[sebhc] H-89 bus pin out Help

Lee Hart leeahart at earthlink.net
Tue Mar 14 15:48:26 CST 2006

The H89 CPU board has *3* different pinouts for the six expansion

The leftmost group of 3 are memory expansion sockets. They were
originally intended to add ROM cards (Benton Harbor BASIC, etc.); thus
they don't have a Write line. They have the data bus (D0-D7), part of
the address bus (A0-A12), 3 decoded chip select lines (RD5,6,7), two
output bits from the General Purpose port (MEM0, MEM1, intended for bank
select), and the +5v, +12v, and -12v supplies and ground. Later of
course, they were used for RAM memory expansion (notably another 16k of
dynamic RAM to bring the total up to 64k). To do this, they added a
jumper wire and a 16-pin flat cable to bring in the missing Write, -5v,
and address lines.

The rightmost group of 3 are the I/O expansion. The rightmost one is
unique, and intended for the hard-sector controller. The other two are
identical. These connectors have the data bus (D0-D7), a few address
bits (A0-A2), Read, Write, Wait, Clock, Reset, four decoded I/O selects,
2 bits from the General Purpose port (I/O#0, I/O#1), 3 interrupt lines,
and the +5v, +12v, -12v supplies, and ground.

The hard-sector I/O slot replaces 2 of 4 decoded I/O selects with a 5th
I/O select, and a write-protect output to write-protect the onboard 2k
bytes of static RAM (only used for HDOS).
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in    --    Leonard Cohen
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net
Delivered by the SEBHC Mailing List

More information about the Sebhc mailing list