This column is, once again, devoted to a lengthy product review. I’m planning a catch-up on resource/supplier listings next issue, but a couple of items deserve immediate mention:
HARDCORE HARDWARE HOBBYIST HANDBOOK
ZX81 hardware expert Bruce Taylor, of Budget Robotics & Computing, Box 18616, Tuscon, AZ 85731, has announced the release of his book, BUILD A MICROCOMPUTER CONTROLLED ROBOT AND OTHER COMPUTER CONTROL PROJECTS. (QUITE A MOUTHFUL).
TAB Books had owned the rights for over a year, then decided not to publish. After this runaround, Mr. Taylor’s company bought back the rights and is now publishing the book. This book presents the design of an award-winning robot in thoroughly documented project form, and promises to be a valuable reference on interfacing just about anything. Printed circuit boards will be available from Budget Robotics for every project in the book. Chapter topics include Adjustable Power Supply, 8255 PIO Board, Digitalker Voice Synthy, Stepper Motor Drivers, Optical Encoders, Polaroid Ultrasonic Ranging kit, and others. Over 200 8.5″x11″ pages, 80+ illustrations, 60+ tables, loosebound. Price: US $13 ppd.
For a sample of previously published work, see “Robotics on a Budget,” p. 18, SYNC Vol. 3 No. 4; Also “Home Control on a Budget,” p. 31, SYNC Vol. 4 No. 1 ; both by Bruce C. Taylor.
Budget Robotics holds rights to an improved version of the Computer Continuum buffered expansion board, as well as the Zodex RX81, an inexpensive general purpose parallel I/O board. Write for details – Bruce replies promptly.
HOLY MODEM, BATMAN
Westridge 2050 Modems, tested, with phone cables but no case or power supply are available form Dave Clifford, 13910 Halldale Av., Gardena CA 90249, 213-516-6648. The price? $25.00 for one, three or more for $22.50 each, ten or more for $20.00 each. Including delivery. At time of writing Dave indicated that he had about 1500 left. However, he also has an undisclosed number of untested boards which are missing a couple of minor parts (an LED and resistor). Plans are to sell these as-is at an even cheaper price, including a complete schematic and parts ID for the hacker.
Needless to say I ordered my modem the same day I heard about Mr. Clifford, as well as a copy of MINI-XMOD 1.7, at $20.00 from WEYMIL corp., Box 5904, Bellingham WA 98227-5904.
MINI-XMOD is terminal software which is said to be the best available for the ZX81/TS1000 (or the 2068). Dave Clifford indicated that he would be working on a version of MINI-XMOD for the 2068, first a 32-column version, then a 64-column version. It would be an interesting challenge to work up a 64 column version for the JLO Video TS 1000!
HARDWARE REVIEW
Subject: Aerco FD-ZX Floppy Disk System
Source: Aerco, 7606 Robalo Rd,
Austin TX 78757
FD-ZX Floppy Disk Controller Board and DOS: $179.00 Auto Boot ROM Board: $ 59.00 Auto Boot ROM Board with Centronics I/F: $99.00
Complete packages are also available. NOTE: See “Bug Alert”, last issue for a discussion of the Boot ROM/Centronics board.
BASIC TECHNICAL SPECIFICATIONS
Drive type: Shugart compatible 3", 5" or
8" drives, 1 or 2 side,SD or DD,
35 or 40 tracks per side.
# of drives: up to 4.
DS/DD Capacity: 400K bytes.
Disk Format: IBM readable, 10 sectors per track,
512 bytes per sector.
#Files per disk: up to 20 (as 4-track pages)
up to 6 (as 12-track pages)
Memory use: 2K DOS in EPROM, and FDC at 12-14K
FDC I/O is memory-mapped.
Format DSDD: approx. 35 seconds.
CopyDSDD-DSDD: approx. 1 minute, 5 seconds.
Avg. LOAD/SAVE: 3 sec. (20K page)
5 sec. (60K page)
Error Reports: WRITE PROTECTED; INSUFF. RAM;
DISK ERROR.
Physical size: Circuit board is approx. 4.5 x 6", gold
finger extensions.
