Menu
| Background: |
| This is the version that works with just a switch to select modes. The theory behind this one is simple, there are two character generators in a TRS-80/SYSTEM-80, one makes the characters and the other one makes the graphics. The hardware decides whether it is about to draw a character or a graphic symbol and lets the appropiate character generator know, it then has a look at memory to see which character/graphic symbol it has to draw, and tells which ever generator is needed, which sends out the first row of dots in parallel (ie. 8 at a time). They are grabbed by a shift register and made to go in single file - (serial). These serial bits go through an OR gate so if there is a dot to be lit up from the graphics generator OR there is a dot to be lit up from the character generator then the gate sends a dot off to the sync section. If there IS a dot to be lit up, it is sent off to another section where it is mixed in with horizontal and vertical sync and sent of to the screen. What we are going to do is reverse all these dots BEFORE the get mixed in with the sync signals. (ie. turn all black dots-white and all white dots- black). To do this we are going to use an I.C. called an "inverter". This nifty little I.C. has SIX inverters in it (hex=six), we are only going to use one, the other five can be used for other mods. You will also need a switch to select modes. |
| What Parts do I Need? |
|
|
| How Do I Do It? |
|
|
| Important Note |
| Some monitors will NOT display inverse video, some of these are RITRONICS and BMC monitors. All tandy monitors and all dick smith monitors WILL work and most others will as well. in all cases an increase of the brightness control will be needed. it is perfectly all right to switch modes while the computer is running. |
Radio Shack technical bulletin - April 25, 1984
Ted Haigh
| This details the conversion, and installation, of a Model III Hires board, Cat # 26-1125, to a Model 4 computer. |
| First, Modify the Model IV |
|
|
| Next, Modify the Graphics Board |
|
|
| Next, Install the Graphics Board |
|
|
| Next, Test The Board |
|
|
Joe Kyle-DiPietropaolo
| The Problem |
| Fairly often I have been asked, "Gosh, why won't my hard disk and hardware clock work together? The hardware vendors say they should 'cause they are addressed differently, but they don't." |
| The Cause |
| On the Model 3 and 4, there is a signal line that is used to reverse the data bus on the external I/O bus so that data can be input to the computer. Normally, the data bus only "points out". When an external device "realizes" that it has been addressed, it can pull this line low to send data to the computer. |
| There are two basic methods by which the external device can control the lead. First, a "normal" TTL or LS-TTL output. This is the one that causes the problem. Second, an "Open Collector" output, which is the correct way. |
| Open collector outputs know how to share. Any device can pull the lead low without conflicting with any other device on the bus. Each may pull the line low, or do *nothing* to the lead. |
| "Normal" TTL signals, on the other hand, may pull the lead low, as above. The problem is that when a "normal" output is not pulling the lead low, it is pushing the lead high, as opposed to just leaving it alone. Now, when there is more than one device in the system, and one of the devices becomes active, it tries to pull the lead low while the rest try to push it high. The result is "contention" and somebody wins. Who wins is up for grabs. If the right party wins, the whole system may seem to work... for now. As the temperature of the components change, the system ages, etc., this can change. |
| If one or more devices in the system are open collector, and some are normal TTL, then some combinations of devices may work and others won't. If only one is TTL and the rest are open collector, the system may seem to work but that TTL device is eventually going to burn out. |
| A technically competent person armed with the schematic for the device in question should understand the problem if given this whole description. They should then be able to look on the schematic for the device and tell whether or not an open collector output was used. If all devices pass this test, then they may all be used together without conflict as long as they are addressed differently. |
| The Solution |
| A 7405 open collector hex inverter can be used to buffer or replace the output device used. Note that the 7405 will invert the logic of the signal, so you must either pick off the signal at a point that it is inverted to begin with, or run the signal through two sections of the 7405 with a 2.2K pull-up resistor on the output of the first section. The pull-up for the final output is already in the Model 3 or 4. |
