Are powered USB hub bus-powered?

I’ve bought many powered USB hubs, even the expensive industrial version like Startech, but they never work reliably when I connect multiple devices that draws quite a bit of power. The behavior is the same (erratic when too many power drawing devices are active within one hub) whether I supply the hub with external power or not.

I suspected the USB hubs wasn’t wired in a way that the USB hub controller understands that I want to have the hub self-powered (powered from an external source like a wall wart instead of drawing the 5V from the USB upstream cable). So I opened up my StarTech and it looked like this:

Love that it’s made in Taiwan and they cut no corners in power management.
10 ports means 3 x 4-port hub chained and they do not share power converters.

On the top left corner, there’s a jumper to set the USB hub to use self-powered instead of bus-powered (default out of the box). I switched the jumpers and the board no longer takes power from the upstream bus and the lights won’t light up until I have the local power connected, which confirms it’s working as a self-powered hub.

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Capacitor Plague for unused New In Stock boards

The conventional wisdom is that the bad capacitors (the ones with stolen chemical formula) bulged because of the hours you put in it. Today I found that it’s not true. I had a batch of new old stock Trinity S2390 motherboard (used in Agilent logic analyzers) and I noticed all the 100uF 6.3V capacitors bulged and one burst:

Reminds me of Duralcell’s Alkaline batteries (never used) that leaked during storage (before the expiration date). Stop blaming the bad chemical formula on how users use them!

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RS-232 motherboard header mapping (DB9)

The pin ordering for RS-232 (DB9) pin is sequential is row-wise (the long side is a row) while the IDC-10 (ribbon cable) header is column-wise (zig-zag pattern).

This might be a little confusing because geometrically, they are in-place on both sides (you can overlay the pins of DB9 on top of IDC-10 and they align perfectly, except pin 6-9 was lowered by half a notch on the DB9 side). I am writing this post so nobody waste their time separating the wires in a ribbon just to find out the DB9 was designed so it aligns with the flat ribbon cable perfectly.

Here’s a great tutorial building your own RS-232 cable which I took the images from to illustrate the point. Please pay them a visit to show some love:

If you split the table on the right in half (cut after pin 5) and place pins 6 (DSR) ~ 9 (RI) on the right, you’ll see it align with the IDC10

1DCD (Data Carrier Detect)
Check if connection dropped
DSR (Data Set Ready)
DTR-DSR Handshaking
2RxD (Receive Data)
RTS (Request to Send)
RTS-CTS Flow control
3TxD (Transmit Data)
CTS (Clear to Send)
RTS-CTS Flow control
4DTR (Data Terminal Ready
DTR-DSR Handshaking
RI (Ring Indicator)
For phone rings
5GND (Ground pin)– (Not connected)10
Rearranged DB9M RS-232 to align with IDC10 has a clearer annotated picture that matches the physical mapping above:

Note that the tutorial itself has Tx(D) and Rx(D) reserved it was building a null modem cable and they skipped all the handshake lines. I’m doing a straight cable (which should be done for internal board header cables where the DB9 socket is male, hence DB9M).

The DTE/DCE might be confusing. Hope these properties can help people make sense out of it (so you can figure it out in your head confidently instead of randomly trying null modem adapters till it work)

  • DTE device (colloquially ‘computer’) has the pattern as shown in the pictures above (receive pins above/before the transmit pins), which is usually the computer end and the port/socket is male. Think of it as the ‘driver/master’ (though it’s arbitrary)
  • DCE device (colloquially ‘modem’) reverses all the sends and receives of the DTE. Can think of it as the ‘receiver/slave’ (though it’s arbitrary). It’s usually the modem and the port/socket is female.
  • For DTE-DTE (like data transfer between two PCs), the send lines on one side should go to the receive lines on the other side. A null modem cable that swaps the send pins with receive pins. You can think of it as making one side DCE. Given that the topology is symmetric, it’s up to the software set up to decide which side is the initiator/client (master) and which side is the reactor/server (slave)
  • The handshaking (optional) and flow control (optional) lines also have their initiator/reactor roles reversed with null modem cable.