Special ICs: 1797 FDC (Siemens); 9216 Data
separator.
What do you see when you open the box containing your newly arrived AERCO FD-ZX? One uncased controller board, a computer bus ribbon cable, one floppy disk, and a 16 page manual. What else do you need? Disk drive(s), drive cables, cases and power supply. Aerco’s manual gives reasonably easy- to-follow instructions for hooking up your system, and has several useful illustrations. The power supply must provide +12V at 2-3 amps and +5 at 3 amps. The drive cable required is a standard IBM/Shugart 34 conductor pc-edge type. If you follow Aerco’s instructions to the letter, you will likely void any warrentee on your new drives. The FDC board gets its +5V and +12V supplies via pins 2 and 34 on the disk drive edge connector. Pin 34 is normally unused, but pin 2 is connected on some drives. Aerco recommends you cut any traces going to pins 2 or 34 on the drive and jumper these over to the power supply pins. This way you connect the power supply only to one drive, and the FDC board and other drives get powered through the ribbon cable. The result is tidy but I hate to hack up a new drive! The alternative would be to cut wires 2 and 34 adjacent to the contoller card, and run separate power leads to everything. I did it Aerco’s way.
Having hooked up drives and power supply, the FD-ZX plugs into the supplied ribbon cable; then the RAM pack, etc. onto the ribbon cable’s male edge extender, and we’re ready to see what’s on Aerco’s “master” disk.
On application of power, the K cursor appears. Following Aerco’s instructions, the master disk is placed in drive A. The command RAND USR 13303 initialises the DOS, and loads “page 1” from the disk. The following menu appears on the screen in about 3 seconds:
WELCOME TO SADOS+ 16K 8/83 C COPY DISK (REQUIRES 2 DRIVES) D DISK UTILITIES E ENTER NAMES IN DIRECTORY F FORMAT DISK IN DRIVE A H HEX MONITOR L LOAD MEMORY FROM DISK Q QUIT S SAVE MEMORY ON DISK
Nothing too surprising here. Some further explortion of the manual reveals some interesting facts. All DOS functions are accessed via RAND USR commands. The “DOS” program supplied on page 1 of the master disk serves two purposes: 1) It is a “user-friendly interface” (oh cliche, cliche!) to the DOS USR routines, and 2) It contains the Disk Directory.
Two different DOS control programs are supplied: a “16K” version and a “64K” version. (All capacities specified here are for a 40 track, DSDD drive.)
The 16K DOS permits 20 programs or data files to be saved on a disk. The disk is divided into 20 4-track pages. Any program saved occupies one page, regardless of whether it is a 1K program or a full 16K program. Each “16K” disk page will actually hold up to 20K full of data. Thus it is extremely unlikely that you will ever use the full potential storage capacity of the disk. The 64K DOS assigns 6 pages per disk, each page having a capacity of 60K bytes. It is otherwise identical to the 16K DOS program.
The Aerco DOS does not record a filename with each program, nor does it automatically maintain a directory. Programs are loaded and saved by page number. It is the user’s responsibility to keep the directory on page 1 updated.
Converting a breakable program to a disk version is quite easy. Aerco uses sequential USR calls for SAVE and LOAD commands: RAND USR (12720 + PAGE) to SAVE a specified page or RAND USR (12290 + PAGE) to LOAD from a specified page. After loading the program from tape, the DOS must be initialised by the command RAND USR 12865. This command initialises the FDC and loads certain DOS parameters into bytes 16507/16508. Thus the lines:
9900 SAVE "PROGRAm"
9910 CLS
9920 RUN…would be replaced by the lines:
9900 RAND USR 12865
9905 RAND USR 12722
9910 CLS
9920 RUNIn this example, a GOTO 9900 would save the program to page 2 on the disk, and autorun. When the program is reloaded, it will come up in the same mode (SLOW/FAST) as when it as saved. To save the program on, say,page 6 instead of page 2, line 9905 would read “RAND USR 12726.”
