craig@unicus.UUCP (Craig D. Hubley) (04/07/88)
The $100 Amiga High-Persistence, High-Resolution Monitor ======================================================== (design by Earl C. Pottinger, Toronto Amiga Developer's Forum) The Commodore 1901 can be hooked up to the Amiga's Digital RGB port (same as the Analog RGB port). This is a green or amber TTL monitor, capable of supporting a 704x470 overscan display (I don't know if there's a PAL version!). The retail cost of this monitor is about $100 CDN, less in the States. The parts to assemble the interface are cheap (under $10 CDN). The downside is, the monitor is of course monochrome and capable of displaying only `white', `black' and one shade of `grey'. The 1901 *can* be modified to display many many grayshades, though. Stay tuned. Earl first saw the 1901 connected to the Amiga at the HyperTech display at the `87 World of Commodore in Toronto. Afterwards, *without seeing the Hypertech interface at all*, Earl assembled his own design, in one night. Earl also wave-solders static RAM successfully, so this is not to be taken as an indication of ease. Apparently this modification will work *only* with the 1901, and not with other monochrome TTL IBM-PC style monitors. Experiences to the contrary should be publicized, though HyperTech told me that they had `blown up' several other monitors. This article will discuss a) the Amiga-1901 interface b) modifications to the 1901 to make it a high-resolution composite monitor. a) The Amiga-1901 interface ------------------------ The Amiga has all the signals needed to drive the 1901, on its DB-23 monitor port (the one you use with non-composite monitors) The 1901 comes with a DIN-6 plug and a cable that connects to this and ends in a male DB-9. On an A1000, it is possible to install a DB-9 female plug *right into the case*, voiding your warranty (if it exists, which I doubt on an A1000), which would let you connect both a 1901 and a colour monitor at the same time. A500 and A2000 owners don't have room in the case, but an A500 with a Zorro II cage, and any A2000, could use a backplate, perhaps in an IBM slot, with a DB-9 female mounted. Parts ----- Female DB-23 plug. Suggestion: saw the last two pins off a DB-25. Female DB-9 plug. Suggestion: you can *always* dig one of these up surplus. 7404 chip. Suggestion: 74F04 works OK, 7404, 74LS04, 74S04 should too. 14-pin mount. Suggestion: don't chintz, this chip is easily fried. Several strands of wire. Of course, a soldering iron and wire-crimps, etc. Interface --------- Connection between ends Signal Name -------------------------- ----------- DB-23 pin 16 DB-9 pin 1 Ground DB-23 pin 17 DB-9 pin 2 Ground DB-23 pin 6 DB-9 pin 6 Intensity (Note 1) DB-9 pin 7 Video-In 7404 pin 9 DB-9 pin 8 Horizontal Sync. (to monitor, modified) DB-23 pin 12 DB-9 pin 9 Vertical Sync. DB-23 pin 18 7404 pin 7 Ground DB-23 pin 23 7404 pin 14 +5 Volts DC DB-23 pin 11 7404 pin 8 Horizontal Sync. (from Amiga, unmodified) (Note 1) The Video-In signal can come from any of DB-23 pins 7,8,9 which are Blue (7), Green (8), Red (9). Which pin to use depends largely on your Preferences settings and individual monitor/machine. Blue, however, can decode strangely, probably having to do with the way in which the monitor interprets what colours map onto white, grey, black. Later, this input can be retrieved from the video jack, giving you composite grayscales (with high persistence), so it's probably best to leave the possibility of connection elsewhere open by using a connector instead of a single piece of wire. This also requires monitor modification (part b) The 7404's only purpose is to invert the Horizontal Sync. signal from the Amiga, which is inverted from the PC signal that the 1901 expects. These chips are common and available anywhere. Diagram ------- This end plugs into the Amiga's monitor port. Alternatively, these lines can be taken from inside the machine. 1 2 3 4 5 6 7 8 9 10 11 12 Female 13 14 15 16 17 | 18 | 19 | 20 | 21 22 | 23 | DB-23 | | | | | | | | | | | | | | ?