The question of tss521

I am assuming that you are using Figure 7 of the TSS521 Datasheet as a schematic reference. Where is Rload, which should be a 100 kOhm resistor from VDD to GND? Was RRIDD calculated using Equation 2? 27 kOhm will allow for a BUS current of a little more than 1.5 mA and STC current of 1.2 mA. RRIS should also have been derived from Figure 3 but is acceptable at a minimum of 100 Ohms.

You should be using the RXI and TXI pins, with TXI connected to UCA0RXD/P1.1 and RXI connected to UCA0TXD/P1.2. Can you confirm with an oscilloscope or logic analyzer that the TSS521 is attempting to send something to the MCU? Are you using ISRs or polling to receive characters in the MSP430G2553 firmware? Are you receiving anything inside of UCAxRXBUF, and if so is the data received not as expected?

Regards,
Ryan

Dear Ryan,

First thank you for replying my post.

In fact,I use Figure 6 of the TSS521 Datasheet as a schematic reference,so a 100 kOhm resistor is not necessary from VDD to GND.The values of Equation 2 are not confirmed,27 kOhm of RRIDD refered to "Recommended Operating Conditions". The value of RRIS should be properly to set 100 Ohm.

To be easy to debug, now I use USB-UART module instead of MSP430 to connect to TSS521. TX and RX of TSS521 connect to RXD and TXD of USB-UART module. When GND of USB-UART do not connect to GND of TSS521, master transmits character, USB-UART module can receive it; USB-UART transmits character, master can not receive it. When GND of USB-UART connect to GND of TSS521, master transmits character, USB-UART module can not receive it; USB-UART module transmits character, master can receive it. In this case, the temperature of TSS521 is very high in a short time. TXI and RXI connect to RXD and TXD, it does not work.

So, why TSS521 can not work normally? Why the temperature of TSS521 is so high? I am waiting for your answer, thanks.

Figure 6 requires a BSS84 p-channel MOSFET and preferably a higher CSTC capacitance. GND should be shared between both ends of the system, regardless of setup. The temperature of the TSS521 is alarming, have you tested on only one device or multiple? What is your BAT and RX voltage level? Can you provide any oscilloscope or logic analyzer screenshots of the RX and TX lines?

Regards,
Ryan

Ryan,

Thank you for your reply. Now I  have discovered the reason of the problem I encountered before. The level of mbus master I use is +15V and -15V, so when TSS521 switches in mbus, GND of TSS521 is about -15V. If GND is shared between TSS521 and usb-uart module, the voltage difference of 15V will cause more than 40mA through TSS521. As a result, the temperature of TSS521 will become very high and can not communicate with master normally. So, I use optocoupler to isolate them like Figure 9 of datasheet. Now, TSS521 become normally and can communicate with master and usb-uart module.

But, I have another problem when I use MSP430 instead of usb-uart module to communicate with TSS521. Because the current modulation from RX or RXI of TSS521 is repeated concurrently as ECHO on the outputs TX and TXI, when MSP430 transmits characters to TSS521, TSS521 will transmit to MSP430 as ECHO, then MSP430 transmits to TSS521. AS a result, the master will receive characters all the time. So, what should I do when programming? 

Glad to hear that you were able to find a solution to your overheating issue. Either wait until the USCI is not busy or disable USCI receive functionality when sending bytes to the master, since you already know that data collision will occur on the TX/TXI lines when both sides are attempting to communicate.

Regards,
Ryan

Hi, look up the TSS521 datasheet, on page 4, there is a line says "Because the TSS521 is configured for half-duplex only, the current modulation from RX or RXI is repeated concurrently as ECHO on the outputs TX and TXI."   Is that mean when MSP430 try to send data to TSS521, then TSS521 will send data back?    

If that's true,when i send more than one bytes,is there any problem? 

On page 10, I use Figure 9 to do experiment (did not add Cvdd and Rload) and I found when I use USB serials port  on PC to send data to TSS521, one byte sended is OK,but when I send more than two bytes ,the data received is wrong. I  use oscilloscope to see TSS521's RXI pin, the waveform shows the first two bytes is right,but the third byte's waveform is not the expected. Down below is the waveform when I send data 0x01, 0x02, 0x55. The red line shows the TSS521's RXI,while the blue line shows

the USB serial port's TXD pin.  You can see the red line ,the last bit is fall down not the right way.