Having saved a program, it must be entered in the directory. So, entering RAND USR 13303 brings up the SADOS menu. Pressing E gets a prompt asking which page you want to name, followed by the entry of the program name. Having done this, you must re-save the updated directory to page 1. This is offered as a Y/N option when you finish making directory entries, or it can be done by pressing the S at the main menu. Now if you hit L at the menu, the directory appears,listing pages 1 through 20 and their contents. Any program may be loaded by entering its page number, or you may copy the directory to your printer.
D, for DISK UTILITIES, provides selection of PROGRAM or DATA mode, of Disk Drive A, B, C, or D; Single or Double Density, and also a Clear Directory command. In Data mode all variables are saved and loaded, without the Basic program.When re-loading data, no test is made for variables space. Variables must already have been dimensioned or declared so there is enough room between VARS and E_LINE for the disk file to load into. With an understanding of these limitations, the Data mode can be very useful. I’ve successfully programmed a version of WSII.5 which loads and saves data files ONLY; up to 20 16,000 byte text files, each saved with a filename string. The program will sequentially search a disk for any named file, and print an automatically maintained directory. I’m sure that versions of ZX PRO-FILE(or other file handling programs) could be modified to sequentially search a whole disk, if necessary to find a entry.
CPUs and BUS LOADING: I had just gotten a few programs saved to disk, and was marveling at the speed of program access, when I realized I had problems- random load errors galore. Bytes were simply dropping out of programs. Suspecting bus speed/loading problems, I singled out 2 possible culprits: my CPU, and the ribbon cable provided by Aerco. The NEC D780C CPU in my computer is a notoriously substandard Z80 chip, so I replaced it with a new Zilog Z80B. The Z80B is designed for operation at up to 6MHz., thus has lower intrinsic capacitance, and “stiffer” bus drivers than the slower versions. At the same time, I nixed the ribbon cable, and plugged everything into my new JLO 6-slot motherboard. A ribbon cable can have 5-10 times the capacitive crosstalk as a motherbvoard the same length, so I figured this would help. Not so simple, though. The male edge connector on the FD-ZX is a mirror-image of that needed by the JLO motherboard! Thus, I had to fashion a “side-swapper” adapter to plug everything together. Having sone all this, the problems completely disappeared, and the disk system has been 100% reliable ever since. I don’t know whether the CPU or the motherboard was the significant factor, but the combination licked the problem.
IN CONCLUSION, the AERCO FD-ZX system has made a “disk convert” out of this ZX81 hobbyist. The hardware is of good quality and is well assembled. The DOS control program is certainly utilitarian, although some resourceful programming can make it do many impressive things. The existing DOS control software simply doesn’t take advantage of the FD-ZX hardware’s potential. Aerco’s documentation is quite thorough, and provides a few clues to accessing more sophisticated DOS commands thru machine code. The incredible speed of reliable access to a large number of programs or files makes it easy to overlook the shortcomings of the DOS.
FROM ACROSS THE POND
Thanks to colleague Larry Chavarie of Ottawa, (who subscribes to even more publications than I do), I will be listing a number of British concerns which continue to support the ZX81 aftermarket. MAPLIN ELECTRONIC SUPPLIES LTD., P.O. Box 3, Rayleigh, Essex, England SS6 8LR, tel. (0702) 554155, is the U.K. dealer for Heathkit, but that’s only part of the picture. Maplin offers no fewer than 9 hardware project kits for the ZX81, including a full-sized keyboard, sound and speech generators, a TV sound/inverse video kit, modem interface, extendi-RAM, and hi-res graphics module. Prices are very reasonable. Most of these are male-only cards which are also compatible with motherboards available in the U.S. from John Oliger and Budget Robotics and Computing. Maplin still offers over 15 books about the ZX81 (with something for everyone), and publish their own electronics project magazine. 12 issues are currently available, and almost every one has a ZX81 project! (How about a ZX81 navigation/autopilot system for your yacht?!)
Larry tells me Maplin gives prompt, reliable service. 450 page catalog is £1.35. If you wish to exercise the plastic, Maplin accepts Visa and Mastercard (called BarclayCard and Access in the U.K.). Check this company out.