| The 7405 is a fourteen pin chip, and may be piggy-backed to any existing fourteen pin chip in the device that uses pins 7 and 14 for ground and power. All leads except pins 7 and 14 are bent ninety degrees so that they point straight out. It is then placed over the "host" chip and pins 7 and 14 are soldered to the corresponding pins on the host chip. Wire wrap wire may be used to connect the 7405 inputs and outputs as necessary. If two sections of the 7405 are to be used (if the inverted sense of the output signal is not available), the pull-up resistor may be run from the output of the first section to pin 14 on the chip. |
| One trace cut should be necessary, to separate the output lead from its existing driver chip. |
| Disclaimer |
| As they say on TV, "This kind of stuff is best attempted by experienced people taking all the necessary safety precautions." If the above explanation and procedure did not make perfect sense to you, I recommend that you do *not* try this yourself. Find a local hardware hacker to help you, or take this explanation and the matter up with your various hardware vendors. |
Bryan Eggers
| Background |
| Here's a simple modification to speed-up some early Model 4s. This does not work on all Model 4's, but it may work on yours. There were a few variations of the Model 4 circuit board, but I've heard that this works on MOST of the early boards. I haven't tried it, so do it at your own risk! If you're successful, you'll eliminate the wait states and your Model 4 will run faster. This speed difference is very noticeable with Orchestra-90. |
| How To Do It |
|
|
| Conclusion |
| That's all there is to it! This supposedly eliminates the wait-states and gives you a "true" 4 Mhz. Perhaps it will work on your board. Larry Payne did this to his system and said that it worked perfectly, and I guess you could call him somewhat of an expert on Model 4's! |
| That's all I know about this little trick. Perhaps some others can tell you more about it. |
| Additional Comments from Ray Pelzer |
| On the later machines (like MINE), you can pull out the Z-80 itself, and bend up pin 27. That cuts it off right at the final in-point. |
Bob Griggs [70045,127] Co-SYSOP "Exclusive-80" TBBS.
June 27, 1985
| For the earlier 4P's with the white screen |
|
|
| For the Gate Array's with the green screen. |
|
Don't rely on page 132 of the 4P Service Manual #26-1080 for the correct IC labels because there are a few typo errors. Change these labels on page 132 before doing the work:
|
|
|
Convert 5" Drive Cable to 8" Drive Cable
| The following pin configuration will effectively translate the 34 pin cable required by the 5" disk drive to the 50 pin cable required by an 8 inch disk drive. In order to run eight inch drives, however, you will need to install a disk controller capable of eight inch operation. Homes Engineering and Micro Mainframe sell such disk controllers. |
34 pin connection 50 pin connection
pin 1...............pin 17
pin 2...............pin 22
pin 3...............pin 19
pin 4...............Cut
pin 5...............pin 21
pin 6...............pin 32
pin 7...............pin 23
pin 8...............pin 20
pin 9...............pin 25
pin 10..............pin 26
pin 11..............pin 27
pin 12..............pin 28
pin 13..............pin 29
pin 14..............pin 30
pin 15..............pin 31
pin 16..............pin 18
pin 17..............pin 33
pin 18..............pin 34
pin 19..............pin 35
pin 20..............pin 36
pin 21..............pin 37
pin 22..............pin 38
pin 23..............pin 39
pin 24..............pin 40
pin 25..............pin 41
pin 26..............pin 42
pin 27..............pin 43
pin 28..............pin 44
pin 29..............pin 45
pin 30..............pin 46
pin 31..............pin 47
pin 32..............pin 14
pin 33..............pin 49
pin 34..............Cut
|
Bob Haynes [73075,77] - September 24, 1989
| Much ado has been made of problems with the solder joints on the power supply boards where the output pins are located. But it's very easy to overlook the harness pin connectors! |
| In the 4P, the p/s connector is well out of the main flow of ventilation. Of course, it shouldn't heat up at all, but a bad connection produces resistance and will cause heat to be generated. This tends to weaken the spring tension of the female connector, the weakened tension causes an even poorer connection, resistance increases, and the problem worsens. You go in, look for the problem, wiggle the wires and the problem disappears. And because the 12v video connector pins are different than the others, they are especially susceptible. You can't determine the problem without actually removing the pins from the plastic connector housing. |
