Pololu TReX - Manual (Page 16)

Unlike with RC signals, the TReX has no way of telling if a channel is receiving a valid analog signal. A disconnected
channel will still produce a result when run through the analog-to-digital converter. This poses two potential
problems:
1. If your analog signal source gets disconnected from your TReX, the TReX could drive the motors in
unexpected and potentially dangerous ways. Because of this, please be very careful when operating the TReX in
analog mode.
2. Unused channels can impact behavior of the TReX in undesirable ways. You may not plan on using the flip
or override channels, but leaving them disconnected could result in their floating to a value that would enable
flipped mode or serial override. One solution to this problem is to use the “ignored channels” configuration
parameter to set unused analog channels as ignored. The TReX considers ignored channels to be fixed at their
neutral positions. An alternate, purely hardware-based solution would be to short any unused motor channels to
a used one and to tie any unused “enable” channels (4 or 5) to Vcc (the 5-volt line). By shorting unused motor
channels to a used one, you should be able to get past the safe-start portion of the TReX’s start-up routine.
4.e. Safe-Start Mode
When the TReX starts up in either RC or analog mode, it first enters safe-start mode. In this mode, the three motors
are shut down and are unaffected by the channel inputs. The TReX lets you know its in this mode by rapidly flashing
its green status LED. To exit this mode, the following conditions must all be simultaneously met:
1. All required channels must have a valid signal. This only applies when the TReX is in RC mode since it’s
not possible to have an invalid analog signal. Required channels are determined by the “required channels”
configuration parameter. Only channel one is required by default.
2. All motor channels must be close to their neutral values (i.e. they must not be instructing the motors to move
very quickly). Channel 5 must be disabling serial override. The TReX will not allow you to start with serial
controlling the motors if you’re in RC or analog mode.
If you have floating channels while running in analog mode, it may be impossible for you to satisfy the safe-start
conditions. Bullet point #2 in the analog section (
Section 4.d ) provides several ways to fix this problem.
Once you have satisfied the safe-start conditions, the green status LED should stop flashing and turn solidly on. This
indicates that the channel inputs are now in control of the motors.
Pololu TReX User's Guide © 2001–2013 Pololu Corporation
4. RC/Analog in Detail Page 16 of 24