Leiy,
Looks like you have a Rev 1 board, that is good.
Your board is set for 300 baud, 8N1.
Transmit out from the board, to your terminal, is on header pin 8 (if pin 1 is on the left)
Receive into the board, from your terminal, is on header pin 7
Looks like you might be wired to pins 6 and 7 by mistake?
Signal ground is on pin 4
The bits and polarity being tested for in Marty's code above are actually for the 6850 ACIA found on the 2SIO board. It won't work for the SIO board. The bits and polarity for the SIO Rev 1 board are as follows:
Transmit ready when bit 7 (MSbit) is ZERO (note: not one, this is inverted logic)
Read data available when bit 0 (LSbit) is ZERO (not one, inverted logic)
Here's echo for the SIO Rev 1 strapped at I/O address 0/1
0000 DB00 loop in 00h ;wait for character
0002 0F rrc
0003 DA0000 jc loop ;nothing yet (negative logic)
0006 DB01 in 01h ;read the character
0008 D301 out 01h ;echo it
000A C30000 jmp loop
Here is the program in octal for easier entry into the Altair:
000: 333 000 017 332 000 000 333 001
010: 323 001 303 000 000
Might be easier to read this by looking at "SIOECHO.PRN" at
http://deramp.com/downloads/index.php?dir=altair/software/utilities/other/.
Some SIO documentation is available at
http://deramp.com/downloads/index.php?dir=altair/hardware/sio_serial_interface/. The file "88-SIO Rev 0 & 1.pdf" is the Rev 0 manual with lots of errata information including the Rev 1 update information. The schematic for the Rev 1 board is also available there.
Mike