String not going out to IP Port

I'm working with a MOXA N-Port device. I'm connecting to it from the controller over IP OK, but for some reason when I try to send data to that IP connection, nothing happens. The code:

dvipproj_12	=	0:4:0 //Room 12

ip_client_open(dvipproj_12.port,rm[2].moxaip,966,IP_TCP)

data_event[dp_all] { //Moxa Proj Port
    online: {
	stack_var integer this_did
	this_did = get_last(dp_all)
	call 'debug' ("'Moxa Proj RM ID=',itoa(this_did),' RM NUM=',rm[this_did].name,'IP=',rm[this_did].moxaip,' Port Open... '")
    }
}

call 'debug' ("'Send_String to Rm ID 2 Name ',rm[2].name,': Type:',itoa(d[2].type),' Cmd ID:',itoa(d[2].lastcmd),' STR:',cmd[d[2].type].str[d[2].lastcmd].dat")
send_string dvipproj_12,"cmd[d[2].type].str[d[2].lastcmd].dat,$0D"

In the DATA_EVENT, I get the debug message that the port is, indeed, open.

In the very last line in the code, if I swap in a spare serial port for the IP port, the string goes out to that serial port OK. But when sending to the dvipproj_12 device, nothing. I can telnet into the controller and send strings directly to any port, and invariably the string will go to any serial port, but not to the IP port.

Can anybody help me out with this one? I'm stumped.

Thanks,