New-Tech Europe Magazine | June 2019

ACE analog input and output may be buffered by a line driver. The output of the DTE is referred to as the transmitter signal (TX), while the input is called the received signal (RX). The interface cable is limited to a maximum length of 15 m. The length of the cable determines the maximum data rate that can be reliably used over the interface bus. The RS-232 interface connects two devices with a full duplex connection, meaning that each device can transmit and receive simultaneously. The RS- 232 serial data packet consists of a start bit, anywhere from 5 to 8 data bits, 1/1.5/2 stop bits, and a parity bit (Figure 2). The minimum cable requirement for RS-232 is for three wires: one for transmit, one for receive, and the signal ground. Ground is the return for both signal conductors. Many of the characteristics of RS-232 are tied to its original application in telecommunications. It uses negative logic with a high state, referred to as a space, and a low state being called a mark. The neutral or rest state is high so the interconnection can be verified remotely. At the transmitter side a 0 state, or space, is a voltage between +5 and +15 volts. The logical 1, or mark state, is a voltage between -5 and -15 volts. At the receiving end, a level of from 3 to 15 volts is a 0, and -3 to -15 volts represents a 1. The transfer is termed asynchronous because no clock signal is transmitted. RS-232 depends on both sides of the

Figure 1: A basic RS-232 system includes data terminal equipment (DTE) such as a computer, and data communications equipment (DCE) like a modem. A UART/ACE interfaces the parallel computer backplane with the serial RS-232 interface. (Image source: Texas Instruments)

RS-232 control signals RS-232 has a number of specified control signals. They report the status of the DTE and DCE devices as well as implement a hardware-based handshake to pace the transfer of data (Table 1). The hardware handshake is implemented using the Request to Send (RTS) and Clear to Send (CTS) flow control signals to ensure that both devices are ready to transfer data and that data has been received by the receiving device. Hardware

bus being set up for a specific clock or baud rate. Baud rate is a measure of the number of symbols transferred per second; for RS-232 it is approximately the clock rate. Common baud rates are 300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200, 230400, 460800, and 921600 baud. The higher the clock rate the more limited the cable length. For example, at 9600 baud, the full 15 m maximum cable length can be used. At higher baud rates the maximum cable length will be reduced.

Figure 2: An RS-232 data packet consists of a start bit, 5 to 8 data bits (8 shown), a parity bit (optional), and 1, 1.5, or 2 stop bits. (Image source: Digi-Key Electronics)

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