• Technically Speaking

Magazine Archive

Home -> Magazines -> Issues -> Articles in this issue -> View

Technically Speaking (Part 2)

Part two in our build-your-own-MIDI link for a BBC and DX21.


The column that builds, literally. Andy Honeybone, a computer literate, continues his construction of a MIDI link for the Yamaha DX21 and BBC micro. Sequencing is only a step away...

Last Month we began the design of a MIDI interface for a BBC micro. In this instalment the hardware description is continued and brief programs for testing are presented.

The hard work of parallel-serial interconversion is performed by the 6850 ACIA chip. The receive and transmit clocks are provided from the flip-flop and the chip select lines CS0 and CS1 are connected to the output of the NOR gate which combines the low and high order address decodes. The modem lines NDCD and NCTS are taken to ground along with the NCS2 chip select. Output enable is driven by the 1MHz clock and the Read/Write line is taken to the bus connection of the same name. This leaves the 8 data lines which are taken straight to the bus along with address line zero and, finally, the NOT Interrupt Request which may be used to generate NMMIS or NIRQs as desired.


It is the optocoupler input that differentiates this circuit from a bog-standard RS232 interface running at a funny baud rate. MIDI-in provides the current to drive the LED of the isolator and the only other circuitry required on the synth end of things is a current limiting resistor and an inverse polarity protection diode. The phototransistor side of the coupler needs just one resistor which you might like to know took me three-quarters of an hour poring over abandoned circuit boards to find. It happens to the best of us.

MIDI-out is even simpler - just a pair of inverters to provide current drive, and a resistor. Note that pin 2 of the out DIN socket is earthed to provide shielding for the twisted pair MIDI cable (1.5 metre HiFi cables worked OK for me). Do not earth the MIDI-in socket or you will have undone all the good of opto-isolated hum loop prevention.

Those of you who will not rest until every element of every IC package is used up will no doubt be wondering what to do with the three spare 74LS04 inverters. Two can be used to provide a MIDI-thru facility by duplicating the MIDI-out buffer and connecting it to the receive input of the 6850. The last spare inverter can be used to provide another MIDI-out by connecting it to the output of the first inverter after the 6850 transmit pin.

Before you all write in to tell me that I have transgressed the Acorn One Megahertz Bus Commandment by putting more than 1 LS-TTL load on each line, let me say a few words in my defence. It works! It has been noted elsewhere that even Acorn do not follow their own teachings. Seriously though folks, building an interface is potentially putting at risk both pieces of the kit you're trying to join up. Brown corduroys are recommended for the first time you power up the BBC and interface and the screen won't come up with a sign-on message. I wouldn't like to have built the thing without having my trusty oscilloscope handy to check each stage of the construction but then I was designing it as I went. You should be able to manage with just a multi-meter for checking shorts.

(Click image for higher resolution version)


Series

Read the next part in this series:
Technically Speaking (Part 3)



Previous Article in this issue

Playing The Perfect Gig

Next article in this issue

Synth Sense


Making Music - Copyright: Track Record Publishing Ltd, Nexus Media Ltd.

 

Making Music - May 1986

>

Should be left alone:


You can send us a note about this article, or let us know of a problem - select the type from the menu above.

(Please include your email address if you want to be contacted regarding your note.)

Topic:

Computing

Electronics / Build

MIDI


Series:

Build a MIDI Interface

Part 1 | Part 2 | Part 3


Feature by Andy Honeybone

Previous article in this issue:

> Playing The Perfect Gig

Next article in this issue:

> Synth Sense


Help Support The Things You Love

mu:zines is the result of thousands of hours of effort, and will require many thousands more going forward to reach our goals of getting all this content online.

If you value this resource, you can support this project - it really helps!

If you're enjoying the site, please consider supporting me to help build this archive...

...with a one time Donation, or a recurring Donation of just £2 a month. It really helps - thank you!
muzines_logo_02

Small Print

Terms of usePrivacy