The MAX33076-9E is a family of robust quad channel RS-485/422 receivers with high common-mode range (CMR), high fault protection, and high ESD for harsh electrical environments. This family operates from a supply of +3.3V or +5.0V.
These devices feature a high CMR of ±25V, which helps in reception of messages when the ground planes between two nodes have large differences or large external interference from motors or other electrical noise sources. These parts have extended fault protection of ±65V where the data lines are protected from accidental shorts to local power supplies. A high ESD human body model (HMB) of ±25kV also protects the data lines from ESD strikes either during production or in the field.
The MAX33076E-79E have true fail circuitry where the receiver output is placed in a logic high state when the inputs are open or shorted. For part numbers MAX33076/7E, the G and
The receiver accepts a RS-485/422 differential input signal on A_ and B_ inputs and transfers it to a single-ended, logic-level output Y_. The RS-485 standard specifies the receiver output state to be logic high or one for differential input voltage of VAB ≥ +200mV and logic low or zero for VAB ≤ -200mV. If the differential receiver input VAB is between ±200mV, the receiver output is not defined and can be either high or low.
The MAX33076/7E enter shutdown when G is low and
Table 1. MAX33076E/77E Outputs Truth Table
| G | (VA-VB) | OUTPUT Y_ | DEVICE MODE | |
|---|---|---|---|---|
| 1 | X | ≥200mV | 1 | On |
| 1 | X | ≤-200mV | 0 | On |
| 1 | X | Open, Short | 1 | On |
| X | 0 | ≥200mV | 1 | On |
| X | 0 | ≤-200mV | 0 | On |
| X | 0 | Open, Short | 1 | On |
| 0 | 1 | X | High-Z | Shutdown |
X = don’t care, High-Z = high impedance
Table 2. MAX22078E/79E Outputs Y1 and Y2 Truth Table
| EN12 | EN34 | (VA-VB) | OUTPUT Y1 OR Y2 | DEVICE MODE |
|---|---|---|---|---|
| 1 | X | ≥200mV | 1 | On |
| 1 | X | ≤-200mV | 0 | On |
| 1 | X | Open, Short | 1 | On |
| 0 | 1 | X | High-Z | On |
| 0 | 0 | X | High-Z | Shutdown |
Table 3. MAX33078E/79E Outputs Y3 and Y4 Truth Table
| EN12 | EN34 | (VA-VB) | OUTPUT Y3 OR Y4 | DEVICE MODE |
|---|---|---|---|---|
| X | 1 | ≥200mV | 1 | On |
| X | 1 | ≤-200mV | 0 | On |
| X | 1 | Open, Short | 1 | On |
| 1 | 0 | X | High-Z | On |
| 0 | 0 | X | High-Z | Shutdown |
The RS-485 standard defines the common-mode range as -7V to +12V for the receiver. For the MAX33076-9E family, the common-mode range exceeds the standard with ±25V. This feature was specifically designed for systems where there is a large common-mode voltage present due to either nearby electrically noisy equipment or large ground differences due to different earth grounds or operating from different power transformers.
This family incorporates true fail-safe circuitry to ensure a known output high state when a fault is detected. The three fault scenarios are: both wires are disconnected or broken and causes an open circuit, a short circuit caused by miswire or insulation breakdown between a twisted wire pair, or when it is connected to a terminated transmission line with all drivers disabled. The output goes logic high when the inputs are between ±50mV for more than 10µs.
To reduce system complexity and the need for external protection, the receiver inputs of these devices are designed to withstand voltage faults up to ±65V with respect to ground without damage. This type of fault typically occurs in the field when service personnel accidentally short the local power supply lines to the input receiver lines. These inputs are capable of tolerating ±65V whether the device is powered or unpowered.