The M577 was based on the M113 armored personnel carrier. From the front to just behind the driver's position and engine compartment, it was identical to the M113, but the passenger compartment was raised by 25.25" (64.14cm) to increase the inside ceiling height to 74.75" (189.9cm). A roof hatch was provided, but there were no vision devices or armament for it. The single fuel tank of the APC was replaced by two 60gal (230L) fuel tanks which were placed on either side of the passenger compartment. The fuel tanks also served as the supports for two folding tables. Map boards, radios, and a five-man bench seat could be installed in the passenger compartment. To the right of the driver, on the front of the raised passenger compartment, was an auxiliary gasoline generator which provided electrical power for the command equipment. The generator could be dismounted, and a 50' (15m) cable was provided for this purpose. A tent was attached to the rear of the M577 to provide more space in which to work. This tent was rolled on top of the rear roof when not in use, and could be used to connect two M577s. The tent provided an additional 120ft² (11m²) of space. M577 was first accepted in December 1962, and 944 were constructed. The M577A1, following in September 1964, used the diesel engine that was introduced in the M113A1 APC. 2693 M577A1s were built. The M577A2 utilized the improved engine cooling and new suspension of the M113A2. Overhaul of earlier command post vehicles to M577A2 standard commenced in August 1979. M577A3 was powered by the RISE powerpack of M113A3. The gasoline auxiliary generator of earlier vehicles could be replaced by a diesel-powered model for M577A3. M577A3 entered service in 1994, by which time FMC had become known as United Defense.