µMD2 is intended for use in hobbyist, experimental, research, and other
applications where a bug in the hardware, firmware, or software, will not
have a significant impact on the future of the Universe or anything else.
While every effort has been made to avoid this possibility, µMD2 is an
on-going development effort. We will not be responsible for any consequences
of such bugs including but not limited to damage to the construction crane you
picked up on eBay for $1.98 + shipping, financial loss from ending up in
the Antarctic when the compass orientation provided by
your home-built ring laser gyro was off
by 1,536 degrees, or bruising to your pet's ego from any number
of causes directly or indirectly related to µMD2. ;-)
Thanks to Jan Beck for selecting the Teensy 4.0 and writing and testing
initial versions of the firmware and GUI. And for getting me interested
in actually getting involved in this project. If anyone had told me
six months ago that I'd be writing code in C, MIPS assembly language,
and Visual Basic - and enjoying it (sort of) - I would have suggested
they were certifiably nuts. ;-) Jan maintains the master GUI source code
as well as slightly different versions of the firmware for µMD2
and µMD1 and a
development blog on the overall projects.
Introduction
The SG-µMD2 kit of parts includes everything necessary for either
a 1 axis or 3 axis µMD2 readout without sensors.
This document provides detailed instructions for assembling the SG-µMD2
PCB Version 1.0.
A similar document will be provided when other versions are available.
No major changes are anticipated but since some parts may change,
it is critical to use the correct manual. Check the marking on the
PCB to confirm the version.
All components are through-hole and except as noted,
should seat flush on the
PCB. They shouldn't be suspended in mid-air swinging in the breeze. :)
Unfortunately, due to a minor error, parts are not labeled on the
silkscreen for V1.00, so the schematic and SG-µMD2 PCB layout diagram
will need to be used to identify parts placement.
A low power soldering iron with narrow tip and thin (e.g., #22 AWG) rosin-core
solder will be required. DO NOT even think about attempting this without
suitable soldering equipment. It's well worth the investment. A Weller
soldering gun or propane torch will not work. :) Rosin core solder is also
essential. And while
I'm quite confident that you never make mistakes, a means of component removal
such as a de-soldering pump (e.g., SoldaPullt™) will be highly desirable.
Screwing up component removal can easily ruin the PCB and is not covered
under the unlimited limited warranty. :-) The total investment should
not exceed $100.
Proper soldering technique will be such that the exposed solder on each
pad should be shiny with a concave profile. It should not be a blob and
just needs to fill the hole. Solder is not glue.
Some excess solder doesn't hurt anything
but looks unprofessional. A 10X magnifier may come in handy for
inspection. Residual rosin can be cleaned off with
isopropyl alcohol or an environmentally-friendly electronic solvent.
However, leaving the rosin alone is also acceptable (if ugly).
Total assembly time for a single axis system should be under one hour
for someone proficient
in fine soldering. Cutting component leads to 1/4 to 3/8 inch before
installation will simplify soldering as the long leads won't be poking
you in the face. :( :) Then trim flush after soldering.
IMPORTANT: All the resistors are labeled using the standard color
as shown below. Normal color vision
is required to be able to identify these reliably. Even then, it is sometimes
difficult to confirm the values that differ in one band or in poor lighting.
And a magnifier may be required to read
some markings on these and other components. If in doubt, have someone
else assemble the kit or assist you.
For those not familiar with the common resistor color code
(Black/0, Blown/1, Red/2, Orange/3, Yellow/4, Green/5, Blue/6, Violet/7,
Gray/8, White/9), two of the resistors near the 8 pin UA9637 ICs in
the layout diagram are are 680 (blue-gray-brown
or 68 with 1 zero) ohms and 330 (33 with 1 zero) ohms. The gold stripe
indicates 5 percent tolerance on the value but for the use here, tolerance
doesn't matter. (It's possible the resistors you use will have 4 stripes
where 3 of them are the value and the 4th is the multiplier, along with one
for tolerance. If in doubt confirm the value with a multimeter.) The chart below is from Digikey. (If the link decays, a Web search will readily
locate another one.)
Resistor Color Code Chart (from the Digikey Web site)
The schematic for the SG-µMD2 Version 1.0 PCB may be found at
SG-µMD2 Version 1.0 Schematic and
SG-µMD2 Version 1.2 Schematic.
(There no longer is a V1.1.)