Here’s an image for the above mapping:

Puertos E/S COM

It turns out the direct geometric mapping (IDC male pins match the relative locations of the DB9 male pins) mentioned above is the less common type of motherboard header configuration. IDC ribbon crimp-on DB9 headers like this:

ฅนบ้ายอ: [34+] Db9 Male And Female Connector Pinout

has to follow the above geometric layout since the pins cannot be remapped (so it has to follow the ribbon order). The soldered version looks like this:

However, the more commonly seen soldered RS-232M to IDC10 header uses a transposed configuration (which DB-9M pin numbers matches IDC-10 pin numbering EXACTLY despite one is row-major and the other is column-major), which has nothing to do with the IDC10 pin layout mentioned above.
(I don’t think it’s a good idea to call it ‘crossed-config’ like CWC did.
It almost mislead me to think it happens to swap the roles of Tx and Rx.
I did the mapping on paper and it didn’t make any sense.
Let’s call it ‘transposed-configuration’)

The crazy thing about the existence of these 2 pin layouts is that there’s no easy way to tell which pin layout/mapping it is until you open the connector up and inspect the solder joints! Taking a pin and probe it with a multimeter is more work than taking the screw posts out and disassemble the connector.

So if you just buy some old scrap parts that came with old motherboards, this might confuse the heck out of you until you tested the pin mapping with a multimeter and realize things doesn’t add up!

Note to self: just open the DB9 side up whenever I see a DB9-IDC cable and mark the configuration on the DB9 end directly on the cable!

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Improved code for Toner Reset SP C250SF/DN

This is based on the Raspberry Pi implementation of the Toner chip reset:

I am using a Raspberry Pi Zero W so the chip is BCM2835 instead and I can use 100Kbps/400KBps instead of 9600 baud as in the original code

The electrical pins we need is clustered on to top left, Pins 1 (3.3V), 3 (I2C SDATA), 5 (I2C SCLK), 9 (Ground)

Raspberry Pi Zero GPIO Pinout, Specifications and Programming language

While looking for the pinouts (, I discovered a useful tool called i2cdetect that allows me to find out the address of the chips which means I can write a program automatically figure out the right image to load to the chip without looking:

sudo apt-get install i2c-tools
sudo i2cdetect -y 1
Sorry I forgot where I got this image from.
Please remind me in the comments section if you find out who should I credit it to.

Since I don’t have cheap pogo pins lying around, I took the 2.4mm pitch (the standard size used in PC, Arduino and Raspberry Pi) jumper block I have (so all pins are set at equal lengths to make simultaneous contact) and hope somehow there’s 4 pins that kind of align with the contact, and it did. See pictures here:

Can press the pins down by using jumpers

You might be worried about shorting into the next pin or hooking something up in reverse damaging the chips, but luckily the chips survived. My guess is that it’s a good design to put the Vcc next to Ground on one side instead of making it symmetric so the polarity can be reversed. When reversed, SCL is set to low (Ground), SDA is pulled up to Vcc while there is no power supply, so no damage is done. Brilliant! The worst case for my poorly aligned jumper block is that SDA and Vcc might touch each other, but it doesn’t matter because it’s a perfectly legal hookup (just not communicating)!

So no worries if you didn’t touch the pins right! The only case it might go wrong is if you intentionally flip the block and slide it by two pins (reversing Vcc and Ground). Other cases (you are likely going to run into) are pretty much data lines getting hooked high or low levels while power lines not getting any supplies.

I’ve designed the program that it’ll detect the chip if you hook it up right and immediately program the chip (takes only a second), so you don’t have to hold the jumper for too long to worry about unstable contacts.


