Forum Rules and Etiquette

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SOL-20 parallel port (Register)

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    SOL-20 parallel port (Register)

    I have been playing around with the parallel port in the SOL. It has separate data in and data out connections. There are two variations depending on the revision of the SOL, for some odd reason on one version they reversed the data input connections, D0 to D7 being on pin 6(D0) to pin 13, and on the other version D0 is on pin 13 and the array is reversed. The schematic in the manual is not correct for my SOL's version and only matches one revision.

    I had used the output data connections before in a program I wrote to put out bytes corresponding to a low frequency sine wave, the output port works well. Since when data is written to the output port, it stays there until updated or the output disabled (there is an input for that) I linked the data outputs to the inputs on a small connector. That way I could test the port. It is like having an additional register, you can write a byte and it is there anytime later !

    In any case what I wanted to do was set up some input sense switches on the port. The SOL does have sense switches but they are awkward to get at. So I made a plug in connector. The input data lines don't actually have any pull up resistors, and there is no 5V available on the 25 pin connector. But in the open condition because they are TTL inputs they assume a high logic state, albeit not very noise immune.

    I once asked about the utility of sense switches. I can see the value of them being able to change data while a program is running. For example, in PT's VDM-1 manual, there is a training exercise to manipulate the switches to see the effects of altering the card's registers on the display. I want to learn that to help me better understand how to scroll displays etc.
    Attached Files

    I just found out something interesting that could also help repairing a SOL-20.

    PT's VDM-1 card will run in the SOL, along with the other effective duplicate circuitry on the SOL mobo.

    The VDM-1's status port can be manipulated with a small program that reads the parallel port in a loop, then using my switches on the parallel port, the upper and lower nibbles of the byte loaded to the status port of the VDM-1 can be manipulated. The lower nibble selecting the beginning displayed line and the upper nibble the first displayed position. When bit 2 is high (bit 1 = D0, bit 8 = D7), the video output of the VDM-1 card exactly matches the SOL-20's video out. The only difference is that the SOL is not controlling the status port on the VDM-1 so the scroll is non operational.

    However, this made me realize that a VDM-1 card is a very useful "diagnostic tool" to have for a SOL-20, in the event that there was a failure in the video generation/character circuitry on the SOL's mobo, the VDM-1 can be used as a tool to see if the problem is in that sector of circuitry, or elsewhere and if it is in that sector of the SOL mobo, the VDM-1 provides a working hardware reference for scope comparisons.


      Actually it is better when running the VDM-1 in the SOL to deactivate its data output port, since it is normally set up at the same address as the video generator in the SOL, otherwise the 8T97 IC's in the VDM-1 can fight the 74LS367's on the SOL Mobo driving the data lines. It is easily done by insulating the connection to PDBIN on the edge connector with some thin polyamide tape and grounding the IC input that PDBIN was feeding with a 100R resistor. This way no tracks need to be cut. Also on my VDM-1 there are no IC sockets, except the character gen IC.