| After this problem became a nuisance on BOTH my 4Ps about a month ago, I really HAD to do something! Eventually I ended up removing the pins, cleaning them with pink pearl and alcohol, then bending them slightly with a small needlenose for more tension. Problems have completely disappeared in both machines. |
Luis M. Garcia-Barrio, 8/N/1 #004
| The USRobotics Courier 2400 Modem uses pin 12 of the RS-232C to indicate 'High Speed' to the computer. When it is set at the 2400 baud rate, (default), the HS LED in the front goes on and pin 12 of the RS-232C interface goes high. |
| The RS-232C interface of the TRS-80 Model 4 does not recognize pin 12 at all: it is connected to ground. |
| When the Courier sends the HS signal, the TRS-80 Model 4 sends it to ground, pin 12 goes low, and the HS LED does not go on. However, transmission is not affected. |
| An easy cure to have the HS LED function properly is to remove the wire that connects pin 12 at either end of the cable. |
Dan Hamilton [70625,1237]
| I bought an ATARI 400 and quickly got sick of that damn monopanel keyboard. Even with communications gear, it was a hit or miss proposition getting copies of programs off CompuServe, because with the small memory I must use unsophisticated terminal software. Little errors in file transfer were causing big problems. | |
| To overcome that I tried to find a way to use my TRS-80 Model III for the download portion, because it supports the CompuServe A protocol, has gobs of memory, and runs circles around the 400 except in the areas of sound and graphics, where the Atari makes the TRS look mighty poor. I assume similar transfer between an ATARI 800 and Model III is possible too, but I doubt there is any compelling need. | |
| So, I made up a cable to go from port #1 on the 850 interface to a black box, which looks fancy, but merely provides a place to hang two DB 25 connectors and some wire. | |
| Here is how to wire two female DB 25's together, and how to make the cable from the 850 to the black box. | |
| 1 - Cable) | Hook pin 1 on a male nine-pin plug to pin 20 on the male 25-pin plug Hook pin 2 on a male nine-pin plug to pin 8 on the male 25-pin plug Hook pin 3 on a male nine-pin plug to pin 2 on the male 25-pin plug Hook pin 4 on a male nine-pin plug to pin 3 on the male 25-pin plug Hook pin 5 on a male nine-pin plug to pin 7 on the male 25-pin plug Hook pin 6 on a male nine-pin plug to pin 6 on the male 25-pin plug Hook pin 7 on a male nine-pin plug to pin 4 on the male 25-pin plug Hook pin 8 on a male nine-pin plug to pin 5 on the male 25-pin plug Don't hook pin 9 on a male nine-pin plug to anything If you already have an Atari modem cable it is likely to be wired this way already. |
| 2 - Black Box) |
Wire two female DB 25's together in the following manner. (note: Model I users can skip this part; a TEL/COM switch on your RS-232 swaps pins for you). The object is to end up with pins 2 and 3 swapped, Wire from one connector's pin 2 to the other's pin 3 Wire from one connector's pin 3 to the other's pin 2 Wire from one connector's pin 4 to the other's pin 4 Wire from one connector's pin 5 to the other's pin 5 Wire from one connector's pin 6 to the other's pin 6 Wire from one connector's pin 7 to the other's pin 7 Wire from one connector's pin 8 to the other's pin 8 Wire from one connector's pin 20 to the other's pin 20 |
| 3 - Connect) | Hook the cable from the 850 to the black box, and hook the cable from the Model III RS-232 port to the other end of the box. |
| 4 - TRS-80 Software) |
Now just fire up a smart terminal program on your TRS. I use STERM, available on many BBS's, because it allows me to select speed of transmission and gives me a huge buffer. |
| 5 - Atari Software) |
Get MINITERM (from XA2) or, if you have more memory, use one of the fancier programs like JTERM or AMODEM, and you are ready to go. |
| 6 - Download) | Go online and use the VIDTEX Model III exec to download error free copies of SIG programs. |
| 7 - Transfer) | Fire up the terminal program on the Atari, turn on the capture buffer, (no pouncing on the START key because YOU choose when to start sending) and watch the goodies flow onto the screen in living color from your poor old monochrome TRS. |
| 8 - Save Buffer) | Then, simply save buffer contents the way your program specifies. You can even use the TRS keyboard to enter programs into the ATARI (as long as ATASCII characters are not needed or can be added later) by using the echo mode on programs like STERM. |
High Res (/HR) Graphic [No Idea What It Is]
[73327,442]
