DS21T09 Ver la hoja de datos (PDF) - Dallas Semiconductor -> Maxim Integrated

Número de pieza

componentes Descripción

Lista de partido

DS21T09

Plug and Play SCSI Terminator

Dallas Semiconductor -> Maxim Integrated 

DS21T09

BUS CURRENT SENSING

The DS21T09 has the capability to use current sensing to determine if the SCSI bus is over- or under-

terminated. A 1Ω=resistor in series with the SCSI bus (preferable the RST line) is used to monitor the bus

current when that line pulls low (Active or “asserted” state). The resistor is between pad R14 and TCS on

the die. Based on the current measured, the DS21T09 will disconnect or connect from the SCSI bus. The

configuration for this automatic isolation technique is shown in Figure 6.

Figure 5 shows a simplified diagram of the sensing circuit. The voltage across the 1Ω=sense resistor is

differentially amplified and converted into a single-ended voltage with respect to ground. This is fed into

a bank of comparators and measured against a reference voltage. The circuit takes a measurement each

time TCS is driven below a 0.8 volts threshold and the outputs are latched on the rising edge of TCS. If

IBUS is greater than 32 mA, the DS21T09 will be isolated from the SCSI bus. It is recommended that the

signal on TCS be asserted for at least 25 µsec to allow the signal (and comparator outputs) to settle into a

known state. A timing diagram of the sensing and latching operation is shown in Figure 7.

It is preferred that the -RST line be used for monitoring the bus termination status because -RST is only

asserted during power up or during a major change in bus configuration. Note that R14 will have a higher

input capacitance than the other lines because of the additional circuitry required for bus sensing.

The DS21T09 will be isolated from the SCSI bus as shown in Table 2.

BUS CURRENT SENSING CIRCUITRY Figure 5

6 of 10

Share Link: