TRS-80 Model II

The TRS-80 Model II is a computer system launched by Tandy in October 1979, and targeted at the small-business market. It is not an upgrade of the original TRS-80 Model I, but a new system.

The Model II was succeeded by the compatible TRS-80 Model 12, Model 16, Model 16B, and the Tandy 6000.

Tandy was surprised at the strong demand for the TRS-80 Model I from business purchasers. The computer was too limited for such use, so the company began development on the Model II in late 1978. It was announced in May 1979, deliveries began in October, and only Tandy-owned Radio Shack stores sold the computer. Tandy advertised the Model II as "a business computer — not a hobby, 'home' or personal computer". It claimed that the computer was "ideal for a small business, and also 'just right' for many time-consuming jobs within larger businesses", including those with mainframes or minicomputers. The base single disk version was US$3,450, and a four disk version was US$6,599.

As a professional business machine, the Model II used state-of-the-art hardware and has numerous features not found in the Model I, such as the high-speed 4 MHz Z80A, DMA, vectored interrupts, a detachable keyboard with two function keys and numeric keypad, and port instead of memory-mapped I/O. It has 80x25 text display and a single-sided 500 KB 8" floppy drive, and either 32 or 64 KB of RAM, along with two RS-232 ports and a Centronics-standard parallel port. The video memory can be banked out of Z80 memory, so that the entire 64 KB address space can be used for main memory. Unlike most computers, it has no BIOS ROM except a small boot loader (the BIOS was loaded off the boot floppy). Because of this and the use of port I/O, almost all of the Model II's memory can be used by software. The Model II runs the TRSDOS operating system (renamed to TRSDOS-II starting with version 4.0) and BASIC. The different disk format and system architecture make it impossible to run Model I/III software on the Model II, and the II's software library is smaller. This is mitigated by the CP/M operating system for the Model II from third parties such as Pickles & Trout; unlike the Model I/III, the Model II's memory map is compatible with standard CP/M. Three internal expansion slots can be used for add-on cards, such as additional serial ports and bitmap graphics.

The floppy drive included with the Model II is a Shugart SA-800 full-height, single-sided 8" drive; like most such drives, it spins continuously whether the disk was being accessed or not and the motor is powered directly off the A/C line. The floppy controller in the Model II is a double-density, soft-sector unit based on the WD 1791 floppy controller. Like with the Model I/III/IV, boot disks on the Model II require Track 0 to be single density. CDC drives are used for the floppy expansion module.

The keyboard is a capacitive keyboard made by Keytronic Corporation. Like most capacitive keyboards, it utilizes a key mechanism with foam rubber disks; these are prone to dry-rotting with age and requiring replacement. A later version of the keyboard was made by Cherry Corporation, but still uses the capacitive technology rather than the more well-known Cherry mechanical keyswitches.

The disk format on the Model II closely follows the IBM 3740 standard, which specifies 77 tracks, 26 sectors per track, soft sector formatting, and a sector size of 128 bytes for a formatted capacity of about 250 KB, but the Model II had a double density controller, so the disk format uses 256 byte sectors and formatted capacity is about 492 KB. If users install a double-sided drive they can get 1 MB of space, but this requires a modified DOS and Radio Shack did not officially support the use of double-sided drives.

There were several hardware revisions to the Model II over its lifespan. The first revision models (1979–80) cannot boot from a hard disk, and the floppy controller requires a terminating resistor pack for the last drive on the chain, instead of the standard method of putting a terminating resistor pack on the internal disk drives. The external resistor pack works by looping back the I/O lines on the external floppy connector to the SA-800 drive's terminator pins. This unusual setup was chosen so the users would not have to remove the cover and install or remove a terminating resistor pack on the floppy drive every time they wanted to remove or attach external disk drives. It proved to be problematic since customers who lost their resistor packs cannot use their machines (Radio Shack sold replacement packs for $50) and Model IIs sold from 1981 onward use a different floppy controller that does not require it. Hard disks offered for the Model II also use a terminating resistor pack. These were sold as master and slave drives, with the master hard disk (which has the resistor pack) needing to be the last one on the chain. Like most hard disks offered on 8-bit computers, there is no subdirectory support and the drive is treated by the OS as a large, fast floppy disk.