S100 Z-80

I then built an S-100 system, with commercial backplane, memory and an 8080 CPU, quickly moving up to a Z-80. First, I just had a borrowed terminal and a paper tape reader, then I built my own serial glass TTY by copying a commercial CRT terminal logic in wire-wrap. I think it was a copy of a Beehive model. I then got a Dura typewriter that had a parallel electrical interface as well as paper tape reader and punch. It was an IBM Selectric made for use with computers and word processors. Slow and noisy, but it could print out a program listing. I got a Persci dual floppy drive with a voice coil head positioner.

Circa 1978, from my apartment in Audubon Park (Brentwood, MO). At the upper right is the paper tape reader. Below that is the power supply for the Persci dual floppy disk drive, which is to the left of the P.S. An HP power supply below that, then the black rectangle is the Z-80 console LED display. Below that is the S-100 bus backplane and cards. Below the Persci floppy drive is a shortwave listening receiver and an RTTY converter.

10 MB Winchester hard drive

From an ad in EDN or similar magazine, I was able to buy a Winchester disk with SASI controller from Memorex on an introductory offer. I think it cost $1600, which was a lot of money, but it was a great unit. I wrote a software driver and simple parallel interface to emulate the SASI bus, the Z-80 had instructions that could read/write a sequence of bytes from an I/O register to/from memory in one instruction, thes was great for this purpose. I discovered you had to read only 256 bytes at a time and then break out of the transfer and check the ready status, as some of the 1K byte CP/M blocks could straddle tracks on the drive and it wouldn't be ready. It performed SO much more reliably than floppies, as well as being faster and capable of holding 10 MB! WOW, that was a lot of disk for a home computer in 1981 or so! I had a major accident while still setting it up, the spindle drive transistors were on the disk drive, and had 24 V on them. These touched the SASI controller board with sparks and smoke. I started debugging with the finger method. When your finger got burned, replace that IC. I then had to use a scope to look for bad TTL levels, and hoped the microcode ROMs didn't get fried. They didn't, and after a few more iterations it fired up again. I quickly made sure proper standoffs were keeping the boards from ever touching again.

This is my home computer room, perhaps about 1983? I have labeled a few things. At the very left is a shortwave receiver (on its side at that moment) sitting on top of a short equipment rack, and a navy RTTY converter below it. On the table, at left there is a color graphics terminal (really a piece of junk, I was lucky to get some cash for that boat anchor). In the middle is an IBM 3101 CRT terminal. To the right edge,below the big tape drive is the S-100 system. I never got that Bucode tape drive connected to a computer, although I got the capstan, vacuum columns and reel servos working.

X-Y CRT display

I also got a Sanders 14" X-Y display that was used in an old hospital info system. It was basically a computer screen CRT with slow phosphor, but used X and Y linear amps to drive the beam around, and a simple character generator. I hooked up some 12-bit DACs and a photomultiplier tube with a fiber optic pigtail to use as a light pen. I had a program that displayed 10 numbers on the screen. When you pointed the light pen at a number, it would turn into a tracking cross and you could drag the number around the screen. When you arranged all the numbers as you wanted, you hit a key and it calculated 8 Bezier splines through the control points. I also had a program that could create the points on some common mathematical curves, I think one was the Lemniscate of Pascal. It took about a minute to compute the list of points, and then drew them at maybe a 1 Hz update rate.


At my home computer, I got a Centronics 101 dot matrix printer (it was one of the first, with an all-steel cabinet and clutches that drove the print head back and forth). I quickly traded it with a guy that had a HUGE Honeywell drum printer with attached tape drive, used as an off-line printer at State Farm Insurance. I got it running and made some test tapes at work to make it print something. While fooling around with it, I did get it to dump the last page printed in its previous life, which was a dunning message to some poor schnuck who hadn't paid his bill. The printer had a core memory as the line buffer, and the last contents were still in it. Wish I could find that page, but I haven't seen it in a long time.

After getting some idea of the signals between the tape drive and the printer, and having all the docs too, I managed to get it working on the CP/M system. It was so huge it had to stay in the basement, so I had to run down, turn it on, run upstairs, print out whetever files I needed to, and then run down to get the printout. But, it was incredibly cool to have a 300 line/minute printer on a home computer!

9-track tape drive on my home system

I also got a Pertec key to tape system from surplus. It had a stock 7" Pertec 800 BPI tape drive and a controller with a keyboard and a field of lights for every upper-case ASCII character. So, you could read a tape and step through the contents a character at a time, and enter data into memory and then write a record to tape. You can see it in the picture above, to the left of the lower reel of the red tape drive. Above it is the original rack of cards that came with it. I think I pulled about the first 4 cards and used the remaining ones.

I found a convenient place to cut into the system and built an interface to read and write data to the drive. The Z-80 couldn't handle read after write, so it backed up and reread every block after writing it. I would write out a tape every night and take it into work to check the quality of the tape format. After a few days, it was basically producing perfect ANSI-D tapes compatible with the VAX. I used it mostly for backups. So, I had a 300 LPM line printer, 10 MB Winchester disk and 9-track mag tape on a CP/M system!