Big Trak Accessory Commands

There are two commands on the Big Trak keyboard that control accessories. The OUT command is documented and functional. But, as you'll soon see, we have to explore the inner secrets of the Big Trak to discover the possible use of the IN command.

The Out Command

The Big Trak has an optional trailer accessory (purchased separately for $13 USD 1979) to cart around small objects behind the Big Trak. When the Big Trak reaches the destination appointed by the user, the OUT command can dump the cargo.

Big Trak Transport trailer.

Big Trak Transport trailer.

Just like the Big Trak, the Transport trailer is simple, yet ingenious. The Transport consists of:

The dump mechanism can be demonstrated by placing an electrically conductive material (paper clip) across the two plug contacts furthest from the plug tip. This completes the motor connection to the battery, causing the motor to turn the worm-driven gearbox. A crank mechanism connected to the final gear dumps and levels the trailer bed over and over again.

When the plug is inserted into the Big Trak expansion plug port and the OUT command is given, one output from the Big Trak’s SN75494 connects the trailer motor wire to ground, just like the paperclip did. (The ground wire from the 1.5 V battery joins the ground wire from the Big Trak’s 9 V battery and 'D' cells). Thus, the Big Trak can control the dumping of the Big Trak Trailer.

The OUT command only powers the trailer motor for a preset amount of time (3 to 4 seconds). The Big Trak Transport trailer plug doesn’t purposely provide electrical feedback as to the angle of the trailer bed, which would have allowed the TMS 1000 microcontroller to detect when to shut off the trailer bed motor.

Rich Harman wrote to me and explained how the trailer is able to return to a level position by itself. A mechanical switch is built into the side of the trailer gearbox. When the trailer is not at a level position, a spring attached to the side of the battery cover pushes the switch contacts together, which completes the motor circuit, causing the trailer bed to go through its motions. The motor continues to turn until the crankpin (a raised nub on the final gear) pushes apart the switch contacts, thus disconnecting the motor.

The OUT command on the Big Trak need only power the trailer motor long enough for the crankpin to move away from the switch contacts, allowing the spring to push the switch contacts together, thus completing the motor circuit until the crankpin rotates around again. With a fresh battery and a light load, it is possible for the 3-4 seconds of the Big Trak OUT command to rotate the trailer bed beyond a single dumping cycle, such that the mechanical switch carries the trailer bed through the end of a second dumping cycle. However, this isn’t a big deal.

The spring on the side of the battery cover has rusted away on one of my Big Trak Transport trailers. The other trailer is missing the spring altogether. So, neither of my trailers are able to continue their dump cycles and return to a level position after the timer on the Big Trak OUT command ceases to power the motor.

The In Command

There’s a mysterious command button on the American-model keypad labeled The IN command key on the Big Trak.. According to the manual, the IN key will be used with an accessory that is not yet available. However, upon internal inspection of my Big Traks, it does not appear that this was ever possible.

The accessory expansion plug port has three possible connections. One is always connected to ground and the second is always connected to an output on the SN75494. The third is not connected to anything on both of the Big Trak toys that I have. That third connection should have been wired to support the IN command.

Three holes for the missing transistor (labeled U1 here) and one hole for the missing wire on the expansion plug port on the Big Trak.

Three holes for the missing transistor (labeled U1 here) and one hole for the missing wire on the expansion plug port on the Big Trak.

Examining the circuit board in the Big Trak, there’s a cluster of three holes that look suspiciously like a place for a transistor. Depending on your Big Trak, there are two unused resistors properly located for pull-up and current-limiting. Near the other two plug wires, there’s an empty hole which connects to the current-limiting resistor which connects to the base lead of the missing transistor. Assuming a PNP transistor (U1) was installed, this would be a perfect way for an accessory to send an input signal by simply grounding the third connector on the accessory plug.

Since at least some of the Big Traks shipped without this transistor and with the third wire on the accessory plug cut off, then those Big Traks were simply not capable of supporting the IN command with an accessory, contrary to the instruction manual.

