <CR> using the old rate, delay 2 seconds, then reply with D <CR> using the new rate

The baud parameter is set using the command

BAUD ###### <CR>

where the command value, ######, is determined from the following table.

Parameter Value Selection
Baud rate 19200 0
  9600 1
  4800 2
  2400 3
  1200 4
  600 5
  300 6

Unit selection

It is possible to control multiple monochromators from one serial port using the Unit command. Serial communication to monochromator controllers may be cascaded as shown in the section on installation. The Unit command selects a particular monochromator controller by its serial number. Only the monochromator that is selected will respond to subsequent commands. All monochromators will, however, watch for the next unit command to see if the selection changes.

The Unit command is

UNIT ###### <CR>

where the command value ###### is the controller serial number.

Format specification

Up to this point, all commands have been described in ASCII FORMAT. An alternative, faster communications format is available. This is BINARY FORMAT.

In BINARY FORMAT, the command values are passed not as ASCII digits but as binary values. Between zero and three bytes are required for the command value depending on the command that is being used. The exact formats are given in the command specification section. These commands are not ended with carriage returns.

The value returned by the External Analog command in BINARY FORMAT is two bytes. No carriage return follows these bytes.

The command understood reply <Y> and the command complete reply <D> are not followed by carriage returns in BINARY FORMAT.

The response to commands may also be programmed in the command format. Both command understood and command complete replies may be suppressed.

The format command is

FORM ###### <CR>

where the command value, ######, is to be selected from the following table.

Command value Format Reply
understood
Reply
done
0 ASCII Y Y
1 ASCII Y N
2 ASCII N Y
3 ASCII N N
4 BINARY Y Y
5 BINARY Y N
6 BINARY N Y
7 BINARY N N

 

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Calibration

Monochromator calibration requires four measurements; determination of the grating groove frequency, determination of the grating reference angle, determination of the grating angle at which white light is transmitted (zero order), and determination of the grating angle at which a know wavelength is transmitted. The effect of minute mechanical flexures (backlash) is removed by determining the zero-order measurement for both increasing and decreasing wavelengths.

Grating Specification

The grating command informs the monochromator controller of the groove density of the grating in the selected turret position. The command has the form

GRAT ###### <CR>

where the command value, ######, is the number of grooves per 10 mm of grating width.

Reference Angle Command

PTI monochromators have an optical encoder that identifies a reference grating angle. This reference position can be determined using the Reference Angle command. The command has the form

REFA ###### <CR>

where the command value, ######, is zero.

If your monochromator does not have the reference angle optical encoder, this command will cause the monochromator to advance by 100 steps.

Zero Command

The Zero command causes the monochromator controller to identify the current grating angle as the angle at which the monochromator transmits white light from entrance to exit. With a white light at the entrance slit, the motor step command is used to rotate the grating until white light is maximally transmitted to the exit slit. The command has the form

ZERO ###### <CR>

where the command value, ######, is zero if the determination is made by stepping the motor toward increasing wavelengths, and one if the determination is made by stepping the motor toward decreasing wavelengths.

For monochromators that have auto-calibration, the ZERO 0 command causes the monochromator to remember the current number of steps from the reference encoder to the zero position. For monochromators without autocalibration, the command causes the monochromator to define the current wavelength as zero nanometers.

For both types of monochromators, the ZERO 1 command causes the monochromator to define the difference between the nominal zero position and the defined zero position as the reverse direction offset (backlash).

Monochromator Calibrate Command

The monochromator calibrate command causes the monochromator controller to identify the current grating angle with a particular wavelength. Using the wavelength or motor step commands the monochromator is set to transmit a known reference spectral wavelength. Then that wavelength is entered in the monochromator calibrate command. The command has the form

MCAL ###### <CR>

where the command value, ######, is the known wavelength value of the reference.

The calibration will use different mechanisms depending upon the presence of the auto-calibration option. If auto-calibration is present. MCAL adjusts the scale factor between wavelength and motor steps. The zero value (Step number at zero wavelength) is not changed. If auto-calibration is not present, MCAL redefines the step number at zero wavelength.

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Included Reference Programs

A floppy disk with examples of monochromator control is included with the controller. Those programs are:

SID-1B.TXT and SID-1B.EXE, source code and executable for a BASIC program that allows monochromator control under MS-DOS or Windosw-3.1.

SID-VB.TXT and SID-VB.EXE, source code and executable for a Visual Basic program that allows monochromator control under Windows-95.

A manual is included with the drivers: sidddemo.doc.

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SERVICE

Troubleshooting

Problem: When power is supplied to unit, the status light does not illuminate.

Solutions: Check the outlet. If the outlet is OK, check the power pack. If the power pack is not outputting 12 VDC, then the power pack is defective. If the power pack is outputting 12 VDC, then the controller is defective.

Problem: The controller and computer do not communicate.

Solutions: If the monochromator does not make sounds indicating an auto calibration when power is first applied, then either the power supply or the controller is defective.

If self calibration occurs, the cable between the computer and the controller may be defective. The cable should be checked for continuity and lack of shorts.

If the cable is known to be good, the communication protocol of the computer should be checked. The SID-101 controller is expecting communications at 19,200 baud, 1 stop bit, and no parity. Baud rates between 300 baud and 9600 baud should also be checked as the baud rate of the controller may have been inadvertently changed. Checking is best done by transmitting the Unit command which will wake up the monochromator if it has been inadvertently deselected.

Problem: The monochromator wavelengths are off significantly.

Solutions: The monochromator turret may have been changed without the controller being set for the current turret value. Use the turret command to specify the current turret position. Also use the grating command to re-enter the know grating frequency.

Problem: The monochromator wavelength is off slightly

Solutions: Perform a reference angle check using the Angle Reference command.

If the problem is not solved, perform a full calibration.

Problem: You have lost the serial number of your controller.

Solution: When the controller is in the BINARY mode, it recognizes an additional query command. If the controller receives a <Q><Q><Q><Q> it will respond with 2 binary bytes that represent the internally stored serial number.

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