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CV/Gate control from RS-323 possible?
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patrickvf1976



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PostPosted: Fri Mar 29, 2013 7:12 pm    Post subject: CV/Gate control from RS-323 possible?
Subject description: talking about vintage home computers e. g. TI99/4A
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I have this really old computer, a Texas Instruments TI99/4A and I was wondering if it was possible to send CV and Gate/Trigger signals from it's RS-323 (or is it centronics?) ? Has this been done before? Is it just a matter of splicing CV/Gate jacks to a RS-232/Centronics cable and program the CV/Gate values in Basic (or Assembler) or what would I have to do/build/download to make it work?

Greetings, Patrick
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JovianPyx



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PostPosted: Sat Mar 30, 2013 4:36 am    Post subject: Reply with quote  Mark this post and the followings unread

My answer off the top is "no" - at least not "easy".

An RS-232 serial port provides an 8 bit data output that is serialized. There are also control signals that are level operated (such as Clear To Send or CTS). CTS might be used as a "gate", but that would depend on what control BASIC or assembly entry points allow. So gate might be pretty easy.

To get a CV you would have to convert the serialized data to a voltage. This would require a DAC (8 bit max since the data stream contains only 8 bits of data) that has a built in UART. I can't remember if I've ever seen something like that. It is true that many if not most DACs these days are serial, but "serial" is a rather broad term. SPI, for example, is a serial protocol, but it will not work with RS232 because SPI is physically clocked by the interface where RS-232 signals have an embedded clocking system.

It is definately possible, but probably requires something like a PIC or other microprocessor (or a lot of CMOS chips) that has a built in UART which can then translate the serial data to parallel so that the data can be presented to a DAC of some kind. And if you're going to use a PIC or other modern microprocessor then using the TI seems a bit moot.

If the TI can support memory mapped I/O, then it should be possible to connect an address decoder and latch to capture 8 bit parallel data and present that to a DAC-08 chip with a small number of parts. Sorry - I don't know the TI-99/4A in particular so I don't know if it has a parallel port - but that would be the easiest way to make CV come out of it.

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richardc64



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PostPosted: Sat Mar 30, 2013 5:27 am    Post subject: Reply with quote  Mark this post and the followings unread

JovianPyx wrote:

If the TI can support memory mapped I/O, then it should be possible to connect an address decoder and latch to capture 8 bit parallel data and present that to a DAC-08 chip with a small number of parts. Sorry - I don't know the TI-99/4A in particular so I don't know if it has a parallel port - but that would be the easiest way to make CV come out of it.


No parallel port, but address, data, and some control signals are available, so decoded I/O seems possible. Some "blocks" are already decoded.

http://www.digitpress.com/library/techdocs/99-4a_technical_data.pdf

Old school interfacing.

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Thomas_Henry



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PostPosted: Sat Mar 30, 2013 10:58 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi,

I actually designed an RS-232 drum interface some twenty-five years ago which provided eight triggers. You'll find it in Electronic Musician, February 1986, pp. 38-40, 42. No microprocessor is required, and the programming was handled by the built-in Basic interpreter of the driving computer. I used a cheapo Radio Shack home computer to fire it.

The main component is the AY-3-1015D UART which used to be available at Radio Shack back then. The circuit could easily be changed to CV output with the addition of a DAC0800.

So, yes, such a rig would be possible without much botheration. However, I think a MIDI interface would be more universal.

Thomas Henry
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patrickvf1976



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PostPosted: Sat Mar 30, 2013 11:58 am    Post subject: Reply with quote  Mark this post and the followings unread

true, but midi doesn't work on old cv/gate machines without an expensive MIDI to CV/Gate device, and since I already have a machine with a general purpose interface, I figured that there might be a dead simple way to control CV/Gate machines with it. Is the UART based circuit in between really necessary? Doesn't the interface already produce a control voltage signal of sorts? Not questioning the necessity of your device, but I was hoping to save a little extra work and use what I have. Do you have a spare one that you might be able to sell for some pocket change?

PS: I just came across this post, which includes a link to a page about a rs-232 to CV/Gate circuit for 486 computers. I wonder if that would work in my TI hmmm...

Last edited by patrickvf1976 on Sat Mar 30, 2013 12:31 pm; edited 2 times in total
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JovianPyx



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PostPosted: Sat Mar 30, 2013 12:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

A serial output from a computer has no control voltage capability built into it. There are only digital signals available.

Thomas Henry's idea is about as simple as it gets for using a computer's serial output to generate voltages. Essentially, by tying the UART (with a bit clock) to a DAC0800, he has created a serial DAC. The control voltage created by the DAC0800 will have a resolution of only 8 bits at best.

Gate can come from a flow control signal, but it will likely need to be limited in some way since it can swing from -15 to +15 volts, however, it is a logic signal and should be settable from within the TI computer.

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gdavis



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PostPosted: Sat Mar 30, 2013 12:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

First you're going to have to straighten out your interface question. RS-232 is serial, Centronics is parallel. Two different beasts so you need to decide which you're actually planing to use before going forward.

If it's RS-232 then yes, a UART is necessary to deserialize the data. If it's Centronics no UART is necessary.

Both interfaces are digital - either on or off - so no CV directly from them, you need some sort of DAC.

A parallel port could be used as gates pretty easily, you might need something to adjust the levels or buffer it. It's only 8 bits though, so for CV you'd need some sort of multiplexing scheme.

I don't know what you consider "expensive" but people have made midi-CV/gate with an Arduino board.

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patrickvf1976



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PostPosted: Sat Mar 30, 2013 12:52 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ok, I got it back out of the attic and took a good look at it - It seems to be neither RS-232 nor Centronics or if at all it's got to be a proprietary version of it. In the manual it's only called an expansion port. weird. Other ports I found are proprietary joystick, tape and video ports. Oh, what was I thinking - I'm not a programmer nor a technician. I guess I'll be better off getting a MIDI to CV device.
PS: What I consider expensive - with my current budget anything over $50 is out of my league
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