# This program detects rewrite the toner chips to "full" for a Ricoh SP C250SF/DN Printer using Raspberry PI (defaults to BCM2835 models such as Raspberry PI Zero)

# The chip data is in file named "black" "cyan" "magenta" and "yellow".
# The pad closest to the edge is GND (-> Pin 9), followed by VCC (-> Pin 1) , DATA (-> Pin 3), and Clock (-> Pin 5).

# Be sure i2c is enabled and installed (it's turned off by default) on Raspbian

# This line is disabled because it takes too long to unregister i2c_bcm2835 to start from a clean slate
# modprobe -r i2c_bcm2835

# This program needs sudo permissios as it involves direct access to hardware
if [[ $EUID -ne 0 ]]; then
  echo "Need root permissions as this code involves direct hardware access. Try sudo"
  exit 2

# Sets the baud rate
modprobe i2c_bcm2835 baudrate=400000

# Create I2C address to color map
COLORS=( [50]="yellow" [51]="magenta" [52]="cyan" [53]="black" )
# Detect chip I2C address
I2C_address=$( sudo i2cdetect -y 1 | grep 50 | sed -e 's/50: //;s/-- //g' )
# Keep the 0x5* address lines since only 0x50~0x53 is valid. Strip the 50: header, discard all "--" entries, and you are left with the detected address

# LED flash function
function turn_off {
  echo 0 > ${target_device}
function turn_on {
  echo 1 > ${target_device}
function toggle {
  # Reads 0 for OFF and 255 for ON
  readin=$(cat ${target_device})
  # Condition 255 to 1 to reduce platform dependency
  # Not equal to 1 is XOR (flip)
  flip=$[$state != 1]
  # Display output
  echo $flip > ${target_device}

function flash_once {

  sleep $period

  sleep $period

function flash {
  for((i=1; i<=times; i++)); do
    flash_once $period

if [ -v COLORS[I2C_address] ]; then
  # Meat
  echo "Detected toner chip for color: $color"

  echo "Short flashes before starting. Long flashes after done. Ctrl+C if you want to terminate get back to prompt earlier during long flashes."
  flash 5 0.1

   # "address" counter sync up with the hex code index in file
   printf "Writing"
   for i in $(cat $datafile); do
     i2cset -y 1 ${HEX_I2C_address} $address $i;
     address=$(($address +1));
     printf .
     flash 1 0
   echo "Done!"
  flash 10 0.5
  echo "Invalid I2C address for SP C250DN/SF toner chips. ${I2C_address}"
  exit 4

Turns out Linux’s dot sourcing (bash calling) syntax is relative to your current folder, not the scripts’ folder. So if you have one script calling another in the same folder when you are not currently at the same folder as the script (e.g. startup automation), the relative paths will be wrong. So instead of ./ , you should use this prefix instead:


I chose to flash the board’s only LED light quickly before starting and blink slowly a few times after it’s done for visual clues. It’s entirely optional. Here’s the guts of the code without the fancy indicators:


# Sets the baud rate
modprobe i2c_bcm2835 baudrate=400000

# Create I2C address to color map
COLORS=( [50]="yellow" [51]="magenta" [52]="cyan" [53]="black" )

# Detect chip I2C address
I2C_address=$( sudo i2cdetect -y 1 | grep 50 | sed -e 's/50: //;s/-- //g' )

if [ -v COLORS[I2C_address] ]; then
  # Meat

  # "address" counter sync up with the hex code index in file
  for i in $(cat ${color}); do
    i2cset -y 1 ${HEX_I2C_address} $address $i;
    address=$(($address +1));
  echo "Invalid I2C address for SP C250DN/SF toner chips: ${I2C_address}"

Download the package. Run program_toner

Just in case if people are wondering. The L01 chip’s datasheet is here:

