Vector 4 - Page 102

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TECHNICAL INFORMATION
9.2 HOW THE SERIAL PORTS WORK
Summary: The SBC uses three 8231 chips. These USARTSs provide
serial ports for a modem, printer and keyboard. . .
A. USARTs
There are three USARTs on the SBC. Each of these chips is a 8251 which is
initialized to operate in the asynchronous mode. The initialization process sets
the USART so that it uses: 8 data bits, 2 stop bits, parity disabled and a baud
rate factor of 16. The bytes sent to the control register (odd numbered port)
are: 00H, 00H, 00H, 40H, CEH, and 27H. °
The 8251 located at U121 is used to link the computer with a modem. In order
to provide this funetion the USART has its handshaking lines connected so that
it appears as a DTE device. The SBC provides a jumper area (Area D) which
will allow you to change the USARTs clock input lines. These lines (Pin 9 and
25 on the USART) are presently attached to the internal clock. Part IV gives
a complete description of this jumper area.
The modem USART is attached to the J1 connector (DB'ZS) The pin-outs of
this connector along with the pin-outs for the other two connectors (using serial
ports) is given in Exhibit II-18.
U122 is the location of the USART which interfaces with a printer which uses
serial protocol. This USART is configured as a DCE device so that it can
communicate with a DTE device. e.g. Printer which uses serial protocol.
There are several types of serial printer protocol. These protocols are listed
below and described in Exhibits [1-19, 20, 21,
Standard Serial Protocol: Uses hardware handshaking
ETX/ACK Serial Protocol: Uses ETX/ACK software handshaking
XON/XOFF Serial Protocol: Uses XON/XOFF software handshaking
The third USART is located at U96 and is connected to the J12 connector.
This USART links the SBC with the keyboard. The actual signal/data lines go
from the J12 connector to a filter board before they are finally routed to the
coiled keyboard cable. These connections are discussed in Section VI and
Exhibits =20, =21
09-01-82 7200-0001 II 5-11