JPS6173443A - Two-wire time division bidirectional communication device - Google Patents

Abstract

PURPOSE:To prevent the generation of a differential waveform in the output of an equalizing amplifier at switching from a transmitting period to a receiving period by providing an offset cancelling circuit between a receiving transformer and the equalizing amplifier. CONSTITUTION:The offset cancelling circuit 17 is constituted of a capacitor 12 and a switch 13. An offset charging period Tc is formed during the period from the start of the receiving period Tr to the arrival of a receiving signal S12. An input offset voltage of is charged in the capacitor 12 by shorting the switch 13 during the period Tc. After the priod Tc, the switch 13 is opened. Since the receiving period Tr has been already started at that time, a switch 15 is shorted and a switch 14 is opened, so that an input signal S12 of the equalizing amplifier 12 is obtained by subtracting the of from a transmission/ reception signal S11 in the receiving priod Tr as shown in S12. Thereby, the output S13 of the amplifier 16 is prevented from the generation of a differential waveform which may be generated in an ordinary example.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an offset cancellation circuit in a two-wire time-division bidirectional communication device.

[Prior art]

A two-wire time-division two-way communication device is a device that performs two-way communication by dividing two wires into two in terms of time and using each as a transmission period and a reception period in order to transmit and receive data. . The configuration of the communication device IO is shown in FIG.

In FIG. 1, 1 is an input/output terminal of a communication device 10, 2 is a transmitting driver, 3 is a transmitting transformer, 4 is a receiving transformer, 5 is a switch for shorting the input of an equalizing amplifier, which will be described later, and 6 is an equalizing amplifier, which will be described later. 7 is an equalizing amplifier.

In the conventional example shown in FIG. 1, as shown in FIG. 2, during the transmission period Tt, the switch 5 is generally shorted and the switch 5 is opened, so as not to apply the large amplitude signal Sla to the input of the equalizing amplifier 70. It is. Now the plus of the transmission output Sla,
If there is an imbalance in the negative amplitude, the equalizing amplifier 7
A DC offset voltage is generated at the input of the

Next, during the reception period Tr, when the switch 5 is open and the switch 6 is short-circuited, the input signal S2 of the equalization amplifier 7 becomes a staircase wave S2a like 52 in Fig. 5S2.

Note that the equalizing amplifier 7 increases the gain in the high frequency range in order to compensate for the high frequency transmission loss of the transmission line 9. Therefore, the output S3 of the equalizing amplifier 7 has a differential waveform like 53 in FIG. 2.

Therefore, the conventional 9! In (10), when the amplitudes of the input DC offset voltage and the input signal S2 become approximately the same, the output S3 of the equalizing amplifier 7 has a DC voltage as shown in S3 in FIG.
There was a drawback that the differential waveform due to the offset and the human input signal could not be distinguished, resulting in incorrect data.

[Problem that the invention seeks to solve]

SUMMARY OF THE INVENTION An object of the present invention is to prevent the above-mentioned drawbacks of the prior art, namely, the differential waveform that occurs in the equalization amplifier output when switching from the transmission period to the reception period.

[Means for solving problems]

The present invention provides an offset canceling circuit that includes a capacitor connected in series between a receiving transformer and an equalizing amplifier of a two-wire time-division bidirectional communication device and a switch that short-circuits the input of the equalizing amplifier. It is.

〔Example〕

The configuration of an embodiment according to the present invention is shown in FIG.

11 is a receiving transformer, 12 is a capacitor for offset charging, 13 is an offset charging switch, ! 4 is a switch for short-circuiting the equalizing amplifier input, 15 is a switch for turning off the equalizing amplifier, and 1B is an equalizing amplifier.

The embodiment configured as described above provides an offset charge period ↑C from the beginning of the reception period Tr until the arrival of the reception signal S12 (see FIG. 4), and in this offset charge period Tc. By shorting switch 13 in FIG. 3, the capacitor voltage is charged with the input offset voltage Vof (see @Vaf in FIG. 4).

When the offset charge period Tc ends, the switch 13 is opened. In this case, since the reception period Tr has already started, the switch 15 is short-circuited and the switch 14 is open, so
The input signal S12 of the equalizing amplifier to is S in FIG.
+2, the transmission/reception signal Sll in FIG. 4 during the reception period Tr.
It is obtained by subtracting Vof in FIG. 4 from .

1. Therefore, the staircase wave S2a in FIG. 2 is no longer seen at the input of the equalizing amplifier 1B, and the output 51 of the equalizing amplifier 1B
3 does not produce a differential waveform due to this (Sl in Fig. 4:
(About Sanfu).

〔Effect of the invention〕

As described below, the present invention has the effect of canceling out the offset cancellation signal at the beginning of each reception period even if it differs from transmission period to transmission period.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a conventional example, FIG. 2 is a timing chart explaining the same, FIG. 3 is a circuit diagram showing an embodiment according to the present invention, and FIG. 4 is a timing chart explaining FIG. 3. It is. ll: Receiving transformer (means) 14.15: Switch 16: Equalization amplifier 17: Offset cancellation circuit 12: Capacitor 13: Switch

Claims (1)

[Claims] 1) A device for bidirectional communication by temporally dividing two wires into two for transmission and reception, comprising: A two-wire time-division bidirectional communication device, characterized in that an offset cancellation circuit is provided between the two wires. 2) The communication according to claim 1, wherein the offset canceling circuit is comprised of a capacitor that charges an offset voltage and a switch that short-circuits the input of the equalizing amplifier means. Device.