80-Bus News


May–June 1984, Volume 3, Issue 3

Page 40 of 51


80-BUS IVC (Intelligent Video Controller) board. Has own on-board 4MHz Z80A to control all video functions. Interfaces to the host computer via three 80-BUS I/O addresses, thus not encroaching on any host memory. Normal mode 80 characters per line by 25 lines, with optional second mode (supplied set to 48x25, but user alterable). System keyboard (parallel type) is normally connected via the IVC which thus provides ‘type-ahead’ facility, and with certain keyboards also provides user-definable function keys (see GM827 and GM852). Light pen input provided. 256 different characters may be displayed, 128 fixed in EPROM and 128 user-programmable, being held in on-board RAM. Many control sequences include cursor addressing, partial screen scroll lock, pixel graphics (160x75), clear to end of line, clear to end of screen, define function keys, define programmable characters, insert or delete character from line, etc, etc. Now replaced by GM832 SVC.


80-BUS CPU/​RAM board. Similar overall specification to GM811 above, but without 8-bit input port and with the ‘byte-wide’ sockets replaced by a 2K/4K EPROM socket (removable from memory map under software control) plus 64K of dynamic RAM. This RAM is switchable as Page 0 under the Nascom style ‘page-mode’ operation. This board also includes the Gemini ‘Extended Addressing’ mode by utilising memory mappers. This allows the user to select the 64K of memory that the Z80 ‘sees’ at any time as any 16 blocks of 4K each, out of a total memory space of 512K. This ‘Extended Addressing mode’ may also be used in conjunction with ‘page-mode’ to provide up to 4 pages of up to 512K, i.e. 2MBytes total. However note that very specialised software would be required to do this. Normally supplied with RP/M (see GM811), but in complete systems (e.g. Gemini Galaxy) supplied with an auto-boot EPROM that detects whether Winchester hard disks or 5.25″ floppy disks are present on a GM809/​GM829 and automatically ‘boots up’.


80-BUS IEEE 488 interface board from EV Computing for (surprise, surprise) connecting to other IEEE 488 devices. Incorporates on-board control software under the ‘page-mode’ scheme that can be brought in and out of the system memory map. Does not support ‘Extended Addressing’.


Single/​double 5.25″ disk drive unit for use with GM809/​GM829, incorporating 1 or 2 Pertec FD250 double sided 48tpi drives and GM804 PSU. Capacity, in double density mode, 350K (formatted) per drive. Discontinued – replaced by GM825.


80-BUS Multi-I/O board. Originally designed by Quantum, and then taken over by Gemini, this board provides 3 Z80A PIOs, a Z80 CTC, and a National Semiconductor 58174 Real-Time-Clock chip with battery back-up. An interesting feature of this board is the internal expansion bus – this allows additional boards to be ‘piggy-backed’ onto the GM816 and make use of the I/O decoding and buffering that it provides. Currently available ‘piggy-backs’ are the GM818 (see below) and the GM663 prototyping board.


An 85 Watt switch-mode power supply unit. Suitable for systems with about 5 boards and 2 5.25″ drives.

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