I bought a Radio Shack Minisette 9 which is the recommended cassette recorder for use with the TRS-80 PC-1. I have two other mono cassette decks, but neither of them work correctly. One doesn't seem to have enough volume (either recording or playback) and the other cannot erase previous recordings for some reason. Troubleshooting did not reveal any obvious reasons, so I decided to buy the Minisette.
The first thing I did was replace the rubber belts as these tend to lose elasticity or sometimes deteriorate alltogether. After doing that I encountered a strange thing. The cassette player was in record mode despite the record switch not being engaged. I discovered that when I had it apart I did not have a hook placed inside the record switch loop, thus the switch could not change state. Once that was fixed it worked perfectly. The first thing I decided to try it out on was on my TRS-80 Model 100. After adjusting the volume and tone, I was able to get the computer to successfully read and write to a cassette tape. After that, I tried using it with my PC-1 Pocket Computer. After some further minor adjustments, it seemed to work just fine. Now I can save any BASIC programs I type in onto tape. Now that I have proven that it works as a storage unit for computer programs, I can use it on my MIL Mod-8. A few months ago, I had a modified Mod8-6 board with cassette interface made up so that I could experiment with Robert Swartz's MIL cassette solution. So far I haven't been successful. The code clearly writes data to the tape, however when it is read back, it doesn't match. There is some troubleshooting left to do on this. Craig Andrews designed and built this amazing EPROM and Bus Monitor board for the MIL Mod8/80. It also include a handy way to single step, automatic slow-step a few other nice features. Check out his YouTube video below for more information. Craig has some other interesting videos, many concerning the MIL Mod8/80 at his YouTube Channel Hello World. With some help from Craig Andrews in the MIL Mod8 Facebook Group, he noted that CE2 had to be HIGH. I should have caught that when designing the board, but I assumed the 2101 was identical to the MIL 2113 except for the pin locations. Apparently on the MIL chip, CE2 was really !CE2, as that was one of the pins tied to ground.
Once that was sorted out, I went to work lifting pin 17 on the RAM sockets and jumpered them to Vcc. Then the board worked 100%. This was a bit of a pain to do, however to fix this for the next run, is not a problem as I updated the Eagle drawings. Today I finished assembling my modified Mod8-6C board. Below you can see how it compares to the standard Mod8-6 input card. As you can see, the cassette interface circuit fits nicely into the top half of the board. I wonder if MiniMicroMart or anyone else used this solution in the past. It has one huge advantage in that you do not have to give up one of the I/O cards to use it.
Next up is assembling the Mod8-9B and testing of both boards. Then board testing. Today I finished up assembling the Mod8-9B board. Below is ab image of how it compares to the original MIL Mod8-9 board.
So far it appears to work fine, except for the RAM portion. For some reason the 1 k block shows up as 377 (FF) when I read it. Writing data has no effect. Troubleshooting will have to wait until tomorrow. Two new Mod 8 prototype boards arrived have just arrived from China. One is a Mod8-9B (left) and the other (right) is a modified Mod8-6 board with cassette interface. I have been waiting to try out Robert Swartz's MIL cassette solution ever since I came across it.
Mod8-9B The Mod8-9 was a combo board that had both RAM and ROM on it. The original board had a 2k 8316 ROM that contained a copy of Monitor 8 along with Robert Swartz's MIL cassette routine (located at 007000-007377), 1k of 1702A EPROM and 1k of MIL MF2113-2P RAM. Since the MIL RAM is nearly impossible to get a hold of, as is a pre-programmed 8316 with Monitor 8, I decided to use the more common 2716 EPROM and Intel 22-pin 2101 RAM instead. Unfortunately neither are a 100% pin compatible with the original chips, so the board had to be modified. No documentation was found for the MIL MF2113-2P RAM, so the pin-outs had to be reverse engineered from the original Mod8-9 board layout. Only two pins that could not be determined with certainty, these were CE2 and OD, however this did not matter for this application. The modified board was given the designation Mod8-9B to distinguish it from the original. Mod8-6C Robert Swartz's MIL cassette interface requires that the card be inserted in one of the two I/O slots in the Mod8-8 board. This would mean that you could not have one of the Mod8-6 or Mod8-7 boards in the system if you wanted to use the cassette to load or save data. Fortunately there is plenty of room on the Mod8-6 board for additional circuitry. It was not too difficult to add the cassette interface to that card. The result was what I call the Mod8-6C (C for Cassette). Over the next few weekends I hope to test these out and make sure they work before I order a production run for sale. Two new reproduction MIL Mod 8 boards on their way. The first one is a modified Mod8-6 board (called the Mod8-6C) which contains Robert Swartz's MIL cassette circuity (see Robert Swartz's Data package in the files section).
The second board is a modified MIL Mod8-9 board that uses a 2716 EPROM and 2101 RAM. The original used a 8316A ROM and MF2113 RAM, which is probably impossible to get. This one I call the Mod8-9B. These should come in about a week or so. Today, I received a copy of a document titled, "Microprocessor Seminar 8008, 8080 : 3-daagse studiebijeenkomst Jaarbeurs Utrecht, 25, 26 en 27 maart 1975". This book came up in a world wide library search using the search term "Microsystems International". It sounded like it had lots of potential. Perhaps it had some information on the elusive MF8080 chip. As it turned out, it did not. It did however have some very in depth information on the 8008 processor, using the MIL Mod 8 computer as an example. Some portions are in English, but the majority of the document is in Dutch. It will need some translating, but I have posted it as is on my Mod 8 documentation page.
The seminar was 8008, 8080 seminar was held at the Jarbeurs Utrecht, which is a sort of technical fair center. It was given by M.P.J. Stevens and B.Veldstra of Technische Hogeschcol te Eindhoven (now the Eindhoven University of Technology).
|
AuthorCharles Baetsen holds a Bachelor and a Master's degree in Engineering Physics from McMaster University in Hamilton, Canada. Archives
February 2024
Categories
All
|