JAIR CPU Board

Joint Altair Imsai Replacement CPU board

Learn about the JAIR CPU replacement board at John Monahan S100Computers.Com site.

This is an Altair and Imsai compatible CPU replacement board designed by Josh Bensadon. I purchased it from him assembled off eBay. Generally this board is only available bare with the user responsible for sourcing parts and construction. It can operate without any cards in an Altair or Imsai equipped with a front panel. I find it very useful in diagnosing other cards while rebuilding S-100 era computers.

Resources

Under construction

Documentation and links

8-inch Systems

These were the first floppy drives available in the mid 70s. They were initially designed by IBM as a method to load microcode into their large DASD disk controllers. Other manufacturers started to manufacture them as storage devices for the then beginning small computer market. Many 8080/8085/Z80 CPM-80 systems used these drives. These drives were large, heavy, and required +5V, -5V and 24V to operate. The media spun continuously although the heads were unloaded when not reading or writing. This introduced the head load and unload time to the FDCs. These drives were available in soft- and hard-sectored models. Media was not interchangeable between them soft- and hard-sectored models.

The first standard disk was the IBM 3740 format 8-inch SSSD disk 241Kb (77 tracks, one head, 26 each 128 byte sectors per track with a skew of six) disk. This was the disk format used for exchange between all the CPM-80 8-inch systems and software providers. http://www.nj7p.org/Computers/Disk%20Subsystems/floppies.html

z80pack

diskdefs

# jha 6 June 2016
# The JAIR post era Imsai Altair replacement S-100 card doesn't use skew

diskdef ibm-3740-noskew
  seclen 128
  tracks 77
  sectrk 26
  blocksize 1024
  maxdir 64
  skew 0
  boottrk 2
  os p2dos
end

Using cpmtools cpmls I read the directory specifying the disk definition to use for 0 skew

jeffa@tx2000ros:~/z80pack/cpmsim/disks$ cpmls -fibm-3740-noskew DISK-D.BIN
0:
asm.com
blinkem.com
ddt.com
dump.com
[...]
view.com
wm.com
wm.hlp
xmodem.cfg
xmodem.com
xsub.com
jeffa@tx2000ros:~/z80pack/cpmsim/disks$