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FD-502 (26-3133)

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Home / Hardware - FD-502 (26-3133)


This is the last Tandy controller. It reverted back to mere solder-tinned edge connectors, for whatever reason (cost?!). Unlike every previous model, this one uses a 28-pin ROM rather than a 24-pin ROM for Disk BASIC. This model was introduced in the 1988 Radio Shack catalog.

All Radio Shack (and some third-party) CoCo floppy controllers use a pair of 7416 open-collector buffer chips to communicate with the floppy drive(s). These chips can fail and cause various strange symptoms. They may be replaced with 7406 chips, which are more rugged (the 7416 is rated at 15V, while the 7406 is rated at 30V). Socketing the replacement chips makes future replacement much easier, should it become necessary.


This drive unit is like the FD-501, with two notable exceptions. This is the only Tandy model ever to incorporate true double-sided drives, and its drive power connectors are like those of typical 3.5" floppy drives rather than those of normal 5.25" drives. The smaller power connectors are also reportedly wired differently than typical 3.5" drive connectors. As noted elsewhere, either a patched Disk BASIC or an alternative operating system is required to gain access to the second side of disks. A cooling fan must be installed for safe operation if a second drive is added.