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1993-04-07
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\SETUP
\LM10,\RM65,\TM3,\BM3
\TEXT
HDEMUL - Hard Drive Emulator interface construction and theory.
by Roger Lacey April 1992
HDEMUL by Peter Welham is a complete substitute for the SIO2PC software.
Here follows a complete substitute for SIO2PC hardware interface which is
both cheaper and more reliable than the original.
Background
1. HDEMUL allows any Atari 8-bit 400/800/XL/XE computer to connect to and
utilise their PC as a file-server connected to the normal 13 pin SIO bus.
2. Access to up to eight emulated drives is enabled by the manipulation
of Atari disk images stored as standard PC MSDOS files. (The files on the
PC hard or floppy drive are accessed directly when loaded).
3. Compatibility with files created with SIO2PC is retained.
4. The disk images are configurable in size from 90 Kilobytes up to 16
Megabytes and so can readily act as a hard drive albeit running at floppy
drive speed.
5. Transfer of files between the two computers is possible with the Atari
software provided with the HDEMUL MyDOS disk image.
6. No special drivers are required at the Atari end as the SIO behaves
as standard thus compatibility with the complete range of Atari 8-bit
computers is retained.
7. Printer interface support is provided for the Atari with the PC
optionally converting Atascii to Ascii, printing to printer, screen or file.
8. Read Peter Welham's detailed document supplied with his HDEMUL software
for an account of this process.
Criteria
The criteria necessary to allow the Atari SIO to communicate with the PC
serial interface is for conversion of the RS232 levels of the PC to Atari
logic levels and vice versa. This involves data inversion performed by the
RS232 driver, necessary due to the fact that RS232 logic is inverse to TTL
data logic. Further, it is essential that the interface presents no loading
to the SIO DATA IN (pin 3) at idle and so allow other devices access to the
SIO. Tri-state logic achieves this and a 74LS368 device was chosen as this
device, due to the large number of gates available and their arrangement.
The RS232 driver chosen is in fact a MC1489 Line Receiver device designed
to convert the plus and minus 12V RS232 levels to logic levels 0V and 5V.
This device is also used as a transmitter and although no minus voltage is
presented to the PC, 0V is still recognised as a valid RS232 Mark signal.
Operation
This is best read with reference to associated diagram.
Find this on this disk as HDHWARE.IMG, a monochrome image file scanned at 200
d.p.i. and containing the vero-strip layout.
The 74LS368 (IC1) contains six inverter gates, four enabled by pin 1 (IC1A),
and the remaining two enabled by pin 15 (IC1B). IC1A is clamped on permanently
and IC1B is used in the circuit to SIO DATA IN.
IC1B gates are enabled by RS232 REQUEST TO SEND (pin 4) going low when there
is data to send, via IC2 13-11, IC1A 10-9 to IC1A 15. Data present on the
RS232 DATA OUT (pin 2) then goes via IC2 10-8, IC1B 10-9 and IC1B 12-11 to SIO
DATA IN (pin 3). RTS is clamped high via R1 so that IC1B presents a no load
tri-state condition to SIO DATA IN so that normal SIO operation is possible
when a PC is not being used.
Note: RS232 DTR (pin 20) can be used as an alternative for IC1B gate enable.
HDEMUL constantly monitors the SIO COMMAND line (pin 7) via IC1A 6-7 and
IC2 1-3 to RS232 RINGING INDICATOR (pin 22), and in it's present revision
(1.2) tends to lock up if this line is not high. As well as going
high when the Atari is switched on this line also provides the data to
control all of the SIO intelligent periferals, and so obviously this line
and it's interpretation is crucial. HDEMUL displays this information
optionally at the bottom of the emulator screen and interprets the commands
to Hexadecimal.
Data is received by HDEMUL constantly, regardless of device, from SIO DATA OUT
(pin 5) via IC1A 4-5, IC1A 2-3 and IC2 4-6 to RS232 DATA IN (pin 3). Response
is dictated by the COMMAND data.
Construction
Building this interface should present no real challenge and requires normal
dexterity with a soldering iron intended for PCB work. Construction is on
Vero-strip which eliminates the need to cut tracks. Layout is un-important,
but the suggested one is easy to follow and has been tested thoroughly.
Remember to run the 0V lines in before fitting the IC's. Sockets are
recommended. I use soldercon pins which give a good stand-off and don't have
bases to restrict access in that respect.
Terminations are via Vero pins pushed through and soldered to the strips. These
are then taken off to a SIO cable centre stripped and prepared to accept the
requisite data and power lines. This may then be enclosed in a suitable box
and left permanently in the SIO chain.
I recommend a 25 way D-type socket to be fitted to this box rather than a
permanent cable connection to the PC. This way a standard PC comms cable can
be used to connect the SIO interface to the PC and when the PC is not
connected the trailing cable can be removed.
A 25-way standard PC comms cable consists of a straight cable consisting of
25-way male - 25-way female with pins 2 through 8 and 20 connected. Not all
pins are required for our application, but are included for full modem
compatibility.
Alternatively PC's use 9-way plugs for their comms ports. If this is the
case a conversion cable can be used as detailed below:-
9-way female - 25-way male D-type connectors.
1-8 DCD
2-3 RD
3-2 TD
4-20 DTR
5-7 Common
6-6 DSR
7-4 RTS
8-5 CTS
9-22 RI
This is the full 9-25 way conversion. Not all pins are required for our
application, but are included for full modem compatibility as above.
Disclaimer
Needless to say, attempting this project is solely your own responsibility
and no-one but you can accept resposibility for faulty construction.
A slip-up here could permanently damage either or both of your computers so
be advised.
Take into account though that as I have succeeded, so may you. If you
anticipate difficulties then it may be possible for me to construct one for
you for a charge of twenty pounds sterling, plus SIO cable, plus postage.
Support
This hardware and software is released as Freeware.
LACE is officially distributing this software and hardware design as a
library special.
Contact LACE for your copy NOW. If you have an Atari 8-bit and a PC you
need this!
LACE: 081-690 2548 or 081-947-8900
156A Swaby Road
Earlsfield
London
SW18 3QY
Please copy this design and distribute it to all interested parties.
Feedback regarding this hardware and software is required. We need to hear
from you.
It really costs little to feedback comments and they are very welcome,
good or bad, believe me!
Or better still, join LACE and come to our monthly meetings.
Regards, Roger Lacey.
EOF
\NP