To make a dedicated Raspberry Pi Zero device to run this script (program_toner) repeatedly on boot, I made a script loop_program_toner that runs the same program as infinite loop. I added 5 seconds of slow flashes after a successful write. To prevent the program from retrying too fast (sucking up computing resources) when there’s nothing connected to it, I wait for 1 second between each retries.


if [[ $EUID -ne 0 ]]; then
  echo "Needs root permissions. Try sudo"
  exit -2

while :
  bash "${BASH_SOURCE%/*}/program_toner"
  if (($?));
    sleep 1;

There are a few steps needs to auto make this automated:

  1. Have a user account dedicate to autorun the program on boot. In the example here, we’ll use Raspberry Pi’s default admin account
  2. Auto-logged on by running sudo raspi-config, select (1) System Option > (S5) Boot / Auto Login > (B2) Console Autologin
  3. Bless the said account with password free sudo rights: Run sudo visudo and add this to the end (replace admin with something else if you chose a different username)

  1. Call loop_program_toner in ./profile (startup script). Can place this at the bottom if your package folder is ~/spc250dn :
sudo ./spc250dn/loop_program_toner

The path can start with . or ~ because you are starting with at home folder anyway.

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Evoluent Vertical Mouse 4 Cable Mod

The mouse cable for Evoluent Vertical Mouse 4 is extremely long, which creates a lot of clutters especially when my keyboard has a USB hub relay built in (it’s the mouse is less than a feet away from it). Instead of splicing the cable, which creates a hard junction that’s not flexible, I modified the mouse to take a micro-USB cable instead.

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AC Coupling (removing DC drift) 3-Amp INA Instrumentation Amplifier (DC restoration)

While clearing out my old data, I came across the teaching materials I’ve helped rewritten as a teaching assistant of Biomedical Electronics Lab (Stanford EE122A).

It’s a generic concept in electronics that often used in EKG/ECG circuits to remove the baseline drift on the fly so the analog signal won’t drift off the rails (exceed the dynamic range limited by the op-amps) before the post-processing filter (whether it’s analog or digital) kicks in to remove the DC component.

This concept is called ‘DC restoration’, which is often not taught in standard electronics textbooks. Instead it’s detailed in one of the instrumentation amp (INA) Burr-Brown (now TI) application notes.

It’s a slick trick but the rationale wasn’t very well explained even in the application note itself. It was presented as a feedback design but it doesn’t tell you intuitively what was fed back and why INA chips, and why the reference pin is the right injection point.

Most textbooks don’t even teach the existence of the reference pin (they always short the reference to the ground without explaining). Application notes talk about the REF pin, but they often jump too quickly into cookbook recipes and equations (likely because customers just want quick answers) so they never tell you the thought process.

This blog post shares my intuition of DC-restoration that’s exposed to EE122A students after I’ve updated the lab document. Hope electronic hobbyists and industry people will find it useful.

Before we get to DC restoration, we must see that the purpose of a 3-amp INA is basically a non-inverting buffer stage (primarily done to increase input impedance) followed by a single difference amp (output) stage.

Non-inverting configuration has higher input impedance as the input goes directly to the high impedance non-inverting (+) input pin without taking material current from the input (loading), so there’s no good reason to lose this property by doing inverting op-amp configuration twice.

Most practical INA chips assign some of the user-adjustable gains at the buffer stage because ‘mirrored-ground’ (superposition) allows one resistor to program the gains for the 2 buffer amps without adding more mismatch, but conceptually the first stage’s primary purpose is a buffer. The rest of the gains can be hard-coded by manufacturing with matched ‘resistors’ inside the IC, mostly at the output difference amp stage.

But for illustration (so not to drown the readers in math), let’s assume the design choice of assigning the gains at the input buffer stage and make the output stage a unity gain difference amplifier (V_pV_n), which I use small letters (n, p) to denote the outputs of the buffer stage internal to the INA chip.

The REF pin (the part of R_4 exposed in Figure 6 above) is often advertised as an offset adjustment pin. This is just one of the many uses if you really understand the idea behind the 1-amp difference amplifier configuration.

