Today is...
Monday, November 12, 2018
Welcome to the Modbus Community, about
the world's leading automation protocol.
Connecting to Deep Sea 7320 Using Modbus
Connecting DSE 7320 to my laptop using modbus RTU

Hi guys,

I'm new to DSE 7320 and maybe this question is not relevant to the topics. But I hope any of you could help me out of this case.

So yesterday, I try to connect DSE 7320 to my laptop using modbus RTU. I was using an RS232-usb converter (trendnet TU-S9) and a null-modem/cross-over cable (as specified in "DSE7320-MKII-Operators-Manual" page 26 of 168) and modscan64 . Unfortunately, I failed to connect.

Could you help advice? Actually, I have one assumption of problem cause. It is stated that "DSE DOES NOT recommend use of USB to RS232 convertor...", in the same page as mentioned above. BUt, is it really the problem? Since for me there is no enough explanation of why they not recommending this.

What I've checked and make sure:
1. it is modbus RTU
2. same baud rate and same byte format (8 bit,No parity, 1 stop bit)
3. working USB-rs232 convertor (tested for other use)
4. properly connected cross-over cable (since we made it)
5. working modscan64 (tested for other use)

Will really appreciate any suggestion. Thanks.

1. I can't find your 168 page manual. The 120 page manual I found
says: default Modbus settings are 19.2K baud, Modbus slave address 10 (presumably decimal).

Are you polling slave 10 at 19.2K baud?

2. The manual I found mentions that Modbus RTU is there for use with a telephone modem for remote communications. A phone modem infers handshaking, not just a cross-over connection. The DSE 232 is probably not just the 3 wire Tx > Rx, Rx > Tx and Gnd > Gnd; it's probably got control signals, too?

I'd be re-checking the pinout numbering of the home-brew 'cross-over' cable. More than once when I've dealt with DB9's I read the pinouts backwards because the diagram was 'viewed from the back' rather than from the front or reversed for male/female. It happens. I once ran into 'bad' molded straight through serial cable out-of-the-box, several of the pins had no connection to the cable. Not common, but it happened.

3. There's a whole section in the manual on RS-232 for a phone modem. You're doing direct connect, not a phone modem. Make sure that the modem is disabled, otherwise comm for the modem complicates things.

"When the DSE7300 series module is power up, it will send 'initialisation strings' to the connected modem. It is important therefore that the modem is already powered, or is powered up at the same time as the DSE7300 series module. At regular intervals after powerup, the modem is reset, and reinitialised, to ensure the modem does not 'hang up'."