I-O Display Systems Basic I/O Product - Manual (Page 20)

Setup & Installation 2
2-3(Vol.1)
COMMUNICATING WITH THE BASIC I/O:
The BASIC I/O is designed to serve as an intelligent I/O front end for a Host computer
(Typically a P.C.). The host and BASIC I/O communicate over a serial link. This interchange
is half-duplex in nature; that is to say the host and BASIC I/O will never be transmitting at
the exact same time. Further, the communications protocol is considered “speak-only-when-
spoken-to”; the Host must poll the BASIC I/O whenever it needs fresh data. This polling is
accomplished when the host sends an instruction to the BASIC I/O. The BASIC I/O will then
generate a reply. Each valid instruction will illicit a corresponding response. The integrity of
this communication is verified using message content checksums.
The serial communication is a form of ASCII printable characters and makes heavy use of the
hexadecimal numbering system. The format of the ASCII characters used is: One start bit,
eight data bits, one stop bit, and no parity.