Using the data obtained from field use of the T200 and T300 series units, SKYNET reached the conclusion that the reason that humans had survived for so long was that they were incredibly adaptable, especially to different terrains and environmental conditions. The very design of the human body allowed a great deal of freedom of movement over a variety of terrain and in areas that the Series 300 units could not access nor could the T200 units gain an effective foot hold.
The heavy pacification campaigns conducted by SKYNET began to drive the human Resistance underground, deep into the ruins. More and more battles were fought using guerilla tactics, a range of tactics that the current models of HKs were ill-designed to utilize or adapt to. Specialized human sapper teams, highly trained in guerilla tactics and ambush techniques, would prowl the ruins, waiting to ambush and defeat the 500 series Machines. Other Resistance units, equipped with a limited stock of liberated pre-War man portable surface to air guided missiles, were recording unacceptable losses against the 400 series Aerial units. Even combined arms doctrine of having the 400 series Aerials escort and provide over-watch for the patrolling 500 series pacification units was only marginal effective in reducing losses between the two units.
It soon became apparent to SKYNET that the humans could readily maneuver and strike from positions that were just not reachable by any of the currently fielded HK series of units. SKYNET began to see that even the 300 series units could not track the humans back to their underground headquarters and staging areas; the maneuverability of the 300 series was on the order of a Ostrich, not a human being, and a locked door or a barricaded entrance way proved more than a adequate defense against even 100 series units. A new unit was needed, a HK unit that could simulate the exact range of movement that a human being could, a unit no larger or wider than a human being; a fully anthropomorphic hunter killer design designed to incur every advantage that the human body enjoyed in movement without any of the weaknesses. Data obtained from the trials of the 200 series and the 300 series units produced the design parameters for the T500 series.
Studying human anatomy, SKYNET produced the first of the endoskeletal based HK units. A microprocessor controlled, hyperalloy armored combat chassis was fitted to servo hydraulic actuators. The T500 looked like a gleaming metal skeleton, it could move faster than a human, had sensors that were lighter yet just as powerful as those of the Series 1200 Scout, and had three modes of operation; direct, automatic, and autonomous. The new T500 could be directed by SKYNET directly like a puppet, it could react automatically to a wide variety of pre-programmed conditions, or the individual units could be relinquished to their own control and act independently for months on end, combing the ruins on extended search and destroy missions. Memory for the T500 series was set to read only, thus its available responses to any given tactical situation were limited and after a time, predictable.
The T500 was armored in a half centimeter hyperalloy sheath which offered superior protection from small arms fire and to a limited extent, from explosions and energy weapons. The T500, thanks to its advanced hydraulics, could perform superhuman feats of strength, never got fatigued, never had to rest, breathe, or eat. It was the perfect hunter, tireless, remorseless, merciless, dedicated, and capable. The T500 stood 2 meters tall and weighed almost 400kg.
The unit was fast, able to maintain a constant run at 60 kph with short bursts of up to 75 kph, but these speed bursts taxed the servos and the joints of the leg assemblies. The T500 could dead lift five times its own weight or punch through concrete and metal with no damage to its reinforced servos. The main armament of the T500 was the Westinghouse M25A1 series plasma rifle but its design and manipulator hands allowed it to use any standard small arm or weapon including stocks of pre-War small arms. The programming of the T500 included detailed information files on all known small arms of the world (circa 1997 A.D.) as well as detailed files on human anatomy, behavior and logic. Resistance casualties were on the order of 80 to 1 in odds against the T500's improved performance, with the odds edging higher in the first few months of introduction when the unit caught the Resistance off guard and mankind was ill-equipped to deal with a brand new design that could track it anywhere.
The T500 was officially classified as a "Infiltrator" and a "Terminator" unit, a first for SKYNET's designs. The Terminator designation was obvious, but the Infiltrator status was special. Now, SKYNET had a unit with which to take the fight back to the Resistance. All previous and post models now would bear the designation "Terminator" but only the T500, T800 and T1000 models would also carry the designation "Infiltrator".
SKYNET would try many new tactics with the T500 line of HK units, including covering the unit in a latex pseudo-skin and trying to pass it off as a human. This tactic worked, but only at range. Up close, the T500-I model failed to pass for human, but it was a promising start for SKYNET and required a dedicated sub-processor assembly to fully analyze the tactical importance of fielding a unit capable of being mistaken for a human.
A notable T500 variant was the T500C or "Drone" unit. The T500C was a maintenance unit, with all combat programming and hardware stripped during production. Sensors were bare minimum required to perform repair and maintenance. T500C units acted as wardens at human concentration camps where armed resistance was almost nonexistent. The T500C could still beat a human in hand to hand combat but also gave SKYNET a unit that could fit into the places that humans had originally designed for themselves to be able to reach when SKYNET was still supposed to serve the human race. T500Cs were never encountered on the battlefield, and when SKYNET went off-line, 99% of the T500Cs in operation died with it. The T500C acted as the eyes, ears, and hands of SKYNET, controlled directly by the super computer.
