I know this is an issue that has been re-hashed over and over, and I still can't get it to work. I am relatively new to this so will convey what I understand at this point.
I have a temperature controller which is 2-wire (literally there is no ground terminal) RS-485 (which means half-duplex). It is set to 9600 baud (fixed), 8 data bits (fixed), 1 stop bit (fixed), even parity, MODBUS RTU (fixed).
I have a rs232 to rs485 converter that does not automatically control the flow, so I need to account for this in my software. I have it configured as a DCE (since it is a go between the PC and the temp controller which are both DTE). ALso I have it set to T-RTS/R-RTS, so that i can look for the RTS signal.
On the rs485 end it has a 4 terminal block, being the T+, T-, R+, R-. I have the T+ and R+, wired together and attached to the (+) terminal of the temp controller, likewise the 2 negative wires are connected to the (-) terminal. This is how the manufacturer of the device says it should be setup, and seems to make sense to me based on what I understand.
Rs232 is full-duplex, meaning that transmit and receive can occur at the same time, since they are on different lines this makes sense. But sinec the 2-wire rs485 is combining the (+) and (-) of the transmit and receive, this would certainly cause collisions.
The rs232 end has a db-25.
Then there is the cable going to the rs232 serial port on COM1 of my PC. Pins 2,3,4,5,6,7,8,20,22 are wired as is standard.
I've got a rs232 tester in-line cause I like to see blinking lights :>)>
My VI first closes the VISA port, then initializes it using the NI MODBUS library, set as I have laid out above. I have tried it with FLOWCONTROL=none, and also =RTS/CTS.
n both cases, immediately after init, my tester shows TD and RD are HI, and all other are LO. When the VISA port is closed, all are HI except CD.
With FLOWCONTROL=RTS/CTS:
if I have serial property nodes to set the RTS signal in between the writes and reads then I get the VISA error -1073807302 (BFFF003A), which leads me to think that I can not combine the 2 approaches. Is this the case? When flow control is set on the init property node does it conflict with trying to do it manually?
when I take those nodes out, and I attempt a write, nothing is read in from the buffer.
With FLOWCONTROL=none
with the RTS signal nodes in, it does not matter which is set to asserted or unasserted, I receive nothing from the read buffer. What is the terminology "asserted" and "unasserted"? why do they trun both RTS and CTS, HI or LO at the same time? Do I need the RTS to be HI or LO when I send a command from the computer, and when I am trying to read from the temp controller? (it seems to me that is should be HI then LO)
when I take the nodes out I also receive nothing in the input buffer.
I've attempted to set the converter to T-on/R-on, and with this I do receive something in the read buffer, but all it is is the exact copy of what I sent. It does not conform to the message structure that the temp controller manual says to expect from the MODBUS response. It has the device address, the function code, the register address, the number of words, and the CRC. It always seems that it is truncating the actual data, or the setup is acting like nothing but a lookback sending my an exact copy of what I send it. i.e \01\03\03\E8\00\02D{ (or in hex 0103 03E8 0002 447B) is returned when I query read 2 words starting at holding register 3E8 on device 1.
I
hope this is clear what I've done and what I am attempting.
thanks in advance
James
