Editing PG31 GPS RS232 Dev Board

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C1 and C3 are decoupling caps that help clean up the output transcients and noise.
 
C1 and C3 are decoupling caps that help clean up the output transcients and noise.
  
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J2 is just a two pin polarized header so that you can either 'steal' 3.3V from the board to power other boards, or have other boards power this board without the need for the external wall wart.
 
J2 is just a two pin polarized header so that you can either 'steal' 3.3V from the board to power other boards, or have other boards power this board without the need for the external wall wart.
  
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[[Image:PG31-SerialConnection.jpg|PG31 Serial Connection]]
 
[[Image:PG31-SerialConnection.jpg|PG31 Serial Connection]]
  
This shows the DB9 connector (also known as a D-Sub 9-pin connector), serial cable, and MAX3232 IC and charge pump caps (all SMD, again, can be done with PTH components). Here is the schematic:
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This shows the DB9 connector (also known as a D-Sub 9-pin connector), serial cable, and MAX3232 IC and charge pump caps (all SMD, again, can be done with PTH c
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omponents). Here is the schematic:
  
 
[[Image:PG31-SerialConnection-Schematic.jpg]]
 
[[Image:PG31-SerialConnection-Schematic.jpg]]
  
Now read-up on the MAX232 IC. It could be the most common IC out there, at least for the embdded world. This IC will have many different model names (ICL232 for example) but all do the same thing, and will most likely have identical pin-outs. This IC is used to take low voltage signals from your micro or PIC or ARM or whatever, and converts those low-voltage signals to higher-voltage signals that the computer can understand. The voltage levels on TTL are also inverted for RS232 so the MAX232 takes care of that as well!
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Now read-up on the MAX232 IC. It could be the most common IC out there, at least for the embedded world. This IC will have many different model names (ICL232 for example) but all do the same thing, and will most likely have identical pin-outs. This IC is used to take low voltage signals from your micro or PIC or ARM or whatever, and converts those low-voltage signals to higher-voltage signals that the computer can understand. The voltage levels on TTL are also inverted for RS232 so the MAX232 takes care of that as well!
  
The MAX232 takes in TTL signals and sends out RS232 signals. It also takes the RS232 signals from the computer and outputs TTL signals to whatever microcontroller you are using. How does it do this magic? With charge pump capacitors. You'll notice four 0.1uF caps attached to the MAX3232. If you're reading closely, you'll also notice we have a MAX3232 not the MAX232. Difference:
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The MAX232 takes in TTL signals and sends out RS232 signals. It also takes the RS232 signals from the computer and outputs TTL signals  
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to whatever microcontroller you are using. How does it do this magic? With charge pump capacitors. You'll notice four 0.1uF caps attached to the MAX3232. If you're reading closely, yo
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you'll also notice we have a MAX3232 not the MAX232. Difference:
 
*MAX232 must be powered with 5V and converts 5V TTL<->RS232
 
*MAX232 must be powered with 5V and converts 5V TTL<->RS232
 
*MAX3232 can be powered from 3V to 5V and converts it to RS232
 
*MAX3232 can be powered from 3V to 5V and converts it to RS232
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So in this application, the GPS unit is running at 3.3V. If we were to send 5V TTL to it, it would be damaged! So we must use the lower voltage MAX3232 part. No big deal, it's functionally identical to the MAX232. From here on out, I will only refer to the MAX232.
 
So in this application, the GPS unit is running at 3.3V. If we were to send 5V TTL to it, it would be damaged! So we must use the lower voltage MAX3232 part. No big deal, it's functionally identical to the MAX232. From here on out, I will only refer to the MAX232.
  
When you first view the MAX232 component, it can be a bit confusing. The pins label T1/T2 are the transmit channels (MAX232 has 2 TX channels and 2 RX channels). T1In/T2In will take *in* TTL and send *out* RS232 out T1Out/T2Out. So for our purposes, we need to connect the GPS unit's serial TX pin to the T1In pin of the MAX232. So when the GPS unit outputs some 3.3V TTL signals, these signals will be converted into RS232 and sent to the computer (TX1Out is connected to pin 2 on the DB9 connector).
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When you first view the MAX232 component, it can be a bit confusing. The pins label T1/T2 are the transmit channels (MAX232  
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has 2 TX channels and 2 RX channels). T1In/T2In will take *in* TTL and send *out* RS232 out T1Out/T2Out. So for our purposes, we need to connect the GPS unit's serial TX pin to the T1In pin of the MAX232. So when the GPS unit outputs some 3.3V TTL signals, these signals will be converted into RS232 and sent to the computer (TX1Out is connected to pin 2 on the DB9 connector).
  
 
Finally, when the computer needs to send info to the GPS unit, it will send RS232 signals down the serial cable, on pin 3 of the serial cable/DB9 connector to R1In of the MAX232. This RS232 signal will be converter to TTL and sent out the R1Out pin. This TTL signal is connected to the RX pin on the GPS unit. Voila.
 
Finally, when the computer needs to send info to the GPS unit, it will send RS232 signals down the serial cable, on pin 3 of the serial cable/DB9 connector to R1In of the MAX232. This RS232 signal will be converter to TTL and sent out the R1Out pin. This TTL signal is connected to the RX pin on the GPS unit. Voila.
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[[Image:PG31-Layout.jpg|300px]]
 
[[Image:PG31-Layout.jpg|300px]]
  
You can see the PCB layout is pretty straight forward. The 'solder jumpers' are a feature that SFE came up with to minimize our manufacturing time while allowing end users to populate the physical jumpers if they needed the ability to connect/disconnect a feature often and quickly. Most users just plug the PG31 onto their computer for quick evaluation. At some point, the MAX232 may need to be disconnected from the circuit so that an external micro or embedded system could be attached. If this is done back and forth often, the end user can clear the solder jumper with a swipe of the soldering iron, install a 2-pin header and a black plastic jumper. This now allows them to jumper/dejumper quickly and keeps us from having to solder the 2-pin headers into a board that may not need them.
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You can see the PCB layout is pretty straight forward. The 'solder jumpers' are a feature that SFE came up with to minimize our manufacturing time while allowing end users to populate the physical jumpers if they needed the ability to connect/disconnect a feature often and quickly. Most users just plug the PG31 onto their comput
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er for quick evaluation. At some point, the MAX232 may need to be disconnected from the circuit so that an external micro or embedded system could be attached. If this is done back and forth often, the end user can clear the solder jumper with a swipe of the soldering iron, install a 2-pin header and a black plastic jumper. This now allows them to jumper/dejumper quickly and keeps us from having to solder the 2-pin headers into a board that may not need them.
  
 
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===Things to Improve===
 
===Things to Improve===
So that's pretty much it. The board works well, and assuming Laipac doesn't change their parts again, we will produce the board for some time.
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So that's pretty much it. The board works well, and  
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assuming Laipac doesn't change their parts again, we will produce the board for some time.
  
 
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