Volume 1, Number 1 – August 1981

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by S. Hope

The main drawback of the standard ROM based graphics for Nascom is that while many programs use the standard “pixel” characters, others, for example Sargon, need special character sets to be most effective. One solution is to have two or more switch-selectable graphics ROMS, but a more effective answer is to store the data which defines the graphics characters in RAM. Any special characters required in a program can then be loaded into this RAM either from tape or by the program itself. In addition, a programmable character generator can be used to simulate bit-mapped high resolution graphics.

The unit described below is simple and cheap to build. It allows up to 128 characters to be loaded into RAM and used as normal graphics characters at any screen location. Each character consists of 16 rows of 8 dots; the data for the character is stored as 16 consecutive bytes in the P.C.G. RAM. A few modifications need to be made to the basic Nascom 1, but the unit does not need a buffer board or any other expansion to be present.

The 2K of RAM is mapped as “write only” memory at addresses £000 – £7FF, coincident with the monitor address space. This reduces the address decoding logic required, and ensures that the unit is compatible with the memory allocation of machines with different software layouts. This is very effective when the device is used purely as a character generator, but has a drawback when it is used in a program which needs to keep track of which dots on the screen are “on”; as the processor cannot read the P.C.G. RAM (reading this address space simply accesses data in the monitor) a copy of this 2K must be kept at some location in the main memory.

The unit is built on a prototyping board. Some soldering ability and considerable patience is required to hard wire all the address and data lines to the RAM. If you are short on either, or have any doubts about the modifications needed, you would probably do better to stick to one of the commercial P.C.G.’s available.

To simplify connection to the Nascom, the original 6576 character generator is resited on the new board, and a Jumper cable runs from this board to the 6576 socket on the Nascom. The state of bit 7 in the V.D.U. RAM determines whether the data for the display is obtained from the 6576 (bit 7 = 0) or from the RAM (bit 7 = 1). Because the outputs of the 6576 have no high impedance state this selection is done by means of a set of two multiplexors (74LS157, IC’s 10 and 11). The 74157 is a four-pole two-way switch contolled by a voltage applied to pin 1.

The P.C.G. RAM is connected to the character and row select lines from the 6576, and the Nasbus address lines A0 – A9, through a further set of multiplexors (IC’s 1 – 3). When the Nascom writes to an address in the range £000 – £7FF, these multiplexors are switched to the Nascom address lines, the chip select signal for the correct RAMs is taken low, and the write enable signal for all the 2114s is low. An 81LS95 octal buffer (IC 4) gates the data from the data bus into the 2114 I/O lines.

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