This includes everything on the board for a three axis homodyne or
heterodyne system with the
OLED display. The only difference between V1.0 and V1.2 is the color
of the signal LEDs and the values for their current limiting resistors,
and labeling for the required wire jumpers for the heterodyne signals.
(The schematic and PCB version numbers do not necessarily match; these
both apply to the V1.00 PCB.)
Printing out the schematic and having it available for reference while
assembling the PCB may be helpful.
Refer to the SG-µMD2 board layout below. Clicking on these
will bring up higher resolution versions in a single separate tab or window
depending on your browser settings:
SG-µMD2 V1.0 PCB Layout Diagrams for V1.0 and V1.2 Schematics (Left) and Minimal Single Axis PCB and Three Axis Populated PCB with OLED Display (Right)
(JB1 and JB2 Jumpered for Homodyne Mode; JB3 and JB4
Jumpered for OLED GND on the Left and VCC on the Right)
(All kits going forward will have red and green LEDs for the A and B signals;
the original prototype was all boring green. Please pay attention to the
values for the current limiting resistors as they differ by more than an order
of magnitude for the red and green LEDs to equalize their perceived brightness.
Where the signal LEDs are all green, the R3,R8,R13,R18,R23,R28 will be 10K
ohms.)
In addition to the resistor color codes and component references, the
layout diagram also shows the signal locations for homodyne
and heterodyne on the Teensy. Sorry about the silkscreen
component references being missing on the actual PCB; this will
have to do.
The populated PCB shows all components required for a three axis system
with the OLED display. The terminating resistors (near the screw terminal
blocks) have not been installed since their value may depend on the specific
configuration, and can often be left out. Five pin headers may be substituted
for the screw terminal blocks if desired. The signal LEDs shown are all
green in the photo, sorry. :)
Print out this document so each step can be checked off ( ) as it is completed.
The parts list below assumes populating the SG-µMD2 for 3 channels
with the OLED display. So for a single channel system, some parts in this
list may not be present and/or there is no need to install those associated
with channels 2 and 3 and/or for the OLED.
The OLED display color may be yellow/blue (yellow for first two lines of text
with blue for the remaining 6 lines), all blue, or all white. In addition,
they may differ slightly in their pinout and mounting hole type/location
as follows (viewed with the pins at the top):
Type 1: GND on the left and VCC (V3.3) on the right.
The mounting holes are round. Their locations match the
SG-µMD2 PCB holes.
Type 2: VCC (V3.3) on the left and GND on the right.
The mounting holes are elongated. Their locations do not match
the SG-µMD2 PCB holes.
There may be other variations. The kits will generally have the Type
1 yellow/blue OLEDs. If you bought a Type 2 OLED, DO NOT drill
holes in the SG-µMD2 PCB to make the screws line up as this risks
shorting the internal VCC and GND planes, use some insulated wires in place
of the screws and Epoxy - or duct tape. ;-) Elongating the holes in the
OLED PCB may be accepatable though.
( ) Confirm that all parts are present and undamaged:
( ) 1x blank SG-µMD2 V1.0 PCB. Confirm the version on the
silkscreen near the location of the USB connector. Inspect for plating
or other defects.
( ) 2x 14 pin SIP male-male socket strip for Teensy. (Only if Teensy
does NOT have pins soldered.)
( ) 1x 3 mm blue LED.
( ) 1x 10 ohm 1/8 W resistor (brown-black-black-gold).
( ) 1x 10K ohm 1/8 W resistor (brown-black-orange-gold).
( ) 3x 5 pin screw terminal block.
( ) 3x UA9637 or UA9639 line receiver (8 pin DIP in foam or tube).
( ) 3x 8 pin DIP socket.
( ) 3x 0.1 µF ceramic capacitor (marked with 104 or 0.1).
( ) 4x 3 mm red LED (3 required, a spare).
( ) 4x 3 mm green LED (3 required, a spare).
( ) 12x 150 ohm 1/8 W resistor (brown-green-brown-gold).
( ) 6x 330 ohm 1/8 W resistor (orange-orange-brown-gold).
( ) 6x 680 ohm 1/8 W resistor (blue-gray-brown-gold).
( ) 3x 1K ohm 1/8 W resistor (brown-black-red-gold).
( ) 1x 10K ohm 1/8 W resistor (brown-black-orange-gold).
( ) 3x 36K ohm 1/8 W resistor (orange-blue-orange-gold).
( ) 1x 40 pin female-male socket strip.
