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Text File | 2010-04-21 | 16KB | 333 lines

DIGITAL JUNCTION ---------------- The right connections are made here. In this column I intend to show you how to interface the ST to various devices and projects via the cartridge or MIDI ports. This will introduce you to an area of computing that you may find as interesting if not more so than playing games or word processing etc and will perhaps encourage you to carry out your own interfacing experiments. Starting with details of the cartridge and MIDI ports I shall move onto some simple interfacing experiments using LEDs (Light Emitting Diodes), switches and relays etc. I will then cover more advanced topics such as ADC (Analogue to Digital Conversion) and DAC (Digital to Analogue Conversion) along with some useful projects. After that? Well, we'll have to see - Maybe a computer controlled weather monitor or some simple robotics such as a computer controlled buggy - I make no promises however. The Cartridge Port ------------------ Unlike earlier micros such as the C64 or Spectrum the ST does not have a general purpose interface or bus extension to allow easy expansion or interfacing of users own projects. The nearest the ST has to offer is the cartridge port which was intended for alternatives or extensions to TOS but has been used for many other purposes, the most common probably being sound samplers or real time clocks for example. Although not as flexible as a purpose built interface or bus extension the cartridge port is quite capable of being used as an interfacing port thus enabling various devices and projects to be connected to the ST. It is even possible to adapt and connect projects that were intended for other micros. There are 16 data lines (D0-15), 15 address lines (A1-15), 2 ROM enable lines (ROM3 & ROM4), 1 Upper data strobe (UDS), 1 Lower data strobe (LDS), 2 +5V lines and 3 ground lines available at the cartridge port. You may have noticed the A0 line is missing, this is due to the fact that the 68000 CPU (Central Processing Unit) can only address even memory locations. However, the lower data strobe can be used to provide the A0 line if needed (This will be covered later). Cartridge port pin-out 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 ---------------------------------------------------------------- | . . . . . . . . . . . . . . . . . . . . | | . . . . . . . . . . . . . . . . . . . . | | | ---------------------------------------------------------------- 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 1 = +5V DC 21 = A8 2 = +5V DC 22 = A14 3 = D14 23 = A7 4 = D15 24 = A9 5 = D12 25 = A6 6 = D13 26 = A10 7 = D10 27 = A5 8 = D11 28 = A12 9 = D8 29 = A11 10 = D9 30 = A4 11 = D6 31 = ROM 3 SELECT 12 = D7 32 = A3 13 = D4 33 = ROM 4 SELECT 14 = D5 34 = A2 15 = D2 35 = UPPER DATA STROBE 16 = D3 36 = A1 17 = D0 37 = LOWER DATA STROBE 18 = D1 38 = GROUND 19 = A13 39 = GROUND 20 = A15 40 = GROUND As the port was designed for use with ROMs (Read Only Memory) there is no read/write line provided so any attempt to write to the port will result in a bus error. Even if a read/write line was brought out to the port an error would still occur as the GLUE chip knows that this is a read only port and objects to any attempt at writing to it. However, all is not lost, a programming trick known as address offsetting is used to write to the port. This places the data to be written on the address lines rather than the data lines and is then latched into the interfacing circuits when needed. Due to Atari using a non-standard pitch for the cartridge port connector it is rather difficult to find a suitable plug to fit it. I have not been able to find a supplier and if anybody knows of one please tell me. I have overcome the problem by installing my own connector in parallel with the cartridge port. Simply, this involved soldering two lengths of 20-way ribbon cable at the points where the cartridge port connector pins enter the ST's main PCB (printed circuit board). I was unable to find a space to install my connector into the case so I brought the ribbon cables out from under the cartridge port and soldered a 37-way D type connector to the ribbons making sure to leave enough length to allow a cartridge to be installed without fouling on them. This does mean that I now have a cable and D connector hanging out the left hand side of my ST but its no problem to me. After all it does mean that I am able to connect any projects or experiments using cheap and readily available connectors. A couple of points should be noted. (1) Carrying out this type of modification would invalidate any warranty that may cover your ST. (2) You would not be able to use any cartridges while having interfaces connected to the additional connector and vice versa of course. If anyone would like further details of my modification then please send a SAE and I shall return full details and diagrams. If enough people are interested then I may include the details in this column. For those that would rather not hack their ST about, after all it's only nutters like me that would even think of such a thing, but would still like to interface projects or circuits via the cartridge port, do not fear, there is a far less drastic alternative. It is in the form of an interface which slots into the cartridge port providing a fully buffered 16-bit wide input, a latched 16 bit wide output and some control lines for advanced applications. The interface also overcomes the non standard edge connector problem by providing a board interface which allows 0.1 inch pitch double sided edge connectors to be plugged in. There are a variety of cheap readily available edge connectors that can be used, Locost PC edge connector 2x25 with closed ends or 50-way IDC socket for ribbon cable for example. The interface, costing about 25 pounds well over a year ago, was designed, built and supplied by Mr. W.M.Kennerley (see 'Suppliers' for address). I have recently been in contact with Mr. Kennerley and he has informed me that he is still able to supply the interfaces. Those who would like one should contact him for details of price and delivery. If writing to him please don't forget to enclose a SAE. I do feel that anyone really interested in interfacing should consider purchasing one of these units, after all, for less than the cost of a full priced game