AM26C31: Input or output clamp current

These drivers are designed to drive a single receiver with 100 Ω … 120 Ω termination. There is no guarantee how much current it can supply at 60 Ω. (The absolute maximum ratings just tell you that the device will not be damaged if you manage to draw 150 mA out of the output.)

When you have two receivers, you should use two drivers, one for each.

The AM26C31 has clamping diodes from GND and to V_CC. The clamping current is the current that flows when you apply an external voltage that is lower than GND or higher than V_CC. It does not matter for normal operation.

Hi Tom,

This is an RS-422 part - so 60 Ohms is out of specification with respect to standard (this is an RS-422 device - which specifies 1, 100 Ohm termination between the differential inputs of the last receiver on the line ) . While it is still within the abs max ratings (150mA)  most of the specifications in the datasheet are not going to go below 100 Ohm loads - so most of the specifications will not be met when operating the device like this. 

The clamping diode current is only when the diodes become forward biased - which under normal circumstances isn't happening.

1. There are clamping diodes to VCC and GND on the A and G pins - you can tell this because the abs max table has a minimum of GND - 0.5V (-0.5V) which would indicate a diode connection and the max is VCC + 0.5V - which is a diode drop above VCC will cause failure (there is clamping current limit) 

2. There is an output clamp diode on the Z/Y pins as the minimum voltage is 1 diode drop below ground (-0.5V) but the max voltage is just a voltage (7V) regardless of what VCC is - so there isn't a diode dependency to VCC on the Z/Y pins.

Regardless - under normal operation these diodes are not forward biased and you don't really need to worry about it. These diodes shouldn't really be used as typical clamps and you should design the system aligned with the recommended operating condition table for best results with the lowest amount of risk.

Best,

Parker Dodson

The line to the second receiver is not properly terminated. This will lead to signal quality problems unless the speed is very low.

For high-speed signals, you can use a linear bus, with the unterminated receivers somewhere in the middle, with the spurs as short as possible:

Hi Tom,

What Clemens said is correct. The unterminated stub length needs to be minimized - for all RS-422 / RS-485 applications the general advise is that the stub should be no longer than the product of 1/10th   the differential transition time and the phase velocity (Speed of light in transmission medium - where the traces/cabling is the transmission media). So Stub Length L <= (t_r or t_f) / 10 * Vp - Where Vp is generally between 70% and 80% the speed of light depending on physical medium -  so the fastest transition time listed is 5ns - so using 75% of c for Vp our max unterminated stub length is ~112mm. 

If your unterminated stub is longer than that there will most likely be significant interference and communication issues.

So the options are:

1. Keep all unterminated stub lengths less that 112mm

2. Create a repeater by using a receiver and driver that will extend bus signal if unterminated node is longer - this will add some delay between these nodes so throughput will be reduced - similar receiver design can be found here: https://www.ti.com/lit/an/slyt480a/slyt480a.pdf?ts=1682356435404&ref_url=https%253A%252F%252Fwww.google.com%252F - it uses RS-485 instead of RS-422 but the same idea can be applied. Also as a note - they use isolated transceivers - not standard - but unless your bus is very long the isolation may not be needed - considering that this bus will be "too long' at 112mm - isolation most likely won't be needed.

3. Terminate every node with 100 Ohms and add additional line driver on driver output to increase drive strength for low impedance bus. All the nodes need to be terminated in this type of topology to prevent EMI issues. We haven't done this on RS-422 parts in the past - but we have on RS-485 using the THS6222 as the additional line driver  - it could work but largely untested with RS-422 devices. 

Those are the main options that you have that should be able to support this application. That being said - option 1 is the best if possible as its the simplest and cheapest implementation of the solution.

Please let me know if you have any other questions!

Best,

Parker Dodson