| 1) | Get a second supply, either a 65W or 35W version, be advised that there are/were two vendors for the supplies for the Mod4. One was Radio Shack, and the other was ASTEC, either supply will work. I personally used a 65W ASTEC due to the fact that the Radio Shack supply has quite a few Technical Bulletins out fixing various "little" problems with the supply, but generally it works if you don't push it too close to the 65W limit. If you do you will see the jiggly screen, and this is due to the supply going into current limiting mode, it literally starts to cut current to save its fanny from frying. |
| 2) | The next step is to open the cabinet, this is accomplished by removing the screws from the bottom of the computer, there should be ten screws when you are done, along with the one right in the middle on the back of the computer. Then you lift the top up "gently" and lay it to the side of the computer, it comes off, or rotates off toward the CRT side of the system. |
| 3) | Now you need to get a supply of MOLEX connectors that go on the new supply, corresponding to the power leads you want to run from the new supply. I would suggest that you run the Video and Floppy drives from one supply and the mother board from the other. The easiest way to shop for the connectors is to take the supply to the electronics store and match it up there instead of guessing. You will need a two position connector for the video, a three position connector for the incoming AC, and a 10 position connector for the floppy drive power. |
| 4) | Now you need to remove the keyboard, this is done by removing the screws you can see around the edge of the keyboard, then gently lifting it up and out of the way so you can get to the power switch located on the right underside of the computer. Solder two wires to the same two places on the switch you see the wires that go to the supply that is already in the computer. Then thread the wire under the drive housing and put the MOLEX connector on (The 3 position job) then you are done with the AC part the connector, the AC is labelled TB1 on the 65W ASTEC and J3 on the 65W Radio Shack supplies. |
| 5) | The video part is even easier, since you may be able to get the video connector to reach the new supply without having to do anything else, be sure to try to get it to reach before you make a new one. To get it to reach (Possibly) try uncoiling a turn or two of the video supply line from the big coil on the iron ring this should be mounted toward the to and on the inside (Near the CRT) of the mother board, just follow the wires till you come to it if you are confused as to what this piece is. The video connector goes on the very right hand side of the big long connector (if you are holding the supply with the big connector at thr top, the letters will be upside down) the Video is labelled COM and V2. So after doing this you are now ready to do the floppy drive connectors. |
| 6) | The floppies are just about as easy, just note which of the wires goes to what to the floppy drives. These are the wires that go up over the top of the supply and into the back of the floppy drives. Cut each wire then solder an extension to each of these and put them back into the same slots in the new MOLEX connector (This will be on the long connector) there will be three wires going to the floppy drives that you will have to cut and extend one will go to V1 one to V3 and one to COM. The connections will be in positions 3, 4, and 9, these numbers correspond to you looking at the connector as if you help the power supply with the plug in section at the top and count from left to right. i.e. position 3 is the third slot/peg from the left side of the power connection stip, or a COM, V1, and a V3 connection, all you need to make sure of is that you get the same wire from the old supply to the same slot on the new supply, wire color doesn't matter. |
| 7) | After you have done all the electrical work you are ready to mount the board. I used 8 nylon stand offs the ones that thread together to form the legs for the supply. I did this by putting one in each corner hole then screwing on another one above the board. This left the lower ones with a threaded hole to allow a screw to fasten in down, I then just lined up two of the legs with some of the slots in the bottom of the computer and then bolted it down by putting screws into these stand offs that were over the slots, you can drill holes to get the other two legs bolted down, but I doubt you would really need to unless you are going to take the Mod4 on a bumpy ocean voyage or something similar. Also don't forget to ground the new supply, you will see on the back of the supply under the corner hole in the upper left side (you are now looking at the back of the supply with the connectors at the top) is a grounding pad, just run a wire from the old supplys ground to this one and you are all done. Plug it all together turn it on and see if you got all of this right. |