( ) 1x 1 pin male-male socket strip to be cut in pieces for
jumper headers (JBx) if desired.
( ) 1x 0.96" 128x64 OLED IIC display in antistatic wrap or bag.
( ) 1x 4 pin SIP male-male socket strip for OLED.
( ) 2x M2*16 screw for OLED mounting.
( ) 2x M2 nut for OLED mounting.
For a single axis system, approximately 2/3rds of the components in the
last block will not be present. And the OLED will not be present, uh, for
the system with OLED. :)
The "Optional" parts identified below
can be omitted if that feature is not being
implemented. The LEDs especially are not really
that useful and with the 10K ohm current limiting resistors, annoying bright.
So you may want to at least experiment with higher values of resistors (like
22K or even 47K) to tame them.
( ) Install R0 (10K ohms) under where the Teensy socket will go.
( ) Install R31 (10 ohms) under where the Teensy socket will go.
( ) Trim the large socket if necessary so it has two rows of 14 pins.
This is for MPB1, the Teensy 4.0 PCB.
( ) Carefully insert it in the PCB confirming no bent pins. Rather
than flipping a coin :), orient it so the large cutout faces the USB to
the left. Then solder
two corners and confirm it seats flat, then solder the other pins. Inspect
for solder bridges and unsoldered pins.
( ) Install the 8 pin sockets for U1 (single axis) and U2,U3 (three axes).
Note orientation - the cutout goes to the right as viewed in the layout
diagram. Solder and inspect for solder bridges and unsoldered pins.
( ) Install C1 (0.1 µF, single axis) and C2,C3 (three axes).
C1,C2,C3 are the oval outlines to right of U1,U2,U3 respectively.
( ) Install R4,R9 (330 ohms, single axis) and R14,R19,R24,R29 (three axes).
( ) Install R5,R10 (680 ohms, single axis) and R15,R20,R25,R30
(three axes).
( ) Install D0 (3 mm blue LED). The anode is the longer lead
and goes to the right as viewed in the layout diagram. The flat is the
cathode and goes to the left. Cut the leads about 1/10" from the body
if the LED can't be inserted to sit flush on the PCB. Take care not
to overheat or stress the leads on the LED when soldering. Be as quick
as possible.
( ) Optional signal LEDs:
( ) Install R3 (36K ohms, single axis) and R13,R23 (three axes).
( ) Install R8 (1K ohms, single axis) and R18,R28 (three axes).
( ) Install D1 (3 mm green LED, single axis) and D3,D5 (three axes).
( ) Install D2 (3 mm red LED, single axis) and D4,D6 (three axes).
The anode is the longer lead
and goes to the right as viewed in the layout diagram. The flat is the
cathode and goes to the left. Cut the leads about 1/10" from the body
if the LED can't be inserted to sit flush on the PCB. Take care not
to overheat or stress the leads on the LED when soldering. Be as quick
as possible.
( ) Install J1 (screw terminal block, single axis) and J2,J3 (three
axes). Make sure the entrance holes for the wires face away from the
PCB! Solder the center pin and confirm they are flat on the PCB, the
solder the others. Check for solder bridges and unsoldered pins.
( ) Install jumper wires (cut resistor leads) at JB1 and JB2 as shown
in blue on the layout diagram. This selects the Homodyne signals.
( ) Carefully inspect for unsoldered pins, solder bridges and
other blemishes. Correct as needed. THIS IS ESSENTIAL!
It would be bad form to blow the brain due to an errant blob of solder. :(
( ) Test the Teensy before doing anything to it. If it fails this test,
contact me before proceeding.
Connect it to a USB port using the USB A to USB Micro B cable.
Assuming the µMD2 firmware has been installed, after a
second or so, D13, the on-board LED, acts as a heartbeat monitor and
should start flashing in some pattern not yet fully determined, but
it will be unlike the "Blink" sketch. ;-) Currently it's a short flash
ever 0.5 to 1 seconds.
It should also be spitting out data via the USB COM port. In the
Arduino IDE, go to Tools->Port and select the port that it is plugged
into. It should show something like: COM5 "Serial (Teensy)".
Go to Tools-Serial Monitor. A window should appear showing data
being sent to the COM port. It will be mostly boring but the 6th value
should be incrementing by 1, probably at around 1 kHz:
(Should you care, the 6th and 7th values are the "Low Speed Code" and
"Low Speed Data", respectively. 10,124 is the firmware version 1.24;
8,100000 is the sample rate of 1,000 x 100, and 20,4099 specifies 3 homodyne
axes + a homodyne multiplier of 4 x 256.)
