This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

THVD1551: Appropriate for use in full duplex multiple node application?

Part Number: THVD1551
Other Parts Discussed in Thread: THVD1512, THVD1511, THVD1552

Hello,

I have designed a system that will eventually have 100+ nodes communicating at full duplex 115kbps using RS485.

In testing, I have discovered that using the THVD1551 as the full duplex RS485 transceiver, the nodes receive perfectly well regardless of how many are connected to the bus. However when more than one node is on the bus (in addition to the "primary" device), the ability of any individual node to transmit no longer functions correctly: the signal becomes significantly attenuated.

Re-re-reading section 9.4.2 Table 3 of the datasheet, I see that since the THVD1551 is always enabled as driver and receiver, the Y/Z pair are actively driven to H/L, respectively, when the node is idle. Therefore, my suspicion is that this is causing the signal problem: one node is idle, therefore driving the Y/Z pair to the H/L state, while the active node is trying to counteract those states during transmission.

Is this correct? If so, is there a solution to this so that multiple nodes can use the THVD1551 to drive the Y/Z pair? The datasheet doesn't indicate that the THVD1551 is inappropriate for use as a full duplex transceiver in a multiple node configuration.

  • "Full duplex" means that one transceiver can send and receive at the same time. Full-duplex transceivers are intended for buses with two separate signal pairs, one for each direction.

    If multple transmitters are connected to the same bus, you must use devices that can disable their outputs, e.g., THVD1512. This is shown in figure 23:

  • Thank you for the response. My application has separate signal pairs for each direction. The only difference between my application and the image shown is that the transceiver is the THVD1511, rather than the 1512.

    Am I correct in thinking that the THVD1511 cannot be used with multiple nodes?

  • Daniel,

    THVD1551 doesn't work here because there is no enable/disable pin for TX. The reason is like what Clemens mentioned. For your application, please choose THVD1512 or THVD1552.