T500R "Reaver" -R variant, psychological warfare unit
Another short lived production variant of the T500 was the T500R or "Reaver" model as it became known by Resistance units. This aberration was designed to frighten and intimidate the Resistance fighters in a bold design and tactical project based on research into using fear in psychological warfare. The T500R used a standard T500 chassis, but mounted the sensors in the upper torso in an armored blister, doing away with the head mount altogether. The forearms were increased in length by fifty percent in order to mount the higher capacity hydraulic rams that powered the heavier duty manipulator unit motors. The five digit manipulative fingers were replaced with slightly exaggerated serrated claws and the finger tips with razor sharp monofilament edged blades which drew the power to maintain their containment fields directly through high capacity leads attached to the power plant itself.
The unit's speed was increased through stronger hydraulics and better mounts and all sound absorption material was removed, creating the loudest of all bipedal mobile units. The T500R was introduced in a limited production run of 200 units. Deployed to the battlefield, the T500R struck terror into Resistance fighters who coined the term "Headless" or "Ghoul" for the unit. T500Rs would acquire targets, intercept them at a high speed lope, and dispatch them with the hydraulically powered blades, often to the point of overkill in ripping the victim into tattered remains. Some T500Rs would slowly kill their acquired targets, especially if other members of the Resistance were known to be watching or hiding in the vicinity; all for the single purpose of instilling fear of the machine into the human psyche. Detailed anatomy and medical files were part of the databases of the T500R, information that was constantly updated through field exercise and SKYNET's own live subject testing procedures done in its automated labs. As the years went by, the T500Rs became some of the most effective killers ever produced, though their lack of ranged armament restricted their use to ambush and lightly defended rear echelon areas and warrens. The T500R was a terror weapon, and it worked. However, possessing no ranged weapons and only close combat / hand to hand armament, the Resistance soon learned that the proper way to deal with a T500R was to engage it as far away as possible with conventional anti-armor or high energy weaponry.
SKYNET reprogrammed the T500Rs tactical doctrine to adopt a guerilla warfare subroutine and soon the remaining T500Rs were going dormant, hiding in debris and destroyed buildings, patrolling the sewers and subway stations, waiting on targets to wander into detection range. By then, it was generally too late for the target. The T500R was adept at lurking in the ruins, its matte finish helping it to blend in with the debris in order to allow it greater camouflage capacity.
The chassis and suspension of the T500R was also modified in the later -B model to incorporate hyper rotational joints which moved on frictionless bearing rings. The legs and arms were further modified by being lengthened some fifty percent and there was initial confusion that the T500R -B variant might have actually been a variant of the T500 "Gaunt" but detailed post combat disassembly by Resistance scavenger teams proved that while similar in appearance, the two models did not use common parts.
The -B models could squat down, tucking their legs and arms in around them to present a very small center of mass, thus presenting a smaller cross section in which to be spotted. Using power conservation management subroutines, the T500R could loiter in a given hunt area for weeks, its sensors passively searching for acoustic and thermal spikes which would represent targets of opportunity entering the area. Once a target was identified and selected as a valid engagement choice, the T500R would rapidly bring power to its primary systems (a process that took 1.67 seconds to achieve) and would unfold from its loiter configuration. The servos of the T500R were purposely designed to whine more loudly when under load. This was achieved by making the servos thinner and increasing frictional load on non-bearing surfaces. Tests in both the field and controlled lab environments showed that the sound of the T500R's servos, especially when the unit was moving and operating at high speed, had a detrimental effect on the morale of Resistance units. A limited number of T500R units were given replicating vocoders so that they could sample the sounds of their victims and then digitally edit, mix, and rebroadcast these sound files at levels approaching 90 standard decibels. The effect of this reintroduction of victim sounds to the ambient environment was shown to further decrease morale among Resistance forces, as proven by both automated lab tests and actual field exercises.
T500-S -S variant, ELINT scout unit
The T500-S was produced with advanced Electronic Intelligence (ELINT) features and an enhanced scout electronic suite. Liquid crystal equipped variable pan and scan optics were installed (an option that would become standard on the later T800 series) with wide angle aperture mountings. Motion and acoustic sensors were upgraded to the -S specifications and the thermal imaging systems were updated with the -V3.79C build, which offered higher resolution and cryogenic cooling of the image processing array. The cranial shell of the T500-S had to be increased in size and was a new mold rather than a modification of the existing unit. SKYNET also situated both the main CPU and the backup CPU of the T500 into the chest cavity with access to the processor through an armored plate on the back of the chassis. The optic mounts were also modified and given a longer track so that they could be moved to cover greater than a 270 degree angle of vision. The high sensitivity acoustic sensors were mounted lower and slightly to the rear of the new cranial form, while the olfactory sensors remained in their standard position. Two sub-processors worked to control and coordinate the ocular arrays, which could be moved independently on their mounts to cover different aspects. Information gathered from the ocular sensors was filtered by the dedicated sub-processor before being relegated to the main CPU for review. While the new ocular mounts afforded the T500-S a great deal of sensor coverage not available to the normal T500 unit, initial tests showed that the movement of the ocular sensors greatly disturbed the live test subjects who observed the operation of the ocular mounts. This reaction was noted and accepted.