Unplug the USB cable.
( ) Assemble the Teensy 4.0 PCB to the pin or socket strips. There will
either be a pair of 14 pin female-male socket strips precut or the male to
male pin strip that needs cutting:
Female to male socket strips: The male pins slip through the
Teensy PCB holes from the top so the female sockets are accessible when
Teensy is plugged into the large socket on the SG-µMD2 PCB.
Male to male pin strip: The short pins slip through the Teensy PCB
from the bottom. (This is what's shown in the photo of the completed
SG-µMD2 PCB, above.)
Solder a single pin near the center and
confirm it seats flush, then solder the rest.
To assist in alignment, the strips can be inserted in the 28 pin socket
taking care not to push any of the individual pins out of position.
( ) Test the Teensy as above before plugging it into the SG-µMD2 PCB
to confirm the soldering hasn't done anything bad. Then unplug the USB
cable.
( ) Carefully plug the Teensy into the 28 pin socket. The USB socket
faces off the left side of the PCB as shown in the layout diagram.
Make sure all pins are seated and none are hanging off the socket.
CAUTION: Make sure all the pins line up with their entry points in the
socket to avoid squashing the leaf sprint contacts.
( ) Reattach the USB cable. The power LED should come on immediately
and after a second or so, the Teensy LED should start flashing as before.
Unplug the USB cable.
( ) Plug a UA9637 or UA9639 IC into the U1 position. The dot or cutout
should face to the right - these ICs are upside-down compared to the
Teensy part labeling as shown in the layout diagram.
( ) Reattach the USB cable. The power LED and possibly one or both
LEDs near U1 (if installed) should come on immediately
and after a few seconds, the Teensy LED should start flashing as before.
( ) (Optional) Here is the nifty bit. ;-) Moisten a finger (doesn't matter
which one) and touch the pins on J1. With some practice, it will be possible
to make the LEDs near U1 to go on and off as the input to the line receivers
cause them to toggle. While the behavior is not really predictable, just
the fact that they change indicates the the line receiver is working.
Since the UA9637 has some hysteresis, it latches but the slight charge
from your electric personality is enough to toggle it. CAUTION: Don't
get carried away, these parts can be damaged by static discharges.
So, no cat's fur and plastic rods, please. :( ;-)
(This will not be possible if the terminating resistors are installed, thus
holding off on them for now.)
If you're wondering how the OLED in the photo, above, can be displaying
such large numbers with nothing attached to the inputs, it was done this
way except the board was plugged in a USB charger, not a USB port. That must
have a lot of ripple relative to my moistened finger, enough to easily
trigger the UA9637 even with its hysteresis.
( ) Start the µMD GUI and select the COM port used to upload
the firmware. The graph should start scrolling. But now, if you do the
moistened finger thing, it should be possible to get the displacement
to change for Axis 1. Once confirmed, unplug the USB cable.
( ) Repeat the previous 3 steps for axes 2 and 3 (U2/J2 and U3/J3)
if desired.
( ) Heterodyne systems ONLY. Add the following wire jumpers on the
bottom of the SG-µMD2 PCB on the Teensy socket as required depending
on the number of axes. Use thin insulated wire and take care to avoid
solder bridges. The second attachment point for each jumper can be to
the appropriate labeled header pad. The following assumes the use of
V2.20 or later firmware as the pin assignments have changed:
Single axis:
( ) Install jumpers for REF: D1 to D0 and D6. (MPB1 socket pins 3 to 2
and 8.)
( ) Install jumpers for MEAS1: D2 to D8 and D9. (MPB1 socket pins 4 to 10
and 11.)
Three axis:
( ) Install jumpers for MEAS2: D3 to D10 and D11. (MPB1 socket pins 5
to 12 and 13.)
( ) Install jumpers for MEAS3: D4 to D12 and D14. (MPB1 socket pins 6
to 14 and 16.)
Note: The original design had the heterodyne signals assigned to pins that
were the same as homodyne signals. But so far it has not been possible to
decipher the control of the ARM Cortex M7 crossbar to put them there, so
they never worked and
the pins above need to be used for now at least. Thus the duplicate
set of signals on the PCB layout diagram. If that gets resolved,
the additional jumpers will not be necessary. But pigs will probably
fly before that happens since all spare brain cells of the members of the
development team have been blown.
