ER-301/Teletype Connection Rev7
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.
Connecting the Teletype to the ER-301
In a nutshell, i2c requires 2 communication lines (SCL and SDA) plus 1 ground line. So there are 3 pins on the Teletype that need to be connected with a custom cable to 3 pins on the ER-301.
There are 5 pins extending parallel from underneath the ER-301 CPU board. You will only be using the top 3. You can also see that the 5 pins are labeled on the ER-301 silkscreen as TX, RX, GND, GND, and 5V. Connect like this:
- Teletype GND to ER-301 GND.
- Teletype SCL to ER-301 TX.
- Teletype SDA to ER-301 RX.
Required Hardware Modification
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.
- 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:
Step 2: Bend it around some needle-nose pliers:
Step 3: You should have a nice boxy U-shape like this:
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:
Step 5: And solder like this: