have the ability to accept optional external input. Programmable keypad There were no LED displays or ways to display program instructions, beyond actually running the program, which was done by pressing "GO". Each command inputted will be added next to the previous command inputted, allowing the operator to build a list of commands up to 16 commands. All programming to BigTrak was done through the keypad shown here: Forward/Backwards: Move forward or backwards in units of body length Left/Right: Turn left or right in units of roughly 1/60th of a full rotation HOLD: Pause in 1/10 of second time units (UK version; P: Pause) FIRE: Fire the light bulb "laser" (UK; Photon Symbol) CLR: Clear the program (UK; CM: Clear Memory) CLS: Clear Last Step (UK; CE: Clear Entry) RPT: Repeat a number of steps (primitive loop) (UK; x2: Repeat key) TEST: Run short test program CK: Check last instruction (UK; Tick symbol) Out: Dump optional trailer accessory In: Reserved for future expansion (UK; missing. Disabled or not implemented on most if not all BigTraks) In and Out commands The Bigtrak is capable of communicating with an optional accessory. It uses an electrical 3.5 mm jack for its communication. Older bigger Bigtrak can also use a Bigtrak Jr optional accessory with an adaptor. However the newer Bigtrak XTR uses a different type of communication port, therefore it is not compatible with any previous Bigtrak accessory. The Out command is used in the 1970s/1980s Bigtrack to dump the cargo on its optional cargo trailer accessory. While in the modern 2010s Bigtrack, it is used to do things such as shooting missiles. However, the hardware on the late 1970s and early 1980s Bigtraks does not seem to be capable of processing any In command. Computer control In 1981 Steve Ciarcia published in his BYTE column "Ciarcia's Circuit Cellar" an article on controlling Big Trak with a personal computer and radio interface, turning it into a home robot. Meanwhile, in the 2000s and 2010s, many people have hacked the Bigtrak by allowing it to be controlled with modern microcontrollers such as the Arduino, or even a single board Personal Computer such as the Raspberry Pi. The Bigtrak XTR is capable of being fitted with a communication module to wirelessly communicate with a computer. Big Trak in psychological research In the early 1980s, the psychology of science community led by the laboratory of David