Motorola USB08 Betriebsanleitung PDF herunterladen (Seite 90)

The pins are labeled with a functional description as follows. 'IN' and 'OUT' refer to the signal

directions as they apply to the DTE:

Number

Abbreviation IN or OUT Description

1 CD (or DCD) IN Carrier Detect. Used by a modem (DCE) to tell the

DTE that it has detected a carrier signal

2 RxD IN Received Data. Data sent by the DTE to the DCE

3 TxD OUT Transmitted Data. Data sent by the DTE to the DCE

4 DTR OUT Data Terminal Ready. Used by the DTE to tell the

DCE that it is operational.

5 GND Ground. Common signal return.

6 DSR IN Data Set Ready. Used by the DCE to tell the DTE

that it is operational.

7 RTS OUT Request to Send. Used by the DTE to signal the

DCE that it may begin sending data.

8 CTS IN Clear to Send. Used by the DCE to signal to the

DTE that it may begin sending data.

9 RI IN Ring Indicator. Used by a modem (DCE) to tell the

DTE that an incoming call has been detected.

In the case of Xplor8, the personal computer host is a DTE and the target board is a DCE. As

explained in Appendix B, the only pins that must be connected are RxD, TxD and GND. Remote

resetting will require a connection to DTR (or RTS) also. No other handshaking signals are needed;

if connected, they will be ignored.

Voltages, Impedances and Biasing

RS232 is referred to as an 'unbalanced' signaling system in that all the signals are referred to a common

ground conductor. This is inherently more susceptible to crosstalk and common-mode sources of

interference than, for example, the balanced system described in RS422. Binary state signaling is used,

with one state represented by positive voltage on the signaling pin of anywhere between +3V and

+15V with respect to the common ground pin (pin 5); and the other state by a negative voltage

between -3V and -15V. Voltages within the transition region from +3V to -3V are meaningless.

The source impedance of a signal driver should be such that accidental cross-connection with another

output cannot damage either driver even where one is outputting a positive voltage and the other a

negative one.

The default state of all signals is de-asserted, i.e. a negative signal voltage. The line receiving devices

should be biased so that they 'see' a negative voltage if the cable is disconnected, or if the cable is

connected but the transmitting end is unpowered. It may be noted that biasing the RxD pin so that it

'sees' a negative voltage in the absence of a connecting cable, and the fact that a negative voltage is

the usual ‘idle’ line condition, means that there is no automatic indication of a break in the cable.

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Motorola USB08 Betriebsanleitung Seite 90