- - ? - -? | | / | | / | / / / / | |/ |/ ::14-----------' / | | | 7404 :::: / / | /| /| :::9--, / / | | | / '-------7::8------' / | | | | | / | | | | ,------------' / | | | | | ,-------------' 1 | 2 | 3 | 4 | 5 Female 6 7 8 9 DB-9 This end plugs into the 1901's monitor cable. If your screen is distorted, adjust the controls on the back of the 1901. [Up to here I have been faithfully repeating Earl Pottinger's printed specs. If you have problems at this stage (as I am!), I suggest that you post your questions rather than directing them to me. I have seen this working, and I have followed directions carefully, but they may contain a misprint! What I see is a sometimes unstable, sometimes rock-steady, image of 5 blurry hands, vertically compressed, holding Kickstart disks. The edges of the picture are jagged, so I suspect a Horizontal Sync. problem. That's what the 7404 is for, so I may have a bad chip, but replacing it didn't help.] b) Turning the 1901 into a composite grayscale monitor --------------------------------------------------- You must have part a) working before going on to this. This part involves changing your 1901 and voiding your warranty. Furthermore, *you are making modifications inside a HIGH VOLTAGE device that may contain dangerous static charges even when unplugged*. If you are not skilled in electrical work, *find someone who is*. The video line, the DB-9's pin 7, is presently plugged into one of DB-23 pins 7,8, or 9. The best source of analog video is the RCA jack labelled Video, or R9 internally. On the Amiga 500 and 2000, use the RCA jack labelled Mono. Hopefully you allowed for this rewiring, as advised, in the original interface. ** Unplug the Monitor's Power Cord and perform all other standard electrical safety precautions (if you don't know what these are, then you shouldn't be doing this!) ** Open the monitor and look for IC-601 which converts all inputs into true digital forms. Find resistors R609, R608, and R601. Disconnect or de-solder R608 and R609 *only* on the side nearest the IC-601. Bend the two resistors up so they are not touching the PCB. Solder a wire connecting the free ends of R608 and R609 to the side of R601 nearest the IC-601. Look at some colour pictures to verify the analog greyscales are working, *after* restoring the monitor to a safe condition before plugging it in. Other things to do ------------------ F02, a two amp fuse, may blow as Earl's did after several days of use. He suspects the overscan screens are responsible. It can be replaced with a three-amp fuse. F06 can be replaced also. Earl also complains of a little ripple down the side of the display, that he thinks can be corrected by replacing all 4 diodes (D101-D104) with larger 5404 diodes or just running a smaller screen like 680x440. His ripple is not very large and could be caused by his monitor, which is old. Replacing the C014 with a larger capacitor would not hurt, Earl says. HyperTech's manual suggests a resistor modification that he doesn't repeat but recommends. Perhaps someone with the HyperTech interface can comment? He adds a final caveat: Please remember that these suggestions are for people who understand what they are doing only. If you don't understand what is going on, you can cause harm, even destroy your monitor if you do the wrong things. And, folks, please post, don't write, if yours doesn't work. You know as much as I do. If I solve my problem, I'll post the solution. If anyone from HyperTech is reading this, may I suggest lowering your price closer to the $10 CDN maximum your interface costs to build, so you get customers among those who don't want to be bothered ? You've already managed to get the naive gee-isn't-this-amazing bunch. Presently they're charging $150 CDN for this. Perhaps $20 is more appropriate. If not HyperTech, then others can do it. Earl places his design in the public domain. Thanks. Questions, and flames will be passed on but don't expect quick answers unless a guru on the net can provide them. Craig Hubley, Unicus Corporation, Toronto, Ont. craig@Unicus.COM (Internet) {uunet!mnetor, utzoo!utcsri}!unicus!craig (dumb uucp) mnetor!unicus!craig@uunet.uu.net (dumb arpa)