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PROM Modifikation

General Information
PROM Option
Monitor Commands
Downloads
Information
References

General Information

Basically, the North Star Horizon does not have a BIOS as one is used to from the later computers. There is only the "first" jump address E800 at which the floppy controller PROM is located and immediately boots the floppy inserted in drive A:.

But if there is a bug anywhere in your system, the RAM board, disk controller or ... you will see absolutely nothing on your screen.

Therefore, a system with ROM is preferable. Basically there are two mods. The original one with a 2708 (1K) EPROM and the modified one with a 2716 (2K) or 2732 (4K) made by Mike Douglas.

PROM Option - 2708

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The original modification is only recommended if you can also program EPROMs of type 2708. Have a look at: Matthew Millman's HVEPROM project. I bought one on eBay.

Bottom left you can see the +12v and -5v regulators, necessary for the 2708 EPROM. I currently have two different 2708 EPROMs available, the Russian K573РΦ1 and the TMS2708. However, with the above-mentioned programmer I can only program the Russian EPROM. There are always problems with the TMS. I have no idea why.

I have replaced the axial capacitor below left (16V, 100 µF) and the PROM regulators 79L05 and 78L12. Both regulators were not working properly. The PROM is configured to auto jump (POJ) address: E000; same as on my other ZPB-A2 boards. The floppy boot sequence (ROM) on the N* floppy controller is located at E800.

ZPB-A2 - Z80 Processor Board with original 2708 PROM modification

The next shown ZPB-A2 board is working fine on my N* Horizon but not good to repair because the ICs are not socketed.

RED belongs to the PROM OPTION (POJ = E800)

The real problem with this 2708 PROM option is finding a suitable monitor ROM that is smaller than 1K. I have asked in the relevant forums, but have not received a satisfactory answer, let alone a ROM.

By chance I found a tiny hint in the old newsgroup (comp.sys.northstar). And this led me to Dave Dunfield and his Northstar Horizon Emulator. A small (482 bytes) and minimalistic monitor ROM named HDM80 is included there. Here are my results with Dave's emulator.

In the emulator, I left the start address of the PROM at F800. On my real N* Horizon, however, it is always E000. That's why I had to reassemble the ROM with this address.

N* Horizon emulator, 2708 ROM monitor (1)

N* Horizon emulator, 2708 ROM monitor (2)

N* Horizon emulator, 2708 ROM monitor (3)

Booting a real N* Horizon with the HDM80 ROM monitor at E000

PROM Option - 2716/32

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Mike Douglas has found a small but nice solution in place of the official PROM option. More details can be found on his website or in the following instructions.

A monitor for the Horizon computer is present in the ROMs subdirectory of the Horizon folder. MON40C.ASM is the Vector Graphic monitor 4.0C ported to run on the Horizon computer. Because of the similarity between the hardware of the VG machines and the Horizon, very little modification of the VG monitor was required. The monitor offers memory examine, modification, breakpoints, an Intel hex file loader, memory test, memory search, and more. The “B”oot command has been modified to jump to the North Star boot ROM at E800 instead of the Micropolis boot ROM at F800.

The current version of the monitor runs at E000 and expects RAM at BF00-BFFF for stack. [1]

Next two pictures: 2716 PROM modification by Mike Douglas. You should definitely make this modification. In the meantime (Mar. 2023) I have converted two boards to this PROM option.

You have to configure the new PROM and auto jump (POJ) address to: E000. According to Mike Douglas you have to install the resistor R10 (1K); this is missing in his working sheet.

ZPB-A2 - Z80 Processor Board with 2716 PROM modification

The advantage of this improved modification is obvious. The larger EPROM (2K), a better monitor program and the omission of the -5V line. The (-) line actually only causes trouble.

ZPB-A2 - Z80 Processor Board with PROM mod

Monitor Commands

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One of the most important commands is TEST (test memory)! Although I must note here that this test did not detect a bent pin on a RAM IC for me. Only by chance and a very close look I found the faulty pin, bent it back and everything was fine again.