The slightly more shallow perspective (more modular or system perspective that I’ve shared with EE122A students) is that if you look at an standard 3-amp INA configuration in regular textbooks like this,

Understanding CMR and instrumentation amplifiers - EDN

the entire sub-circuit (3 amp INA) is floating so it has absolutely no idea what the reference (‘ground’) would have been if we did not ARBITRARILY define it through R_4 by tying it to the CM ground, by forcing REF pin (V_{REF}) to be 0V (relative to the common mode ground shared with the inputs)!

In other words, we now have an INA ‘ground’ and a Common Mode (CM) ground, which they do not have to be the same unless we force them to be equal by shorting the REF pin to the CM ground.

This means whatever voltage V_{REF} we set the REF pin to be, it’s the baseline of the system (amplifier) and the whole output shifts moves up and down by whatever V_{REF} relative to common mode ground we are feeding into the REF pin for the moment.

The DC restoration takes advantage of the user-definable baseline (INA ‘ground’) by extracting a low-frequency (drift) portion of the output signal V_o with an INVERTING low-pass filter (LPF) with frequency response L(\omega), and re-define it as the INA’s ‘ground’ level. This is the LPF:

e.g. if the signal’s baseline drifted up by 1V, a -1V is generated by the inverting LPF and the INA ‘ground’ respond by moving from 0V down to -1V, which pulls the entire signal down by 1V, cancelling the 1V increase in baseline. All the voltages used here are relative to the common mode ground.

As with any AC coupling circuit, there is no precise definition of what ‘DC’ or ‘baseline’ is. It’s up to the experimenter to consider what cutoff frequency in the LPF is close enough. Technically DSP engineers can call a running window of trimmed-median the baseline if they wanted to.

The feedback (how fast the INA ‘ground’ is readjusted) is as responsive as the phase delay introduced by the LPF’s time constant. If you only consider anything below 0.00001Hz to be DC, you have to pay a price for the long delay catching up to the changes which might or might not be considered a baseline drift (it’s an application specific context).

I also have an alternate view of DC restoration which does not use the concept of INA ‘ground’ (not taught in EE122A). This is based on seeing the final stage op-amp (1-amp INA) not just as a simple difference amplifier, but as a 3-input arithmetic circuit (summing and subtractions) through super-position (setting one input to 0V at a time and add the results up).

This is the gut of a 1-amp INA difference amplifier

We can break it down into a (1 input) inverting amplifier plus a summing (2 inputs) non-inverting amplifier.

The equations for inverting amplifier and non-inverting amplifiers are not symmetric! The core part of the feedback gain in EITHER CONFIGURATION are ALWAYS set at the feedback branch which ONLY goes to the inverting input (-), aka R_1 and R_2!

  1. Inverting amplifier portion do not care about the resistors at the non-inverting input (+), but
  2. Non-inverting amplifier portion’s gain is determined by the 2 resistors at the inverting input (-)! The resistors at the non-inverting inputs (+) never boost the amplifier gains! They only attenuate signal from external sources (like voltage dividers). The gain boost happens ONLY at the inverting branch!

Why? By superposition (short out other inputs you are not considering)

  1. Inputs to R_3 (V_p) and R_4 (V_{REF}) shorted to the common ground gives an inverting amplifier. They don’t matter to V_n. The gain to V_n is -\frac{R_2}{R_1}.
  2. Input to R_1 (V_n) shorted to the common ground gives a non-inverting amplifier, which the gain boost (1+\frac{R_2}{R_1}) is relative to voltage V_+ showing up at the non-inverting input (+), which is the result of attenuating V_p through R_3 and V_{REF} through R_4.

