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Optimum Line Receiver Threshold Voltage Generating Scheme

IP.com Disclosure Number: IPCOM000089564D
Original Publication Date: 1977-Nov-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 3 page(s) / 65K

Publishing Venue

IBM

Related People

Hong, JH: AUTHOR [+2]

Abstract

In a data communication network (Fig. 1), the input voltages at successive receivers decrease because of the line voltage drop. When the resistance of the cable is sufficient, a single threshold voltage for all the receivers is not possible with an emitter follower or open collector line driver. To overcome this problem, it is necessary to adjust the threshold voltage of each receiver as a function of cable resistance measured from the driver.

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Optimum Line Receiver Threshold Voltage Generating Scheme

In a data communication network (Fig. 1), the input voltages at successive receivers decrease because of the line voltage drop. When the resistance of the cable is sufficient, a single threshold voltage for all the receivers is not possible with an emitter follower or open collector line driver. To overcome this problem, it is necessary to adjust the threshold voltage of each receiver as a function of cable resistance measured from the driver.

In Fig. 2, a single threshold arrangement is shown to aid in the analysis. The output voltage from the emitter follower driver is V(1) volts. Then the input voltage at the kth receiver is:

(Image Omitted)

When the driver changes its state to a logical zero, the transient voltage waveform at the driver is as shown in Fig.3. Note that

(Image Omitted)

Let V(od) = Alpha V(1). Then (8) becomes R(dc) </= 2R(s) over (1 + 2 Alpha) + 1 - 2 Alpha over 1 + 2 Alpha R(o) (11)

If the first receiver is close to the driver, Rs = 0 and the maximum cable resistance for a single threshold voltage system is given by R(dc(max)) = (1 - 2 Alpha over 1 + 2 Alpha) R(o) < R(o).

In Fig.4, an implementation of the automatic variable threshold voltage is shown. A driver sends a logical 1 level prior to the selection sequence and at each receiver a voltage which is equal to half of the received voltage is generated by a closed loop threshold voltage generator. Then at the kth receiver, the thresh...