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TMUX4827: Request for suggestions in selecting a suitable part.

Part Number: TMUX4827
Other Parts Discussed in Thread: TMUX136, TMUX1072, TMUX2889, ISOW1432

Hi TI,


We require your support in identifying a Mux/Switch/SSR For switching between RS422 signals.

Description:

We are in the process of developing a customized board for military and aerospace applications. The board involves two pairs of RS422 signals: a primary pair and a redundant secondary pair, both operating at a rate of 2.5 Mbps(2.5MHz) the secondary pair acts as a backup and is activated only if the primary pair malfunctions. The driver end utilizes AM26LV31ESDREP. For isolation purposes at the receiver end, we employ an optocoupler to convert the RS422 signals to LVCMOS signals, which will then be connected to an FPGA. To switch between primary and secondary pairs, we've opted for the TMUX4827YBHR IC. The optocoupler requires a minimum of 1.8V across Anode and Cathode to detect a high voltage. An image of depicting the electrical connection of the circuit is attached with this query.

Issue:

The output voltage of the TMUX4827YBHR matches the input voltage across the frequency range of 10KHz to 120KHz. The input, coming from AM26LV31ESDREP, is approximately 4Vpp (differential).

The TMUX4827YBHR output stays equal to the input up to about 120KHz. Beyond this frequency, the output voltage starts to deviate from the input and begins to decline. Around 290KHz, the TMUX4827YBHR output reaches approximately 1.75V(Differential), which is just enough for the optocoupler at the receiver end to detect the signals, despite the input still being 4Vpp (differential).

However, beyond 290KHz, the output drops significantly below 1.75V. Consequently, the optocoupler at the receiver end struggles to recognize the transactions occurring above 290KHz.

Also, we are using a current limiting resistor at the anode pin of opto-coupler. Since the O/P voltage of TMUX4827YBHR is changing from 4Vpp differential (@10KHz) to about 1.2VPP differential (@2.5Mhz), the forward current If is also varying significantly.

Query:

Can you please suggest a suitable part, such as a switch, multiplexer (mux), or solid-state relay (SSR), that would better fit this application and address the issue we are facing with the TMUX4827YBHR?

Regards,

Sameep

  • The TMUX4827 has a −3 dB bandwidth of 18.3 MHz. Any additional resistance or capacitance will reduce the bandwidth further.

    Please show a detailed schematic that includes all components on the RS-422 lines. Also specify the length and capacitance of any cables.

  • Hey Sameep,

    Just reiterating Clemens point here. The insertion loss of the TMUX4827 at 290kHz should be similar to the insertion loss at 120kHz. Looking at the insertion loss curve, you really shouldn't see the impact of the TMUX4827 bandwidth until roughly maybe 5MHz when it starts to decay and at 18.3MHz you'll hit that -3dB point. 
    However, that doesn't mean that the overall bus isn't near it's limits here. Doubling what Clemens has said, a schematic and the length and capacitance of the relevant cables would be useful here to identify the issue.
    Additionally, have you run the tests bypassing the mux and seeing the results?

    Thanks,
    Rami

  • HI Rami ,

    HI Clemens,

    I have attached the image of detailed schematic in this post.

    "Just reiterating Clemens point here. The insertion loss of the TMUX4827 at 290kHz should be similar to the insertion loss at 120kHz. Looking at the insertion loss curve, you really shouldn't see the impact of the TMUX4827 bandwidth until roughly maybe 5MHz when it starts to decay and at 18.3MHz you'll hit that -3dB point."

    ---Does this means that the output voltage(across D1 and D2) should be equal to that of the input Voltage(across S1A and S2A / across S1B and S2B) across a frequency range of 10Khz to 5Mhz?

    However, that doesn't mean that the overall bus isn't near it's limits here. Doubling what Clemens has said, a schematic and the length and capacitance of the relevant cables would be useful here to identify the issue.

    ---We are currently prototyping the board using Dot matrix board. For all RS422 lines we are using 26 AWG twisted pair wire with length not exceeding 15cm.I am not sure on what exactly "capacitance of the relevant cable" means and how to find it.

    Additionally, have you run the tests bypassing the mux and seeing the results?

    --When the mux is bypassed and only one channel is connected directly to the optocoupler (receiver end), It works fine and is having no issues. We have tested till 5Mhz, and it is working fine.

    Also, In the data sheet of TMUX4827 "Figure 6-22. Maximum Signal Swing" Sayas that the maximum swing after 50khz is 2V. What does this maximum swing mean? is this the maximum voltage swing at output?

    Regards,

    Sameep

  • R253/R254 would be needed only for a bidirectional bus; remove them.

    Termination must be at the end of the bus, so R255/R256 are at the wrong place. Remove them, and add termination right in front of the optocoupler.

    HP's Optocoupler Designer's Guide suggests the following RS-422 receiving circuit for high-speed applications:

    Your application probably does not need the second optocoupler and flip-flop; ignore them.

    The 1.62 kΩ resistor limits the DC current. Adjust it so that you get the required forward current for your ACPL-K74T (10 mA?).

    The 20 pF capacitor counteracts the capacitance of the cable and the LED by increasing the current during switching. You can try adjusting it if the edges are too slow, but it should not be larger than about 100 pF.

    R1 and C1 terminate the differential line. R1 is the termination impedance, about 120 Ω (but might need to be somewhat higher because it is in parallel with the optocoupler input); C1 prevents DC currents. For details about AC termination, see section 4.3 of the RS-422 and RS-485 Standards Overview and System Configurations application report; it suggest C1 = 1000 pF, but you can try to make it smaller if the edges are too slow. You could also try completely removing R1 and C1.