( ) Optional OLED:
( ) Install the 4 pin female-male socket strip for the OLED
between JB3 and JB4 and HDR3. If longer, it will need to be trimmed.
( ) Install wire jumpers to select V3.3 and GND for the
OLED at JB3 and JB4 depending on which version you have. The outer positions
select V3.3/VCC on the left while the inner positions select V3.3/VCC
on the right. DO NOT plug in the OLED until correct power connections are
confirmed with a DMM:
OLED OLED
VCC GND SCL SDK GND VCC SCL SDK
o o
|
JB4 o JB4 o
|
o o
o o
|
JB3 o JB3 o
|
o o
CAUTION: DO NOT jumper the middle pins together by accident, that will
short V3.3 to GND. :(
( ) If the OLED does not already have a 4 pin header attached,
cut off a 4 pin section of male-to-male pin strip and insert it under
the OLED with the short side through the OLED PCB just as with the
Teensy. Solder one pin, confirm it seats flush,
and solder the rest. Inspect for solder bridges and unsoldered pins.
( ) Insert an M2*16 mm screw through each of the bottom holes of the OLED
and loosely thread an M2 nut onto each.
( ) Plug the OLED into the 4 pin socket strip.
The holes in the PCB are a snug fit for
these screws so it should be possible to thread them in without nuts. Set
them so the OLED is level and tighten the nuts just snug.
( ) Power up the PCB via the USB cable. After a second or so, the OLED
should display something like "µMD2 V1.xx" on the top line and
the sequence number on the 3rd line incrementing at the sample rate
(probably 1 kHz). These OLEDs can display 8 lines of text; for the version
provided in these kits, the top two are yellow while the other 6 are blue.
It is NOT a color display.
( ) Using the moistened finger trick :), it should be possible to fool the
system into thinking there is activity on each of the installed axes
at which point lines will appear on the OLED with the relevant axis ID
and count. This can be more fiddly than might be assumed as the Quadrature
counting hardware of the Teesny will ignore sequences of A and B that don't
make sense.
( ) (Optional) Install R1,R2.R6,R7 (single axis) and
R11,R12,R16,R17,R21,R22,R26,R27
(three axes). The terminating resistors included in the kit are 150 ohms,
which is generally satisfactory. However, your specific situation may
differ. If in doubt, cut the 40 pin female-male socket strip
into pieces and solder them in so other value terminating resistors
can be swapped in without desoldering.
Note: If normal (not high brightness) LEDs are installed in unused axis
locations, there may be slight "ghost" glow from them even when they should be
off due to leakage from +5V through to the the +3.3V of the Teensy pin. If
you're a purist and that bothers you (or just don't want to be distracted),
add a 1K ohm resistor between +5V (pin 1 of the UA9637/9) and the LED
cathode (pins 2 and/or 3).
Congratulations, you're all set to go. Order that construction crane
in need of a controller with free shipping on eBay. ;-)
Where the input signals are differential with approximately equal average
levels and an amplitude more than about 0.5 V, the UA9637 RS422 receivers
are all that's needed. This includes Quad-Sin-Cos which will automaigically
convert to digital.
But where the input signals are single-ended such as normal TTL or only
one polarity of a Quad-Sin-Cos, there are locations on the SG-µMD2
PCB for a reference voltage divider.
The threhsold voltage should be selected to be approximately mid-way between
the nominal high and low levels. For standard TTL, this would be 1.4 V. The
resistor values can be in the 10K range with C4 of 0.5 µF.
These are the required parts for up to a 3 axis system.
Some parts like the LEDs (along with their
associated current limiting resistors) can also be omitted if desired.
Refer to the schematic for more details.
