Instruction manual

9
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UNDERSTANDING RS232 CONNECTION & SIGNALS
RS232 Connections
A straight-through cable should be used to connect a DTE (e.g. computer) to a DCE (e.g. Amp, Matrix or Modem), with
all signals from one side connected to the corresponding signals on the other side in a one-to-one basis.
A crossover (null-modem) cable can be used to connect two DTE directly, without a modem in between to cross
transmit and receive data signals between the two sides. There are numerous variations on how control signals are
wired:
RS232 Logic Waveform (8N1)
The graphic (p.8 bottom left) illustrates a typical RS232 logic waveform (Data format: 1 Start bit, 8 Data bits, No Parity,
1 Stop bit). The data transmission starts with a Start bit, followed by the data bits (LSB sent first and MSB sent last),
and ends with a “Stop” bit.
The voltage of Logic “1” (Mark) is between -3VDC to -15VDC, while the Logic “0” (Space) is between +3VDC to
+15VDC.
RS232 connects the Ground of 2 different devices together, which is the so-called “Unbalanced” connection. An
unbalanced connection is more susceptible to noise, and has a distance limitation of 15m/49ft.
Straight-through (DB-9
(DTE) (DCE)
1 DCD ------- DCD 1
2 RxD ------- TxD 2
3 TxD ------- RxD 3
4 DTR ------- DSR 4
5 GND ------- GND 5
6 DSR ------- DTR 6
7 RTS ------- CTS 7
8 CTS ------- RTS 8
9 RI ------- RI 9
Crossover (Null-Modern) (DB-9
(DTE) (DCE)
1 DCD ------- DCD 1
2 RxD ------- TxD 3
3 TxD ------- RxD 2
4 DTR ------- DSR 6
5 GND ------- GND 5
6 DSR ------- DTR 4
7 RTS ------- CTS 8
8 CTS ------- RTS 7
9 RI RI 9
0 3 61 4 72 5
stopstart
LSB MSB
Mark (1) -12v
space (0) +12v
DTE Pin Assignment (DB-9) DCE Pin Assignment (DB-9)
1 DCD Data Carrier Detect 1 DCD Data Carrier Detect
2 RxD Receive Data 2 TxD Transmit Data
3 TxD Transmit Data 3 RxD Receive Data
4 DTR Data Terminal Ready 4 DSR Data Set Ready
5 GND Ground (Signal) 5 GND Ground (Signal)
6 DSR Data Set Ready 6 DTR Data Terminal Ready
7 RTS Request to Send 7 CTS Clear to Send
8 CTS Clear to Send 8 RTS Request to Send
9 RI Ring Indicator 9 RI Ring Indicator