AMIS-49200 Datasheet, PDF (15/22 Pages) AMI SEMICONDUCTOR – Fieldbus MAU Chip

AMIS-49200 Fieldbus MAU
Data Sheet
4.3 Transmit Block
The transmit block contains four sub-blocks:
1. MDS-interface – decodes input signals to generate internal control signals.
2. Tri-level modulator – generates current signals used as inputs to the slew-rate controller.
3. Slew rate controller – converts current to three distinct VDRV voltage levels (V_S, V_H, V_L).
4. Current drive amplifier – op amp designed to drive current drivers for 31.25kbps voltage-mode medium.
4.3.1. MDS-interface
The MDS-interface decodes input signals to generate internal control signals. The POL pin, is used to select the polarity of TxE
(transmit enable). The TxE and TxS (transmit signal) are the MDS-MAU interface signals. These three signals are CMOS logic signals
powered by the V_DD supply voltage. When POL is connected to GND, TxE is assumed to be active high (positive logic). Likewise, if
POL is connected to V_DD, TxE is assumed to be active low (negative logic). See Table 11 to see how MDS_CTRL Pin 26 can be used
to control MDS interface operation. The following table shows the resulting VDRV output for the various combinations of interface
signals.
Table 11: MDS-interface Logic
POL TxE
TxS
Low
Low
High
Low
High
Low
High
High
Low
High
Low
High
Low
High
VDRV
V_S
V_H
V_L
V_H
V_L
V_S
4.3.2. Tri-level Modulator
The tri-level modulator switches current signals into a summing node. The slew rate controller converts the current to a voltage signal,
VDRV. The DC level of silence (V_S) is nominally 2.5V. Transmission high (V_H ) is nominally 2.9V and transmission low (V_L) is nominally
2.1V, yielding an amplitude of 0.8V.
N_VL
N_Vs
Tri-Level Modulator
&
Slew Control
Active Low
4R
80K
Active Low
4R
80K
R
20K
VCC
20R
400K
A3
VMID
VDRV
1.2K
21
1.2K
CRT
19
1.2K
Figure 8: Tri-level Modulator
AMI Semiconductor – Jan. 07, M-20532-003
15
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