The output contributions, if R_1=R_2 and R_3=R_4,

  1. [Inverting amplifier gain of -1 relative to V_n] contributes -V_n to the output
  2. [Non-inverting amplifier gain of 2 relative to V_+] if R_4 is set to ground through V_{REF}, R_3 and R_4 forms a 1:1 potential divider which halves V_p to give \frac{V_p}{2} at V_+. Doubling (2x gain) the halved input gives an overall gain of 1, therefore contributing V_p to the output

So the overall output equation is V_p - V_n if V_{REF} is grounded to 0V.

The intuition for DC restoration is to untie the REF pin (going to R_4) from CM ground and treat it as equals to V_p pin (going through R_3, so instead of a potential divider, they form a non-inverting summing amplifier:–r1-q34558104

So the DC restoration circuit can be seen as a 3-input arithmetic amplifier that gives the equation V_{REF}+V_p-V_n and you can subtract the baseline by setting V_{REF} to be whatever baseline your inverting LPF feedback branch judged. The overall AC-coupled system response is \frac{1}{1+L(\omega)}.

Note that ALL 3 inputs (V_p, V_n, V_{REF}) should be driven from sources with low output impedance. V_p and V_n is the outputs of a buffer op-amp so they already have good low impedance outputs feeding to the last stage. We’ll need to do the same for V_{REF} by using an op-amp to lower the output resistance whether it is an active low pass filter or active potential divider, because V_+ do not see V_p differently from V_{REF}. Noise showing up from high resistance output driving REF pin do not simultaneously appear on V_n, so it’s not canceled and therefore it’s worsening the common mode rejection (CMR).

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XFX TS Series 550W Power Supply – Made In China – Bulging Capacitor because it was installed backwards

I opened up my ATX Power Supply as I had it for quite a few years but it has been stowed away and used intermittently until I use it a lot more in my office computer in recent years. I just don’t trust any power supplies Made in China, even from a reputable brand as a couple of decades of working with computers tells me that they are bound to break after a few years, and very often it is the capacitor that rotted and the rest are collateral damages. Lo and behold there is one:

After I took the capacitor out, I noticed something odd: the polarity marker on the circuit board is the reverse of how the capacitor was installed! Holy smokes! I just want to verify if the PCB markings is right or the installer was right, so I installed wires to the capacitor to lift it up so I can connect the multimeter leads across it to measure the voltage polarity. This picture also shows the PCB’s capacitor orientation marking:

And the multimeter reads -5V following the original orientation of the capacitor before I took it out. This means the polarity was reversed!! No wonder the capacitor bulged. I was lucky that it didn’t blow up after a few years of use! Probably it was rated 16V yet only -5V was passed to it so the electrolytic capacitor rotted slowly.

To give XFX Force credit, they didn’t slap the power supply together with the cheapest white label components from the gutters. It uses proper Nichicon and Hitachi capacitor, so it might be the reason that reversed capacitor lasted so many years.

It’s the workmanship in China. If you go with a Red Chinese (Yellow-Soviets) brand, they might use junk components, but don’t think you are safe with foreign companies that has a solid process and design. The cheap labor in China who doesn’t give a crap can still manage to fuck it up. So trust nothing


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HP 54502A Datasheet typo about AC coupling

The cutoff frequency of 10Hz on the datasheet is a typo. Better scopes at the time claims 90Hz. 10Hz is just too good to be true.

Found the specs from the service manual:

Don’t be fooled by the -3dB cutoff and ignore how wide the transition band can be (depends on the filter type and the order). Turns out this model has a very primitive filter that AC couple mode still messes square waves below 3kHz up despite the specs says the -3dB is at 90Hz. You better have a 30+ fold guard band for old scopes!

Remember square wave pulse train in time domain is basically a sinc pulse centered at every impulse of the impulse train in frequency domain superimposed. Unless you have a tiny duty cycle (which is not the case for uniform square waves, they are 50%), the left hand side of the sinc function at 1kHz fundamental still have sub-1kHz components that can be truncated by the AC coupling (high pass filter).

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