4 ms·

I want to know how when you see a stack of pager, each one knows its position in the stack so that when the stack decides to go into "attract" mode, it animates

I want to know how when you see a stack of pager, each one knows its position in the stack so that when the stack decides to go into "attract" mode, it animates them one by one, like an elevator going up. Somehow when they stack, they know where they are relative to the other pagers (at least something does).

jrussino 2y ago
If I recall, they have female port connectors on the bottom and little male prong connectors that mate with them on top. I assume that is mainly for power connection, so you can put a whole stack on a charger and charge them all simultaneously. But if they can sense a signal on the female pins then that's sufficient to enable the behavior you're describing without directly knowing the positions of each pager, right? Pager on the bottom sees a signal on its female pins, blinks its light, send the signal on its male pins, then the next pager above sees the signal on its female pins, blinks its light, sends the signal on its male pins, etc...
- Cerium 2y ago
  I think that is the whole trick - they look like they are all in parallel on four wires, but really two of the wires are daisy chained through each device (like serial LEDs). In this configuration you can do the light tricks without knowing the order, but you can also enumerate the order. I have seen addressing based on hop counting, or by appending your ID to "who are you" message. Eg: Host sends "who are you" first device appends an ID number and then propagates the message forward in the chain. Each device does the same, until the last device in the chain (which detects electrically or via a timeout that there are no more devices) decides to loop the message back to the host.
ssl-3 2y ago
Looking at photos on Google Images, it looks like the pagers in question have just two pins on the top, and two more mating pins on the bottom. Obviously, that's for power delivery: They stack to charge. But it seems like data must be sent (in one direction) through the stack at the same time that charging happens -- to set IDs, and to let attract mode work. One way I can conceive of to allow this to happen is by reversing polarity: Send power with one polarity for a 0, and in the opposite polarity for a 1. Once a pager has some data, it can pass it along to the one stacked above it by doing the same thing -- in a manner not dissimilar to how things like RGB LED strings do. Charging can happen at the same time as this data is received (and potentially modified) and then retransmitted by using a bridge rectifier in each unit for the charging circuit, so that (as far as the battery bits can see) the polarity remains constant. (This sounds complex and slow, but using things like diodes and MOSFETs is cheap at mass production PCB stuffing scales, and the data rate needn't be fast. It may even be cheaper to do it this way than a more complex assembly with dedicated data pins.)
- kragen 2y ago
  this is a great idea. you don't even need a bipolarity driver, just an h-bridge. though if you do have plus and minus rails plus ground, you only need two transistors
  - ssl-3 2y ago
    How do you mean? There are only two contacts between any device and the device below it, according to the photos. This trend continues upward for all subsequently-connected devices. An interface with +, -, and ground implies three contacts.
    - kragen 2y ago
      i mean you connect one wire to ground and switch the other wire between - and + with two transistors, thus reversing polarity on it in a way that can be used to signal information while also efficiently transferring power (though only a quarter of the power you could transfer with an h-bridge across rails of the same voltage) basically rs232 signaling with just tx and ground
      - ssl-3 2y ago
        I like it. That's related to, but more elegant than, my off-the-cuff idea was.
        kragen 2y ago
        i think that to the receiving device it looks exactly the same as the h-bridge approach which i think was your initial version, unless it has some other path to ground, maybe a capacitive one. but if it does have such a path, it's probably useful for it to have such a genuine ground pin
- fragmede 2y ago
  Or you could have a fixed clock and sample based on that, so no having to deal with reverse polarity. This is the restaurant industry though, so assume they'll get wet and the the worst possible combination of pins get shorted, so design the circuit to be as tolerant of that as possible. Also consider the 4 pins getting into conductive fluid electrified with AC.
yencabulator 2y ago
LED strips have 3 pins (power, data, ground) but if you don't care about exact power levels maybe you can modulate power to send data. Protocol-wise, WS2812B for LEDs works basically by having every LED along the way take the first X bytes, interpret as a command to themselves, and forward the rest -- and that's how you can address a 100 LEDs on one data line.

jrussino 2y ago

If I recall, they have female port connectors on the bottom and little male prong connectors that mate with them on top. I assume that is mainly for power connection, so you can put a whole stack on a charger and charge them all simultaneously. But if they can sense a signal on the female pins then that's sufficient to enable the behavior you're describing without directly knowing the positions of each pager, right? Pager on the bottom sees a signal on its female pins, blinks its light, send the signal on its male pins, then the next pager above sees the signal on its female pins, blinks its light, sends the signal on its male pins, etc...

Cerium 2y ago
I think that is the whole trick - they look like they are all in parallel on four wires, but really two of the wires are daisy chained through each device (like serial LEDs). In this configuration you can do the light tricks without knowing the order, but you can also enumerate the order. I have seen addressing based on hop counting, or by appending your ID to "who are you" message. Eg: Host sends "who are you" first device appends an ID number and then propagates the message forward in the chain. Each device does the same, until the last device in the chain (which detects electrically or via a timeout that there are no more devices) decides to loop the message back to the host.

ssl-3 2y ago

Looking at photos on Google Images, it looks like the pagers in question have just two pins on the top, and two more mating pins on the bottom. Obviously, that's for power delivery: They stack to charge. But it seems like data must be sent (in one direction) through the stack at the same time that charging happens -- to set IDs, and to let attract mode work. One way I can conceive of to allow this to happen is by reversing polarity: Send power with one polarity for a 0, and in the opposite polarity for a 1. Once a pager has some data, it can pass it along to the one stacked above it by doing the same thing -- in a manner not dissimilar to how things like RGB LED strings do. Charging can happen at the same time as this data is received (and potentially modified) and then retransmitted by using a bridge rectifier in each unit for the charging circuit, so that (as far as the battery bits can see) the polarity remains constant. (This sounds complex and slow, but using things like diodes and MOSFETs is cheap at mass production PCB stuffing scales, and the data rate needn't be fast. It may even be cheaper to do it this way than a more complex assembly with dedicated data pins.)

kragen 2y ago
this is a great idea. you don't even need a bipolarity driver, just an h-bridge. though if you do have plus and minus rails plus ground, you only need two transistors
- ssl-3 2y ago
  How do you mean? There are only two contacts between any device and the device below it, according to the photos. This trend continues upward for all subsequently-connected devices. An interface with +, -, and ground implies three contacts.
  - kragen 2y ago
    i mean you connect one wire to ground and switch the other wire between - and + with two transistors, thus reversing polarity on it in a way that can be used to signal information while also efficiently transferring power (though only a quarter of the power you could transfer with an h-bridge across rails of the same voltage) basically rs232 signaling with just tx and ground
    - ssl-3 2y ago
      I like it. That's related to, but more elegant than, my off-the-cuff idea was.
      - kragen 2y ago
        i think that to the receiving device it looks exactly the same as the h-bridge approach which i think was your initial version, unless it has some other path to ground, maybe a capacitive one. but if it does have such a path, it's probably useful for it to have such a genuine ground pin
fragmede 2y ago
Or you could have a fixed clock and sample based on that, so no having to deal with reverse polarity. This is the restaurant industry though, so assume they'll get wet and the the worst possible combination of pins get shorted, so design the circuit to be as tolerant of that as possible. Also consider the 4 pins getting into conductive fluid electrified with AC.

yencabulator 2y ago

LED strips have 3 pins (power, data, ground) but if you don't care about exact power levels maybe you can modulate power to send data. Protocol-wise, WS2812B for LEDs works basically by having every LED along the way take the first X bytes, interpret as a command to themselves, and forward the rest -- and that's how you can address a 100 LEDs on one data line.