    The capacitance of twisted pairs is very roughly 40 pF/m. Your cable should have a datasheet.

  • Hi Clemens,

    Your swift response is much appreciated.

    Ill implement the changes what you have said and update the results here.

    But I am having few confusions.

    1.When we bypassed the MUX (We didn't change any resistors. All were mounted) and connected the output of the driver to the opto-coupler ,It was working fine up to 5MHz.

    a. We didn't remove the mux from the circuit. We tapped the connection from S1A and S2A pins using a small, twisted pair wire (less than 10cm) and connected it to the second channel of the optocoupler (ACPL-K74T)

    b. At the First channel output of ACPL-K74T (Input is given through MUX) it was observed that the transactions were happening up to 290KHz.Above which the voltage was dropping considerably such that the opto-coupler was not able to recognize it as a high (LED was not turning on)

    c. At the second channel output (mux bypassed), there were no issues. In this case it is working fine across frequency up to 5Mhz. (We haven't verified above 5Mhz as it was not in our area of concern).

    d. I wonder as you mentioned the issue in our design is all about terminations and capacitance, how bypassing the MUX (Not removing it from the circuit. Just tapping the input) would provide a proper output from the opto-coupler?

    2.As asked in the previous post can you let me know " In the data sheet of TMUX4827 "Figure 6-22. Maximum Signal Swing" Sayas that the maximum swing after 50khz is 2V. What does this maximum swing mean? is this the maximum voltage swing at output?"

    3. Meanwhile, Ill rig up the new circuit along with the changes you have suggested and come with the results I observed.

    Regards,

    Sameep

  • I have no explanation what the physical reason for the limits in figure 6-22 is. But your observation confirms that the TMUX4827 does not support high voltages at high speeds (worse than indicated by the specified bandwidth).

    Why did you choose the TMUX4827? A simpler switch like the TMUX136 would not have such a signal swing problem.

  • HI Clemens,

    I have no explanation what the physical reason for the limits in figure 6-22 is. But your observation confirms that the TMUX4827 does not support high voltages at high speeds (worse than indicated by the specified bandwidth).

    I am testing TMUX4827 as a standalone device by giving input a frequency range of 10KHz to 5MHz with 5Vpp and observing the Corresponding output. I am Using few decoupling caps. Other than that, no passives are being used. I'll keep this thread updated with my observation.

    Why did you choose the TMUX4827?

    Our final objective is select a mux/switch or a solid-state relay which shall be used in a 2:1 configuration.

    There is no solid point with me to say why we choose TMUX4827 in first place. But here are few things we observed in the datasheet of TMUX4827 when we selected this part.

    a.  As we were having 5V generated on the board, " Supply range: 1.8 to 5.5 V " was suitable for us.

    b. As we were using this to switch between 2 RS422 signals, "Beyond the supply signal range: -12 V to 12 V" property gave us some Voltage room to play around with.

    c. It was having Low Ron

    d. it was having 1A of output current capability.

    e. Moreover, under its application section, it specifies that it can be used for "RS485/232" applications.

    A simpler switch like the TMUX136 would not have such a signal swing problem

    As you suggested I just gone through TMUX136 datasheet once. But I'm having fer questions here...

    a. TMUX136 has a maximum VCC Voltage of 4.8V.I don't have any 4.8V rail in my board.

    b. Thus to use TMUX136 I must add one more regulator to generate 4.8V. Which I think is not the best solution.

    c. Also, in datasheet, under recommended Operating condition its specified that "Input-output DC voltage" (WRT GND) can be up to 3.6V. As in RS422 Voltages Can be up to 6V. Can you please clarify the same.

    d. I'm having 3.3V rail in my board. Is it ok to power TMUX136 using 3.3V rail and to connect IO with RS422 signals? Are the inputs being tolerant to be powered more than VCC voltage?

    If TMUX136 is not the Best fit, can you please let me know of any possible solutions available That can be a direct fit as a RS422 Switch/Mux. (Working at 2.5Mhz)? 

    Is there any Training material available with TI which helps me better understand about RS422 protocol and the necessary termination strategies. If available, please provide us the link for the same.

    Regards,

    Sameep 

  • Oops, the TMUX136 was marked as supporting 5 V in the search function, but it is actually a 3.3 V device. A switch that does support 5 V would be the TMUX1072.

    The TMUX4827 or the lower-bandwidth TMUX2889 are the only switches that allow voltages beyond the supply rails. I do not know how much of a voltage offset you will see between your boards.

    There is lots of documentation about RS-422, but it always assumes a normal (high-impedance) receiver; there is not much information about optocouplers in this situation. Maybe consider using an isolated differential receiver like the ISOW1432.

  • Hi Clemens,

    Thanks for suggesting TMUX1072.

    Before we use TMUX1072 directly in our design, we want to thoroughly assess the capabilities and compatibility of the TMUX1072 to determine its suitability. Could you please provide us with information regarding the availability of TMUX1072 samples? Additionally, if there are any specific requirements or procedures for requesting samples, kindly guide us accordingly.

    Regards,

    Sameep

  • I am not a TI employee; I can just mention to the "Request sample" link on the TMUX1072 product page.



  • Hey Sameep, looks like there's good inventory of the RUT package on ti.com currently. 
    As Clemens mentioned, you can get samples through the request sample link on the product page