In Experiment

I added a wire to the third connection on the accessory plug and connected it to the empty hole on the motherboard. I then installed a 2907A PNP bipolar transistor to the group of three holes on the motherboard. This particular Big Trak already had the resistors and jumper traces.

According to the manual, the IN command does nothing other than skip the next step. I had hoped that with this extra hardware installed, that the IN command would act as a conditional statement (like an IF) by not skipping the next step when the third connection on the plug is grounded.

The Big Trak performs normally until the IN command is reached. At that point, it either skips the command (third connection not grounded) as usual or pauses indefinitely (third connection is grounded) until the third connection is not grounded. This demonstrates that this portion of the circuit definitely affects the behavior of the IN command.

Regardless of what command followed the IN command (OUT, FIRE, HOLD, move), the next step was skipped. I’m perplexed as to what value the next step provides.

At this point, it looks like the only action performed by IN command is to pause and wait for some sort of input to continue.

In Questions

It seems odd that the resistors, jumper traces, holes, and the third plug wire (which was then cut off) remained in production for some Big Traks when the lack of the transistor made those components completely worthless. At some point the manufacturer apparently realized this and completely eliminated all of the unused parts.

The support resistors and jumper wires (left) disappeared (right) from later Big Trak boards. The color change in the capacitor is unimportant.

The support resistors and jumper wires (left) disappeared (right) from later Big Trak boards. The color change in the capacitor is unimportant.

Shortly after writing this web page article, I acquired my third Big Trak, serial number 1007748. Given the early date on the chip (7923) and the pre-printed “1”, I’ve come to the opinion that this is actually the 7748th Big Trak manufactured.

This early Big Trak includes the “missing” transistor, a 3906! The 3906 is a PNP bipolar transistor, which matched my educated guess. (However, I had figured it would be a 9112 to keep the parts consistent and save money due to volume.)

Now comes the surprise. Despite the inclusion of the resistors, jumper traces, and transistor, the third wire to the accessory port was cut off! Why? Why do that? Why incur the cost of including all of the parts, only to remove the required wire? This pretty much rules out my hypotheses on the manufacturer originally eliminating the IN function to save manufacturing costs.

Grounding the third wire on the early Big Trak causes the IN function to pause indefinitely, just like my other Big Traks. That would be a pretty lame feature and doesn’t explain gobbling up the next command step.

So, here is my current favorite hypothesis:

The designers and manufacturer intended for the Big Trak to include a proper IN (input) command. The box, manuals, keypad, and circuit boards were all designed with this in mind.

A large batch of custom TMS1000 MP3301A microcontrollers was ordered.

Board assembly began, with passive components (jumper traces, resistors, etc) added first, and delicate semiconductors (transistors and chips) added last.

When the custom TMS1000 microcontrollers arrived and were installed, it was discovered there was a bug or defect that prevented the IN command from working properly. Since this couldn’t be corrected (ROM isn’t modifiable) and since the custom microcontroller was probably one the most expensive and longest-lead-time parts, they decided to go ahead as-is.

The third wire on the accessory plug might have been cut off to save the labor cost of soldering it on, assuming it was one of the last items added. Or, they might have been concerned that the IN command defect would cause interference with the trailer accessory. In any case, some boards still included the transistor because they had already been completely assembled. And, some boards included only resistors and jumper traces (but not the transistor) because they had already been partially assembled.

The keypad on a later Big Trak is missing the hole for IN key.

The keypad on a later Big Trak is missing the hole for IN key.

In later Big Traks, the keypad plastic was altered to eliminate the hole necessary for the top pad and bottom pad to make contact for the IN key. The pictures I’ve seen of the European Bigtrak even eliminate the image of the IN key on the keypad.

In other words, pressing the IN key (or blank spot where that key should be) does nothing on these Big Traks, since the plastic insulator prevents the key from making an electrical connection. Keep this in mind if you decide to gut your Big Trak and replace the motherboard with your own design. The IN key may not be available to your new circuit if you have a newer or European model.

Speaking of gutting the Big Trak, there are some simple repairs that you can perform with replacement parts on the next page...