JPH07231324A - Transmitter - Google Patents

Abstract

PURPOSE:To provide a transmitter for satisfactorily transmitting signals by preventing lowered impedance even when this device is connected to a bus line. CONSTITUTION:Concerning the transmitter for exchanging signals through a bus line L while being connected from a transmission circuit TX through a transformer T to the bus line L, this device is provided with a switch S between the transformer T and the bus line L for switching the connection between both of them. This switch S is changed over by the enable signal of the transmission circuit TX. Only when the transmission circuit TX is in a transmission enable state, the transformer T and the bus line L are connected. Thus, the impedance of the transmitter is prevented from lowering when it is observed from the side of the bus line L.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transmission device for transmitting and receiving data and the like via a bus line.

[0002]

2. Description of the Related Art Generally, in a bus type balanced transmission system based on the RS-485 standard using a bus line such as a twisted pair line as a transmission medium, a plurality of transmission devices TRX1, TRX2 ,. , TRXN are connected in parallel to the bus line L, and the terminating resistor RT having a resistance value equal to its characteristic impedance is provided at both ends of the bus line L.
1, RT2 is connected. The resistance values of the terminating resistors RT1 and RT2 are 120Ω, and the transmission devices TRX1, TRX2, ...
The maximum number of TRXNs is 32. However, as the number of transmission devices increases, the bus line L
The degree of equilibrium is reduced, and unwanted radiation increases.

In order to eliminate this drawback, as shown in FIG. 5, a transformer T is provided between the transmission circuit TX and the reception circuit RX in the transmission device and the line connection terminal C for connection to the bus line L. By connecting them, the unwanted radiation was reduced by increasing the balance.

[0004]

However, in the transmission device having such a configuration, the transformer T lowers the impedance seen from the line connection terminal C to the transmission device. As a result, the received signal level at the transmission device drops,
There is a problem that the number of transmission devices connected to the bus line L is limited.

The present invention has been made in view of the above points, and an object of the present invention is to provide a transmission device capable of preventing a decrease in impedance even when connected to a bus line and performing good signal transmission. Especially.

[0006]

The invention according to claim 1 is
In a transmission device in which signals are transmitted and received via a bus line, and the transmission circuit is connected to the bus line via a transformer, a switch is provided between the transformer and the bus line to switch the connection between the two. The switching of the switch is performed by an enable signal of the transmission circuit, and the transformer and the bus line are connected only when the transmission circuit is in the transmission enabled state.

According to a second aspect of the present invention, in the transmission device according to the first aspect, a waveform shaping diode is connected between the signal line between the switch and the transformer and the reference voltage terminal. It is a thing.

[0008]

According to a first aspect of the present invention, there is provided a transmission device in which a signal is transmitted and received through a bus line, and the transmission circuit is connected to the bus line through a transformer. A switch for switching the connection between the two is provided, and the switch is switched by an enable signal of the transmission circuit so that the transformer and the bus line are connected only when the transmission circuit is in the transmission enabled state. By doing so, it is possible to prevent a decrease in the impedance of the transmission device viewed from the bus line side.

According to a second aspect of the present invention, in the first aspect of the present invention, the transformer is formed by a signal line between the switch and the transformer and a waveform shaping diode connected between high level and low level reference signal terminals. The waveform distortion of the signal output from is corrected.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram showing a transmission device according to an embodiment of the present invention. This transmission device is a transmission device TRX1, TRX2 of the bus type balanced transmission system shown in FIG.
... is used as TRXN. TX is a transmission circuit that outputs a data signal input from the data input terminal DIN based on a control signal from the enable terminal EN. That is, when a high level signal is input from the enable terminal EN, the output has a high impedance and the data signal is not output (disabled). When a low-level signal is input from the enable terminal EN, the output has a low impedance and the data signal is output (enable). T is a transformer, the primary side of which is connected to the output terminal of the transmission circuit TX. A switch S has one terminal connected to the secondary side of the transformer T and the other terminal connected to the line connection terminal C. Here, the switch S is configured to be opened and closed based on a control signal from the enable terminal EN. RX is a receiving circuit, which is directly connected to the line connection terminal C and receives a data signal transmitted from another transmission device via the bus line L. The line connection terminal C is connected to the bus line L.

Next, the operation of this embodiment will be described. At the time of transmitting the data signal, a low-level control signal (enable signal) is applied from the enable terminal EN to the transmission circuit TX and the switch S. In the transmission circuit TX, the output impedance becomes low impedance and the data signal is output. Here, since the switch S is controlled to be closed by the low-level control signal (enable signal) from the enable terminal EN, the data signal output from the transmission circuit TX is the transformer T, the switch S, and the line connection terminal. It is sent to the bus line L via C. When the data signal is not transmitted, a high level control signal (disable signal) is transmitted from the enable terminal EN to the transmission circuit TX.
Is applied to the switch S, the switch S is in an open state, and the connection between the bus line L and the transformer T is disconnected.

Therefore, in this embodiment, the switch S is closed only when the enable signal is output.
At other times (disabled state), the switch S is opened, so that only the impedance of the transmission device in the enabled state is lowered and the impedances of the other transmission devices are kept high. Transformer T
Since the impedance of the line connection terminal C inside the transmission device does not decrease, the number of transmission devices connected to the bus line L can be increased.

FIG. 2 is a schematic block diagram of a transmission device showing another embodiment of the present invention. In this embodiment, the transformer T 2
A diode D is connected between the secondary terminal and the switch S in the forward direction from the signal line to the reference terminal V _H having the high level voltage of the data signal, and has the low level voltage of the data signal from the signal line. The diode D is connected in the opposite direction to the reference terminal V _L.

According to the present embodiment, the diode D functions to shape the waveform, and the signal output from the transmission circuit TX via the transformer T has no waveform distortion as shown in FIG. 3 (a). The normal pulse signal waveform as shown in FIG. 3B is transmitted to the bus line L via the line connection terminal C.

[0015]

According to the first aspect of the present invention, in a transmission device which transmits and receives a signal via a bus line and is connected from the transmission circuit to the bus line via a transformer, A switch for switching the connection between the two is provided between the bus lines, and the switch is switched by the enable signal of the transmission circuit, and the transformer and the bus line are connected only when the transmission circuit is in the transmission enabled state. By doing so, the impedance of the transmission device as seen from the bus line side is prevented from decreasing, so it is possible to provide a transmission device that can prevent the impedance from decreasing even when connected to the bus line and perform good signal transmission. It was

According to the invention of claim 2, in the invention of claim 1, a waveform shaping diode is connected between the signal line between the switch and the transformer and the high-level and low-level reference signal terminals. The waveform distortion of the signal output from the transformer is corrected.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a transmission device according to an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of a transmission device showing another embodiment of the present invention.

FIG. 3 is a signal waveform diagram according to the above.

FIG. 4 is a system configuration diagram showing a bus type balanced transmission system.

FIG. 5 is a schematic configuration diagram showing a conventional transmission device.

[Explanation of symbols]

 TX transmitter circuit RX receiver circuit T transformer S switch C line connection terminal D diode TRX1 to TRXN transmission device RT1, RT2 termination resistor

Claims (2)

[Claims] 1. A transmission device, wherein a signal is transmitted and received via a bus line, and the transmission circuit is connected to the bus line via a transformer, the connection between the transformer and the bus line is switched. And a switch for switching the switch by an enable signal of the transmission circuit, and the transformer and the bus line are connected only when the transmission circuit is in the transmission enabled state. Transmission equipment. 2. The transmission device according to claim 1, wherein a waveform shaping diode is connected between a signal line between the switch and the transformer and a reference voltage terminal.