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AM26C32-EP: Does AM26C32MDREP need termination in long line transmission?

Rodolfo Lima
Prodigy 10 points
Part Number: AM26C32-EP
Other Parts Discussed in Thread: AM26C32


Hi.

Question:
I would like to confirm that AM26C32MDREP CAN USE this 120 Ohm termination resistor.
Besides, I would appreciate an explanation of the role and rationale for such termination in the circuit.


Explanation:
I need to transmit a TTL signal from a subsystem to another subsystem 25m away.
I decided to use AM26C31MDREP transmitter and AM26C32MDREP receiver in a 25m shielded twisted pair (in a cable assembly) transmission (TTL-RS422-TTL).

In the past I used the obsolete AM26LS31ACN/AM26LS32ACN, which would frequently get faulty and need receiver replacement.
This issue stopped when the receiver was terminated with a 120 Ohm resistor, as sugested in the old data sheet.

Well, the present data sheet for AM26C32MDREP (AM26C32-EP) in Texas Instruments website is dated 2007:
This data sheet has no mention of a termination resistor.
Nonetheless, the AM26C32 family data sheet in Texas Instruments website is dated 2018 and has somewhat more detailed electrical information.
In the AM26C32 data sheet Section 9.2 Typical Application, page 10, the termination resistor is mentioned (see Figure 4 Differential Terminated Configuration) and recommended values are 80 to 120 Ohm, just like the old AM26LS32ACN.
I am inclined to use the resistor termination since the old system would frequently get faulty receiver without it. My design colleague does not want the termination since he believes it will increase current, power consumption and decrease efficiency.

I would like to confirm that AM26C32MDREP CAN USE this 120 Ohm termination resistor.
Besides, I would appreciate an explanation of the role and rationale for such termination in the circuit.



Here I add the directions to the data sheets in TI website.

AM26C32-EP Enhanced Product Quadruple Differential Line Receiver Data sheet
www.ti.com/.../am26c32-ep.pdf
www.ti.com/.../slls870.pdf


AM26C32 Quadruple Differential Line Receiver datasheet (Rev. L)
www.ti.com/.../am26c32m.pdf
Section 9.2 Typical Application, page 10
Figure 4. Differential Terminated Configuration


Thank you very much for your support.

  • 0
    TI__Guru 53555 points

    Hi Rodolfo,

    In short - The purpose of the termination is to prevent adverse transmission-line phenomena, such as reflections. 

    This is a common question regarding the RS-422 and RS-485 standards. Termination networks can be implemented in several ways that can make referencing existing designs confusing as to their purpose and how they fit in to your own design. For this reason, TI has many technical documents and application reports on the subject to help explain the purpose termination plays in these systems and different techniques so that a proper scheme can be chosen for your unique design. 

    RS-485 basics: When termination is necessary, and how to do it properly

    AN-1057 Ten Ways to Bulletproof RS-485 Interfaces – Section 6 and 7

    AN-903 A Comparison of Differential Termination Techniques

    Common applications for RS-422/RS-485 devices are to carry serial data over a distance of cable (such as the system you describe) or through an electrically noisy environment. In cases where fast transitions (high data-rates) or long cables are used, signal reflections can greatly impact the signal quality seen at the receivers in a system. A reflection is where some of the energy of the signal is literally reflected back through the transmission line when it reaches a terminal. This energy can constructively or destructively interfere with bits transmitted later on and may cause glitches or misinterpreted bits at a receiving node. Adding termination resistors that match the impedance of the cable allows this energy to be dissipated, protecting the bus from reflections. This is likely why you saw improved performance when adding 120-ohm termination to your past system.

    120-ohms is typically used because it is a common cable impedance value. Matching this impedance optimizes reflection dampening.

    Because adding termination does increase the overall loading of the bus (decreases total resistance between differential lines) it does decrease the voltage differential created by the drivers (output current is relatively fixed – Resistance decreases – voltage must decrease). You will also see a slight increase in power draw from the drivers as well, though many factors will impact consumption of these devices (signal rate, capacitive load, number of nodes, temperature, etc.). Because termination can provide great improvements in signal quality, this power is often well worth the increase in data reliability. If power consumption is a concern, you may look into what optimizations can be made in other areas.

    I hope you find this information helpful. Let me know if you have any other questions regarding my comments or the linked content.

    Regards,

    Eric