With the intention to develop a reliable and powerful QL compatible computer which could be used countinuously in an industrial environment a modular computer system has been developed. It has five main components.
While the QL has its expansion cards area in memory from address 0C0000 to 0FFFFF the Primus has its one directly under 16MByte and it is nearly 1MByte big (from F00000 to FFBFFF). The expansion cards in this area are recognized by the MINERVA operating system without any alterations. QDOS has to be altered slightly to take care of the new area when searching for expansions.
The operating system, up to three QL ROM expansions and the boot loader are placed within a 128kByte EPROM. This EPROM can be replaced by option by an EPROM with a maximal size of 1MByte for additional data.
The boot loader loads the operating system and the QL ROM expansions into the 16Bit wide RAM. There the CPU can access it without any wait states. Also, while bootloading, date and time will be transmitted to the QL clock from a real time clock. The RTC is powered by a special capacitor (1F) while the system is off line.
The microcomputer itself has a socket for an EEPROM (electronical erasable PROM) with a size of 128kByte that can be used e.g. for important user specific data.
A watchdog timer is used to control the program flow. If the normal flow of operation is disturbed by any reason and thus the watchdog timer is not reseted periodically any more it causes a RESET. The original program can be restarted by your boot program.
Less important for a QL compatible system are the alterable interrupt priorities for external and RTC interrupts, the wait states and the error handling after bus errors.
Three 9-pin SUB-D sockets for the two serial ports and a CGA monitor connector are connected via IDC plug boards. In the same way connected is a 25-pin SUB-D socket for the matrix keyboard. The matrix keyboard is used, because it allows you easily to use custom made keyboards. Such custom made keyboards can eventually be integrated into the 19 inch case. Optionally the Primus comes with a Di-Ren keyboard interface and an IBM AT compatible MFII keyboard.
The DSMCL signal line is not used anymore and therefore not necessary anymore!!!
The most important expansions are controller cards for hard disk and floppy disk drives. For use with control applications cards with analogue and digital input and output are a matter of priority.
Contrary to the QL the bus has a higher phase, therefore expansion cards have to be able to treat the higher velocity. For example the access time for EPROM's should be less than 150 ns.