Is this possible? (Serial sniffing on PCB)

[rarick123]

I'm wondering if something like this is possible... I've got a device that has an onboard RFID reader, and it sends the output of the tag via serial to a microcontroller to "do it's thing" with. I know the output is serial, because the datasheet of the RFID chip actually has the pins labelled as "serial in", "serial out", and "serial clock". It's one of those stupid painted boards, so I can't completely follow the traces to see where they're going, but I'm thinking I shouldn't need to.

I tested this in theory by using a desktop PC connected to a Cisco router via RS-232 console cable. I spliced into the cable, and attached the "receive" line from a second cable (connected to my laptop) to the "transmit" line from the router. If I open up HyperTerm on both the desktop and laptop, and then power on the router, sure enough I can see the router's output in both terminal windows. So, in theory, this is doable.

Now, I don't know what levels or anything the transmission is happening at on these chips. I've tried attaching the same cable mentioned above, as well as an FTDI TTL-232R (5V) cable to the "serial out" pin on this chip, and then brought the RFID chip into antenna range. The biggest pitfall seems to be that I have no idea what speed I should be setting HyperTerm at... on the Cisco router, I already know it's 9600/8/N/1, but how would I determine it in this other scenario? I've got the full data sheets for both chips, but I'm kind of at a loss.

I have a known-value transponder, but no matter what baud rate, stop bits, etc. I choose, and no matter which of the two cables I try above, I get nothing but garbage in my HyperTerm window.

Any help would be GREATLY appreciated!

 

[Pommie]

The fact that there is a serial clock line suggests that it is using synchronous serial which is completely different from RS232. I suggest you google "synchronous serial" for a better explanation.

Mike.

 

[3v0]

Then ID the RFID chip and look at the datasheet to see what it is transmitting.

 

[rarick123]

The RFID chip is a Philips PCF7991AT, and it's sending the contents of an Atmel TK5530 transponder to a Fujitsu MB90F497G.

 

[Nigel Goodwin]

As suggested, it's not RS232 (asyncronous serial), it's syncronous serial, a completely different thing - you need to read the datasheets to find out how to connect to it, but it's probably pretty simple, and you have to provide the syncronous clock yourself. You probably can't do it with a PC serial port very well, but you can with a PC parallel port check out PIC programmers which are a pretty similar thing.

 

[rarick123]

If I provided data sheets and maybe even an engineering sample or two, would someone be willing to help me with this? I'm not an EE or anything, just trying to work through this as best I can.

 

[Boncuk]

SDA and SCL are used for ATMEL controllers for ISP (in system programming).

 

[kchriste]

What you need is a protocol analyzer. Here is an Old HP version which you might be able to find cheap:
**broken link removed**
A newer version (Don't let the name fool you):
**broken link removed**
These will show data from both the DTE and DCE equipment on the serial line and monitor various buss types.