Motorola USB08 Betriebsanleitung PDF herunterladen (Seite 91)

Logic Levels

It is this aspect which causes the most bewilderment among users, largely because of the counter-

intuitive naming conventions used. The negative voltage is referred to as a logic 1 or 'mark' condition

and a positive voltage is referred to as logic 0 or 'space' condition.

A driver transmitting data on the TxD pin idles in the logic 1 or 'mark' state, i.e. with a negative

voltage output. In the absence of data being actively sent from the remote device, the voltage on the

RxD pin will also be negative, because that is the idling state of the driver at the 'far' end of the link.

In the case of the control signals, a de-asserted signal is one in the 'mark' or negative voltage state. For

example, DTR is asserted if it goes from the negative (logic 1, mark) state to the positive (logic 0,

space) state, as here:

It may be noted that all of the common RS232-to-logic level converters (MAX232, 1488,1489) invert

the polarity of the transmitted and received signals. So a TTL 'high' level, logic 1, +5V, corresponds

to a negative RS232 voltage (somewhere between -3V and -15V).

Data Protocols

Serial data on an RS232 link is transmitted asynchronously, i.e. a separate clock signal is not required

and the start of a data byte can occur at any time. To achieve this, a logic 0 start bit is sent, followed

by the data bits, and ending with a logic 1 stop bit. I will only consider here the commonest 8N1

format - a start bit followed by Eight data bits, No parity bit and One stop bit. The data bits are sent

in least-significant-bit first order. All the bits (start, data and stop) have a duration equal to the

reciprocal of the baud rate. Note that the technical definition of baud rate is more complex, and it only

happens that baud rate and bit rate are equal for the simple binary signaling system used on RS232

links. To send a single byte takes 10 bit times (1.042 ms at 9600 baud). Since a start bit can

immediately follow a stop bit, at 9600 baud the maximum throughput is 960 bytes per second.

Here is the letter 'M' in ASCII coding (hex 4D, binary 01001101) sent according to the 8N1 format

at 9600 baud, as it appears on the TxD pin:

1 2 ... 86 87 88 89 90 91 92 93 94 95 96 ... 99 100

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Motorola USB08 Betriebsanleitung Seite 91