The T500-S was able to go where the Series 1200 Scouts could not and T500-S models worked closely with other larger units to track large movements of Resistance units as well as venturing below ground to gather tactical and strategic data on Resistance strong holds and movements. Often, when SKYNET deployed special operations (SPEC-OPS) braces of endoskeletons, the braces were made up of one T500-S acting as point and brace coordinator, followed by four to five T500 (later T800) series units with one of those invariably being either a -T variant or a -W variant. Although more costly in time and materials than the Series 1200, the T500-S had many advantages over the older scout series.
The T500-S was more maneuverable, faster, better armored and could also be armed with the same weapon load-out that the standard T500 unit could which meant that it could be issued, operate, or obtain any standard small arm or light support weapon in use at that time.
Only three thousand and sixty five T500-S units were produced and over half were still in service by the time of the Final Victory.
T500-T -T "Tank" Heavy Support Variant-
The T500 series chassis proved to be so adaptable that SKYNET implemented many designs on that particular model, some of which saw field use and some of which did not. The -T variant was a limited production unit which met with a high degree of success in both controlled and field operations. SKYNET's need for a medium to heavy support unit for its small unit tactics was a given. Initial protocols were based around designing large scale, rapid fatigue plasma weapons and issuing those to selected endoskeleton models. The initial development of the heavy plasma guns found that the existing T500 series did not have the capacity to fully utilize this weapon to its full potential and initial field exercises met with mixed results. The chassis was not strong enough to handle the output of this weapon, nor was the power plant of the unit. Self contained power plants were introduced, mounted in armored carriers and attached to the rear of the chassis, but their mass caused undue stress on the suspension and the failure of three units so equipped in the field resulted in termination of that design branch.
The T800 series of endoskeleton was in pre-planning but SKYNET needed a small unit support element before the T800 would ever see production. In a fit of desperation, SKYNET took a standard T500 and removed the forearm and manipulators from the design. In place of each forearm and hand, SKYNET incorporated dual rapid pulse phased plasma guns in a single combined mount with dedicated power leads and extra capacity fuel cells. The dual weapon mount housed a M95A1 plasma gun and a coaxial M20-A plasma gun, both stripped down to their component elements and packaged in a cooling sleeve that prevented overheating. A high intensity white spot light was mounted as part of the weapon pod and could provide white light illumination along the same path that the weapon pod was being aimed. An alternate active infra-red source projector was also mounted in the pod and used to illuminate areas when the white spot might give the unit away.
The power supply of the T500-T was also upgraded, being moved higher up in the frame and used the same unit found in the T500R -B model. Dedicated power and high flow fuel leads to the heavy plasma guns were attached at the bottom of the weapon mount and linked to the rear chassis of the unit by a flexible armored coil cable a meter and a half long and three centimeters in diameter. The armored cables were the one weak link in the design as severe damage to the cables would prevent the plasma guns from operating or lead to feedback that could damage or destroy the unit. The cables, though armored, were highly susceptible to the effects of explosions, both proximity and especially contact.
Another weakness was found in a initial design parameter specifying only two centimeters of superalloy armor plating on the rear power cradle housing. This parameter was changed after several T500-T units were lost to Resistance sharp shooters who engaged the units from behind, targeting the somewhat weaker armor plating protecting the hydrogen power cell. The modified production parameter called for three and a quarter centimeters thickness which proved more than adequate at protecting the hydrogen power cell in field trials and actual combat.
In order to offset the increased mass of the larger power cell which had been moved up and to the rear, the T500-T had a chassis modification that made it stoop forward and walk with a slight but noticeable lope. This stance, along with the enlarged armored shell on its back housing the larger power supply gave the unit the nickname of "Hunchback" among Resistance units. The nickname "tank" was also used on occasion, though the nomenclature is unclear except as a possible reference to the heavier armament.
The electronics of the unit were also upgraded to allow for multiple targeting and multiple target engagement (a protocol that would not be implemented on a series wide basis until the introduction of the T800 series years later). The CPU was aided by two high speed co-processors, each controlling fully all the functions of one weapon system of the T500-T model. In the event of the loss of a co-processor, the main CPU could take over the load of weapons management but at reduced efficiency. The optic sensor arrays were enhanced with faster motion tracking and rapid target lock sequencers.
A total of 1800 T500-T models were produced and with the introduction of the T800-T models years later, the older T500-T models were relegated to installation defense and rear echelon resource raiding operations. By the time of the Final Victory, only 397 T500-T units remained in operational service, with another 48 awaiting repair and parts in depots scattered throughout the Occupied Territories.