| 8) | If you do not feel comfortable doing this modification see if there is a friend around that tinkers with electronics and is willing to help. It really is a fairly simple operation, but if you get wires crossed you can cause severe damage to your computer, so be careful, and if you are not sure call someone who knows. The Radio Shack Repair Center nearest you should be able to give you any extra needed advice, I have always had no problem with the Repair guys just the salesmen. Good luck!! |
Lyle Piner
| The process itself was not difficult, but I did encounter a couple of 'pitfalls' that resulted in not getting a working computer back until Tuesday evening (yes, I was getting worried!). To start at the beginning, when I visited the tent sale I noticed a large display of cassette based Model 4's on sale for $50.00. After picking out one that seemed to have all of its parts intact and lugging it into the store and plugging it in to see that it brought up ROM BASIC ok as evidenced by the familiar 'Cassette?', 'Memory?' and 'Ready' prompts, I paid my money and took my prize home. | |
| My first thought was to simply install the Model 4 motherboard and keyboard in my Model III. After some consultation and reflection, however, I decided it would make more sense to move the drives, drive controller, and RS232 board from the 3 to the 4. This offered the benefit of taking advantage of the newer electronics in the 4 and it would LOOK like a 4. With this plan in mind, I disassembled both machines and set about the task of removing and installing parts. The first difficulty I encountered was the fact that the ribbon cables that connect the floppy drive controller board and the RS232 board to the motherboard were different for the two machines. On the III, the connectors at each end of the cable are exposed on the same side, while the 4 required that the connectors be exposed on opposite sides. This is a function of a difference in the plugs on the motherboard. | |
| The plugs on the III have teeth on both sides of the plug, while the 4 plugs had teeth on only one side. The original cables are also a bit flimsy and don't lend themselves to successful modification. This meant putting everything away for the night with plans to call National Parts the next morning. The young lady I spoke with at National Parts was quite helpful and looked up the part number for me. She explained that they could ship the new cables that day and I would have them in two or three days. I explained to her that she was speaking with a desperate man and asked her to try to visualize my den floor with parts from two computers scattered about. She very kindly arranged for me to pick up the cables that afternoon at the Northside Drive outlet store (next door to National Parts). After picking up the cables (about $7.00), I headed home confident that my 'new' Model 4 would be soon be up and running! | |
| Here's where the REAL problem began! After reassembling everything and double checking all the connections, I plugged in the 4 and hit the power switch. No smoke, but no drives either! The Model 4 simply refused to recognize that the drives were there! I disassembled EVERYTHING and put it all back together slowly,rechecking every connection as I went - with the same result! The machine would come up in cassette basic just fine, but the drives wouldn't spin. More phone calls and consultations. Finally, it dawned on me that I had forgotten one of the basic rule of electronics (though I had been warned): Make sure nothing is grounded where it shouldn't be! | |
| The culprit in this case was the floppy drive controller board! It didn't have the insulators on the back. The mounts on the Model 4 chassis are slightly different from their counterparts on the Model III chassis - enough different to cause a ground (short). I fabricated suitable insulators, reinstalled everything and the 4 booted just fine with drive 0 spinning and the 'Diskette' message appearing on the screen. | |
| I did have to make one more trip, this time to the auto parts store to get a supply of wire ties before putting everything back together (those wire bundles looked like they could be a real problem if not securely fastened). | |
| Would I do it again? You bet, but I'd do it a little differently of course as a result of what I learned in the process. The end result is very much worth the investment of $57.00 and the time required to swap parts. I've only been using the 4 for a few days now, but I LIKE it! From a hardware standpoint, the Model 4 keyboard is greatly improved over the 3 and of course the 80 by 24 display is much more convenient. Model 4 software (public domain) seems to be much more plentiful as well and you can still run Model III software in the Model 4's 'Model III' mode. | |
