Heathkit H89a

Be waned that the hardware manual is a fairly thick book, and has a separate pile of sheets that contain the schematics, PCB layouts, and so on. It's an excellent manual (as you'd expect from Heathkit), but it won't be trivial to copy. The H88 and Z90 are essentially the same computer. Basically, 'H' models are Heathkit, and came as partial kits (the disk drive, CPU board, and 'terminal logic board' were ready-built, you got to solder up the monitor and PSU). The 88 was a cassette-based machine, the 89 has an internal disk drive. The 90 has the full 64K RAM as standard. Or at least that's how I interpretted the manuals. Yes, it's a H19/Z19 terminal with a computer inside. The Z19 is a VT52/VT100 compatible terminal using a Z80 as the control processor and a 6845 for video. That's what's on the 'terminal logic board', which stands vertically right at the back of the case. In the H89 (etc), that's unchanged. There's a CPU board mounted in front of the terminal logic board. It contains another Z80 and 48K DRAM. And a boot/monitor program in ROM. The computer communicates with the terminal via an RS232 link (I kid you not) between the 2 boards. Plugged into the CPU board you are likely to find a triple serial board (8250-based IIRC) and a disk controller or 2. And maybe a 16K RAM board if you've really got a Z90. There were several disk controllers. One was certainly for hard sector 5.25" disks, and used a USRT + a bit of simple logic. Another handled double-density soft sectored 5.25" disks -- I think this was based on the Western Digital 1793 chip. There was, I think, a 8" drive controller and even a hard disk interface. Oh yesm, it runs CP/M or HDOS (which is a bit like a DEC OS at the user level). I hope you got a boot disk... -tony