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ZBasic System Library 74 ZX Microcontroller Family DefineCom Type Subroutine Invocation DefineCom(channel, rxPin, txPin, flags) DefineCom(channel, rxPin, txPin, flags, stopBits) Parameter Method Type Description channel ByVal Byte The serial channel being defined. rxPin ByVal Byte The pin which will serve as the receive line. txPin ByVal Byte The pin which will serve as the transmit line. flags ByVal Byte Configuration flags. See the discussion below. stopBits ByVal Byte The desired number of stop bits. Discussion This routine configures a serial channel, preparing it to be opened using OpenCom(). If the specified channel is already open, this routine does nothing. Likewise, there is no effect if the specified channel is invalid (see the ComChannels() routine) or if either of the rxPin and txPin parameters are invalid or both are zero. Note that either rxPin or txPin may be zero, allowing you to define a transmit-only or receive-only serial channel. If the specified channel is a hardware UART (e.g. Com1), the rxPin and txPin parmeters must both be zero. Otherwise, if they are valid, the pins specified by rxPin and txPin are automatically configured as input and output, respectively. However, if the rxPin and txPin parameter values are equal and non- zero, the pin is initially configured as an input to support half-duplex, bussed operation. In this mode, the the pin will be made an output when transmitting a zero bit if configured for non-inverted operation or when transmitting a one bit if configured for inverted operation. A pull-up resistor (non-inverted mode) or pull-down resistor (inverted mode) is required for bussed operation since the pin will only be actively driven in one of the two output states. The flags parameter contains several bit fields used to specify some of the details of the operation of the serial channel. Serial Channel Configuration Flag Values Function Hex Value Bit Mask Inverted Logic¹ &H80 1x xx xx xx Non-inverted Logic &H00 0x xx xx xx Ignore Parity Bit &H40 x1 xx xx xx Even Parity &H30 xx 11 xx xx Odd Parity &H20 xx 10 xx xx No Parity &H00 xx 00 xx xx 7-bit Data &H07 xx xx 01 11 8-bit Data &H08 xx xx 10 00 ¹ Applicable only to software-based channels (3-6). The remaining bit values are currently undefined but may be employed in the future. When Non-inverted Logic is selected, the idle state of the transmit line will be logic 1. When a character transmission is begun, a “start bit” of logic zero will be sent for one bit time (the inverse of the baud rate). Next the data bits are sent, each for one bit time, beginning with the least significant bit and continuing through the eighth data bit or parity bit as the case may be. Finally, one or more “stop bits” of logic one are sent, each for one bit time. With Inverted Logic, each of these elements is complemented – the idle state of the transmit line is logic 0.