or serious software you are getting a really useful bit of kit. As mentioned earlier it saves any hacking of the ST plus the system busses are protected by the on-board buffers thus saving you having to include them in your own designs and Mr. Kennerley has provided the full sixteen bit wide input and output which makes the unit very useful indeed. I shall be supporting this interface with software and projects throughout this series and will do like wise for those that would rather connect directly to the cartridge port. The MIDI ports -------------- If, like myself, you are not into music or don't own any MIDI (Musical Instrument Digital Interface) equipment then ST's MIDI sockets would appear to be of little use to you. This, however, is not so. The MIDI ports work in the same way as any other serial port. The serial data format used on the MIDI ports has the same specification as the RS-232C protocol used on the serial or modem port except that the MIDI transfer rate is fixed at 31.25KBaud. When data is sent or received it starts with a 'start bit' which is always a 0 then follows the eight data bits (one byte) starting with the LSB (least significant bit) and finishes with a 'stop bit' which is always a 1. From this it can be seen that the MIDI ports could be used for general serial communications. There is a multi-player game, MIDI MAZE, that allows up to 16 STs to be linked together as a network and play against each other through the MIDI ports. Programming the ports to send or receive data is catered for in the BIOS routines bconout, bconin and bconstat. GFA BASIC v3 accesses these routines with the following commands:- bconout = ~BIOS(3,3,b%) where b% = data byte to send bconin = r% = BIOS(2,3) where r% = data byte read from port bconstat = r% = BIOS(1,3) Interrogates input buffer r% = 0 if no byte available r% = -1 if a byte is ready In order for this port to be of use to us for interfacing, some circuits capable of serial to parallel and parallel to serial conversion will be needed. This is fairly easy to do using a UART (Universal Asynchronous Receiver/Transmitter) but there is a less expensive way of accomplishing this by using some standard CMOS ICs. I have come across a project first published by M. Breuer in the April 91 issue of Elektor Electronics magazine that does exactly this by turning the MIDI ports into an 8-bit in/out interface. This interface cannot pass data as fast as the cartridge port nor is it as flexible in use but it is perfectly capable of most interfacing tasks and is an ideal project for inclusion in this series. Like the cartridge port, I shall be supporting this interface throughout the series. I chose the MIDI interface project as an alternative for the following reasons:- (1) It puts to use the MIDI ports that would have otherwise remained idle (at least in the case of those that don't have MIDI equipment) (2) An opto isolator has been included on the interface board MIDI input which complements the isolator already built into the ST's MIDI input. This has the very desirable effect of protecting the ST from damage due to any mishaps that may occur with projects attached to the interface. (3) You can still carry out interfacing but still have your favourite cartridge plugged in plus it is fairly simple to program. The interface has been designed to be built on a double sided PCB which is available from Elektor Electronics readers services but this will cost £13.34 including VAT and P&P. It is possible to build the interface on a piece of stripboard (Veroboard) costing about £1.35. I have designed a stripboard layout for the project which I hope to have built and tested in time for the next article. Perhaps I should point out that as far as possible I intend to make sure that it will only be necessary to use the cartridge port OR MIDI port interface to follow this series. The choice as to which one to use is yours but I do recommend the cartridge port as your first choice as it is the more flexible of the two. However,as I said earlier, the MIDI interface project is capable of carrying out most interfacing tasks and is an ideal alternative particularly if your cartridge port is tied up with other devices that you may wish to keep on line. Close Box --------- A lot of blurb in this first article but we will get down to some practical work next issue so get your soldering irons ready. Next Issue ---------- Full details of parts required and building of the MIDI Interface. More on buffering the cartridge port. Stay connected...... NOTES ----- All modifications, experiments, hardware and software discussed in this column are tested on an Atari STFM with high resolution monitor but should work on other set ups. If you spot an error or know of a better way of achieving something in the hardware or software then please let me know and I will pass the information on through this column so that we may all learn. This, after all, is what STEN is all about. Whilst every effort has been made to make sure that the information given in this article is correct neither I or STEN can be held responsible for any damage which may arise due to you carrying out modifications or connecting any devices directly or indirectly to your ST. "By Order of der Management" ************************************************************************* * Suppliers * * * *Cartridge port Expander from:- * *Mr W.M.Kennerley. 90 Lingfield Road, Stevenage,Herts. SG1 5SN * *Tel. 0438 352847 * *-----------------------------------------------------------------------* *MIDI 8-Bit Interface * *Elektor Electronics (Publishing), Down House, Broomhill Road, London * * SW18 4JQ* *PCB £13.34 inc. * *Software on disk £8.64 inc. * ************************************************************************* ************************************************************************* * Bibliography * * * *Concise Atari St 68000 reference guide by K.D. Peel * *Atari ST Internals by Bruckmann, English, Gerits * *ST Applications- Internals series by Paul Rossiter * *GFA BASIC V3.6 Manual * *April 91 issue of Elektor Electronics * ************************************************************************* ************************************************************************* * Dave Price. 42 Foxglove Green, Willesborough, Ashford, Kent TN24 0RJ.* ************************************************************************* -oOo-