80-Bus News


September–October 1983, Volume 2, Issue 5

Page 42 of 67

Internal Layout

Internally the layout of the Galaxy is simple and compact. At the rear of the case is the fan and power supply unit. The fan ran reasonably quietly and was certainly an improvement over the fans fitted to earlier Galaxy 1’s. The power supply is a switch mode one, fully RF screened and with impressive mains supply filtering. No effects of mains glitches were observed even under the ‘dirtiest’ of mains conditions. The drives are fitted to the front panel immediately in front of the power supply and solidly secured to a sub-chassis on the base of the computer case. To the left of the drives is a card cage which contains six card slots. The three lower slots contain the three modular cards needed for the standard Galaxy, the upper three are empty and are available for expansion. As mentioned earlier a blanked off hole is provided on the back panel adjacent to the card cage to allow entry for leads which might need to be connected to any envisaged expansion cards. Air flow through the case is dictated by the position of the ventilation slots on the sides and to the front of the case top wrapper. There are no slots on the top of the case, so the computer is reasonably safe against spilt coffee, etc. Having removed the top cover, it is only too easy to replace it the wrong way round and so restrict the ventilation although no harm seems come if this accidentally happens, indicating more than adequate cooling.

The standard Galaxy is fitted with three cards, the CPU/​RAM card, the disk controller card and the video card. As is to be expected, all three cards are engineered to the highest standards, the pc layout whilst compact, is orderly and neat, with gold flashed connectors, pc solder resist and component legend screening on the fibreglass boards. The boards are flow soldered with the ICs socketed where necessary. Good quality components are used throughout

The CPU Card

The CPU card contains the main processor, a 4MHz Z80A. The clock speed is 4MHz without wait states. The board also incorporates 64K of 4164 dynamic RAM and the 2K simple monitor/​boot EPROM. A Z80A PIO device takes care of the parallel I/O, normally used for the ‘Centronics’ protocol parallel interface, whilst a WD8250 serial controller provides RS232 I/O and full modem control. A number of links are provided for reconfiguring the board for specialist purposes which are fully detailed in an optional manual for the card. All I/O to the bus is fully buffered with considerable expansion in mind. The RAM and ROM are paged and can be selected and deselected from software. The software is so arranged that the EPROM is normally paged out of the system on boot up, leaving the whole of the 64K RAM available for CP/M. The RAM is similarly paged, with extended addressing to 2M byte. This means that implementation of CP/M Plus (CP/M 3.x) will present no particular problems as the additional RAM required for this operating system could be provided by the inclusion of the Gemini GM802 64K expansion RAM card, which already exists within the Gemini range of expansion cards.

The Disk Controller Card

The disk controller card features the Western Digital WD1797 three chip disk controller set, and incorporates software controlled clock rate select for 5.25″ or 8″ drives. Connectors are provided for the standard 5.25″ and 8″ drive connectors, whilst a third connector is used for the SASI interface to Winchester hard disks. This card is extremely flexible in its own right, although normally only used to drive the internal 5.25″ drives. 8″ drives and a Winchester drive can be fitted externally and because of the software selectable clock rates and the SASI interface, given the correct

Page 42 of 67