ULN Modifications for Serial Comms, NPN Inputs, and POT/RCTIME Functions

[JonnyMac]

EFX-TEK accessories use OT (open true) communications which means that the serial line is pulled high (through a resistor) and "start" and "0" bits are created by pulling the line low.  The ULN2x03 high current driver takes an active-high input, so it can in fact put a load on any pin used for serial communications that interferes with the messages being set by the Prop-1/Prop-2/Prop-SX and the external device (e.g., RC-4).

We recommend using P7 (P15 on Prop-2 and Prop-SX) for serial communications as this allows the simple replacement of the ULN2803 with the 7-channel ULN as shown on the right side of this illustration:



CRITICAL NOTE: When installing the ULN2003 it is bottom-aligned in the socket.

If you don't have a ULN2003 you can remove pin 1 of the ULN2803 to accomplish the same thing.  On the left side of the illustration you can see the relationship of the ULN2803 to the Prop-1 I/O pins.  Use this reference if you have more than one serial channel.  For example, I have a prop that uses the Prop-1 with the Rogue uMP3 player, and the uMP3 has two serial pins.  In my configuration I do this:

P7 - Serial in from uMP3/VMUSIC  (remove ULN pin)
P6 - Serial out to uMP3/VMSUIC (remove ULN pin)
P5 - Serial to EFX-TEK devices (remove ULN pin)
P4 - Trigger (leave ULN intact)

So, I remove pins 1 - 3 (for P7 - P5) from the ULN2803 to prevent it from interfering with the EFX-TEK boards and the uMP3. 

Tip: ULN is pretty inexpensive (especially if you buy them in bulk from big electronics vendors like Mouser) and it protects the Prop-1 from the outside would; this is the reason we put it into a socket: if it goes bad you can change it.  Since it can be changed, don't fear the modifications above as you can replace the ULN should you need to reconfigure your prop.

The ULN circuitry will also interfere with the Prop-1 POT function and the Prop-2/SX RCTIME function -- remove pins as required for POT and RCTIME.

When using active-low (open-collector) sensors with the Prop-1 the associated trigger pin (usually P6) must have the ULN pin clipped and the SETUP jumper moved to the UP position.