Reference Type Part/Value Function
-------------------------------------------------------------------------------
C1 Capacitor 0.1 µF U1 5V bypass
C2* Capacitor 0.1 µF U2 5V bypass
C3* Capacitor 0.1 µF U3 5V bypass
J1 Terminal Blk 5 pin 1A/1B/REF/MEAS1 input
J2* Terminal Blk 5 pin 2A/2B/MEAS2 input
J3* Terminal Blk 5 pin 3A/3B/MEAS3 input
D0 LED 3 mm HB LED Blue Power LED
D1+ LED 3 mm HB LED 1A/REF LED
D2+ LED 3 mm HB LED 1B/MEAS1 LED
D3*+ LED 3 mm HB LED 2A/MEAS2 LED
D4*+ LED 3 mm HB LED 2B/MEAS3 LED
D5*+ LED 3 mm HB LED 3A LED
D6*+ LED 3 mm HB LED 3B LED
MPB1 CPU Teensy 4.0 Teensy 4.0 soldered to header
PCB1 PCB SG-µMD2-PCB Blank SG-µMD2 V1.0 PCB
R0 Resistor 10K ohm, 1/8 W Power LED current limiting
R1 Resistor 150 ohm, 1/8 W IN1 termination
R2 Resistor 150 ohm, 1/8 W ~IN1 termination
R3+ Resistor 10K ohm, 1/8 W 1A/REF LED current limiting
R4 Resistor 330 ohm, 1/8 W 1A/REF 5V->3.3 V level shift
R5 Resistor 680 ohm, 1/8 W 1A/REF 5V->3.3 V level shift
R6 Resistor 150 ohm, 1/8 W IN2 termination
R7 Resistor 150 ohm, 1/8 W ~IN2 termination
R8+ Resistor 10K ohm, 1/8 W 1B/MEAS1 LED current limiting
R9 Resistor 330 ohm, 1/8 W 1B/MEAS1 5V->3.3 V level shift
R10 Resistor 680 ohm, 1/8 W 1B/MEAS1 5V->3.3 V level shift
R11* Resistor 150 ohm, 1/8 W IN3 termination
R12* Resistor 150 ohm, 1/8 W ~IN3 termination
R13*+ Resistor 10K ohm, 1/8 W 2A/MEAS2 LED current limiting
R14* Resistor 330 ohm, 1/8 W 2A/MEAS2 5V->3.3 V level shift
R15* Resistor 680 ohm, 1/8 W 2A/MEAS2 5V->3.3 V level shift
R16* Resistor 150 ohm, 1/8 W IN4 termination
R17* Resistor 150 ohm, 1/8 W ~IN4 termination
R18*+ Resistor 10K ohm, 1/8 W 2B/MEAS3 LED current limiting
R19* Resistor 330 ohm, 1/8 W 2B/MEAS3 5V->3.3 V level shift
R20* Resistor 680 ohm, 1/8 W 2B/MEAS3 5V->3.3 V level shift
R21* Resistor 150 ohm, 1/8 W IN5 termination
R22* Resistor 150 ohm, 1/8 W ~IN5 termination
R23*+ Resistor 10K ohm, 1/8 W 3A LED current limiting
R24* Resistor 330 ohm, 1/8 W 3A 5V->3.3 V level shift
R25* Resistor 680 ohm, 1/8 W 3A 5V->3.3 V level shift
R26* Resistor 150 ohm, 1/8 W IN6 termination
R27* Resistor 150 ohm, 1/8 W ~IN6 termination
R28*+ Resistor 10K ohm, 1/8 W 3B LED current limiting
R29* Resistor 330 ohm, 1/8 W 3B 5V->3.3 V level shift
R30* Resistor 680 ohm, 1/8 W 3B 5V->3.3 V level shift
R31 Resistor 10 ohm, 1/8 W V3.3 protect
SKT28 Socket 28 pin, 600 mil Socket for Teensy 4.0
SKT8 Socket 8 pin, 300 mil Socket for IN1/2/REF/MEAS1 line receiver
SKT8* Socket 8 pin, 300 mil Socket for IN3/4/MEAS2/3 line receiver
SKT8* Socket 8 pin, 300 mil Socket for IN5/6 line receiver
SKT40+ Socket 40 pin, SIP Socket strip for HDR1,HDR2,JP6
U1 IC UA9637 or UA9639 REF/MEAS1 line receiver
U2* IC UA9637 or UA9639 MEAS2/MEAS3 line receiver
U3* IC UA9637 or UA9639 MEAS2/MEAS3 line receiver
"*" denotes parts that can be omitted for a sigle axis system.
"+" denotes parts that are not required for µMD2.
Notes:
The only LEDs that is really desirable is Power, D0. And that is blue.
The line receiver signal LEDs may be either super bright white or green, my
choice. And even with the 10K ohm series resistors limiting
current to less than 0.3 mA, they may still be annoyingly bright. ;-)
Feel free to substitute LEDs having your preferred decorator colors,
modifying the values of the current limiting resistors as needed to
equilize brightness.
The 0.1 µF bypass caps do not have reference designators but
they are next to pin 1 of U1, U2, and U3.