| If you decide to tackle such a project, I've included some tips below that you might find helpful. | |
| 1. | Get the required parts first: |
| 2. | Find something else for your six year old to do instead of 'helping'! |
| 3. | Stake out an area you can control. (I chose the floor of my den, but leaning over on your hands and knees does get painful after awhile.) |
| 4. | Carefully disassemble the machine by placing it on its side and removing the screws from the bottom (10) and the middle of the back (1). Three different size screws are used on the bottom so keep track of which screw came out of which hole. After the screws are removed, right the machine and GENTLY lift off the top. The back of the CRT is vulnerable in this step. You must lift the cover straight up or angled slightly towards the front so that the back of the tube will clear the wiring and the aluminum shield housing the boards. On your Model 3, you can look through the drive 1 hole to make sure it clears. If you feel resistance, don't force it. Once the cover is up and the CRT is clear, lay the cover over on its side to the left of the base. There are two connectors (one ground, one card edge connector) connecting the video to the base. Disconnect these and place the top in a safe place out of the way. |
| 5. | Remove the screws holding the aluminum panel in place and remove the panel exposing the mother board. |
| 6. | Disconnect all the plugs around the edges of the mother board. |
| 7. | Remove the screws holding the mother board in place and gently remove the mother board, setting it aside in a safe place. |
| 8. | After the above steps are performed on both machines, disconnect and remove the drives from the 3. |
| 9. | Unplug the power supply on the side of the drive tower and remove the drive tower. Reinstall the drive tower in the 4 case and reconnect the power supply (make note of the orientation of the plug in the 3 and plug it in the same way in the 4). |
| 10. | My 4 had some type of network board installed behind the mother board. If your's is the same, remove this board and all associated wiring (here's where you'll have to cut the old wire ties). |
| 11. | Remove the floppy drive controller board and RS232 board from the III and reinstall in the 4 making sure to insulate between the back of the board and the aluminum frame. The RS232 board plugs into the main power supply and the FDC board plugs into the power supply on the drive tower. Plug your new ribbon cables into these boards making sure that the exposed conductors will be in the correct position to contact the model 4 board's contacts when the cable is looped over the top of the board (opposite from the side they were on in my 3). |
| 12. | Reinstall and reconnect the drives. |
| 13. | Reinstall the model 4 mother board (again making sure of proper insulation behind the board) and reconnect all plugs. |
| 14. | Secure all wiring with wire ties. |
| 15. | Using a pair of wire cutters, cut the plastic around the edges of the plastic blocks covering the drive holes in the top of the Model 4 case. Gently 'break' out these plastic covers, remembering that you will want to use them to cover the holes in the 3. |
| 16. | Reconnect the two leads to the Model 4 video. This would be a good point to plug in the 4, cross your fingers, and apply power. If all is well, drive 0 will light up and spin and the monitor will display the 'Diskette' message. On the 4, the drive will keep spinning until you insert a disk. Insert a system disk and make sure it boots normally. If it doesn't perform as above, its time to unplug and go back and check all your connections. |
| 17. | If you got through step 14 ok, replace the aluminum cover over the boards and replace the top (again making sure you don't damage the back of the tube), replace the screws, and begin enjoying your new Model 4. Don't get so caught up in the 4 that you forget to put the 3 back together, remember, it's still a good machine and can run cassette based software (a present to your six year old for staying out of the way?). |
| BUT .... | |
| According to page 5 in the hardware section of the manual, a motherboard configured for 16K operation (jumper positions and capacitors) uses three voltages (+12, +5, -5) for 16K RAM Chips while 64K rams require only +5. When power is applied to the replacement motherboard with the jumpers and capacitors positioned for 16K operation, the 64K rams will get 'zapped' when hit with +12 volts. Modify the board to 64K operation before using non-16K chips or you WILL lose all your RAM chips! | |