ER-301/Teletype Connection Rev7

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The first few runs of the ER-301 did not have a dedicated i2c header. Instead users would perform a small hardware modification (described here) so that they could use the exposed UART header instead.

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If your ER-301 board revision is revision 10 or later then this modification is NOT necessary.


The goal of this hardware modification is to short out the diode labeled D3 on the red CPU board on the back of the ER-301. The diode's original purpose was to provide level-shifting for the RX pin of the UART port. Shorting out this diode allows for this pin to be used for the SCL (clock) pin of i2c.

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It is not necessary to remove the CPU board in order to perform this modification.


Required tools:

  • Soldering iron (ESD-safe, reliable temperature control) and solder (recommend no-clean flux)
  • Short length of solid core wire (or waste lead from a through-hole resistor or LED or similar)
  • needle-nose pliers (optional)

Step 1: Cut a 7mm piece of solid core wire or use part of a lead from a resistor or LED:
1046491ead3f3fa1920923b12850da2e5fef64ed_1_690x450.jpg

Step 2: Bend it around some needle-nose pliers:
14892e8891d2d0dd8dfdbf746645a1bccc26c3ba_1_531x499.jpg

Step 3: You should have a nice boxy U-shape like this:
eeeea02955370ae9d6eab3024646a3a870252045_1_690x398.jpg

Step 4: Place around diode D3 like this with the bottom of the U oriented towards the bottom of the board so that we do not short resistor R62:
b8b5fbaf21b5454184a83681f3aa02a05087df3e_1_690x397.jpg

Step 5: And solder like this:
0a3722da3150d3243fa63971b0b5cedc7da948cf.png

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Do not use a solder sucker on this board. If you need to remove excess solder, please use copper braid.