Serial Port Complete - Latest Microcontroller projects

Network Programming

Firmware is responsible for determining the value of the ninth bit to transmit

and interpreting the value of a received ninth bit. The hardware has no built-in

support for setting and detecting address/data bytes. Firmware must also define

an 8-bit address for the device.

To use address detecting with 9-bit data, in the RCSTA register, set ADDEN =

1. A computer that wants to communicate with the device must begin by send-
   ing a 9-bit word consisting of the device’s address with the ninth (most signifi-
   cant) bit = 1. In the PIR1 register, RCIF will be set only after receiving a word
   whose ninth bit = 1. When RCIF = 1, firmware should read the received byte. If
   the byte matches the device’s address, firmware sets ADDEN = 0 to enable
   reading data. All received words that follow with bit 9 = 0 contain data
   intended for the device. On receiving a word with bit 9 = 1, firmware should
   again check for a matching address. If it’s not a match, firmware should set
   ADDEN = 1 to disable setting RCIF on any data that follows.
   Other microcontrollers, such as Maxim Integrated Products, Inc.’s DS89C420,
   can store a node address and a broadcast address and respond only when a
   received address matches one of these values.



The UARTs in PCs typically don’t have full hardware support for 9-bit proto-

cols, but applications can use Mark and Space parity to implement a soft-

ware-assisted 9-bit protocol.

Before changing the parity type, an application should ensure that all data writ-

ten with the previous parity setting has transmitted. Chapter 6 described meth-

ods for finding out when the data has finished transmitting to determine when

it’s OK to disable an RS-485 driver. The same methods are also useful for deter-

mining when it’s OK to change the parity type. An application that sends 9-bit

data might also need to add delays to ensure that the receiving computer pro-

cesses received bytes in the correct order, as described below.

To use 9-bit data in a .NET application, set the SerialPort object’s DataBits

property to 8 and set the ParityReplace property to zero to disable the par-

ity-replace feature. The ParityReplace property can specify a byte that will

replace any byte received with a parity error. With 9-bit data, you want to pre-

serve the data rather than overwrite it.

 myComPort.DataBits = 8
myComPort.ParityReplace = 0
myComPort.Open