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RS-485 ASCII

Created by: subin at: 12/26/2014 5:18 AM (19 Replies)

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12/26/2014 5:18 AM	Rate (0)
subin Posts: 84 Rating: (20)	Sir, I am having a Weighing controller with RS-485 ASCII I need to communicate with S7-1200 PLC with CB1241 RS485 communicaion board. I have done earlier for modbus communication with two wire connection. But this is new to me. In the weighing controller it have four terminal. Two terminal(SD A-, SD B+) for send two terminals(RD A-, RD B+) for receive. Please help me understand how to do a four wire connection. and sample program for the same. Thank you.
subin Posts: 84 Rating: (20)
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12/26/2014 5:49 PM	Rate (0)
Marko Bursic Platinum Member Joined: 9/8/2009 Last visit: 7/17/2023 Posts: 1410 Rating: (154)	That's RS422, look in the manual if you can connect them (4 wires) together for rs-485 operation (2 wires). If you can't, then you may use rs-232 moodule instead, and rs232 to rs422 converter.

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12/27/2014 3:15 PM	Rate (0)
subin Posts: 84 Rating: (20)	Sir, But the weighing controller says its RS-485. I hereby attach the wiring diagram for the weighing controller. Is that RS-485 is always 2-wire? This weighing controller have RS-232 port too. But in PLC i only haveCB 1241 RS485. Attachment unipulse.jpg (742 Downloads)
subin Posts: 84 Rating: (20)
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12/27/2014 3:35 PM	Rate (0)
subin Posts: 84 Rating: (20)	Sir, I herby attach the manual for the weighing controller communication option Attachment Option.pdf (645 Downloads)
subin Posts: 84 Rating: (20)
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12/28/2014 12:15 PM	Rate (1)
Marko Bursic Platinum Member Joined: 9/8/2009 Last visit: 7/17/2023 Posts: 1410 Rating: (154)	subin Sir, But the weighing controller says its RS-485. I hereby attach the wiring diagram for the weighing controller. Is that RS-485 is always 2-wire? This weighing controller have RS-232 port too. But in PLC i only haveCB 1241 RS485. It says also fullduplex, this is not good for your app. Looking forward in the manual it just receives a command and sends the result (half duplex). It could be 4 wire or 2 wire, in a multidrop network master<->slave it is easier to connect 2 wires only and normaly the comm. is half duplex: one device sends a query to a specified device (ID), the selected (ID) device responds, all othersare in silence. Therefore 2 wires are just enough., however fullduplex is not possible. Try to hook together SD A- and RD A-; SD B+ and RD B+. Put a 120 ohm resistor on each side.

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12/28/2014 3:34 PM	Rate (0)
subin Posts: 84 Rating: (20)	Sir, can i connect as SD A- (Send Data) to TA , SD B+ (Send data) to TB, T/RA (Receive Data) to RD A- and T/RB (Receive Data) to RD B+ respectively. will it be same? Also i am new to ACSII communication. Is it i need to used SEND_PTP and RCV_PTP to request and receive data? Is there any sample program available? I have only one Weighing controller and i just need to get gross and net weight to the PLC.
subin Posts: 84 Rating: (20)	Last edited by: subin at: 12/28/2014 3:57 PM
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12/28/2014 9:28 PM	Rate (3)
danw Gold Expert Joined: 3/28/2010 Last visit: 2/21/2025 Posts: 1116 Rating: (218)	This site went down last night and I couldn't post this last night, and Marko has since replied with essentially the same assessment as I have: Page 17 of the F805AT-CK Loadcell indicator option manual has the specs for the RS-485 interface, Transmitting method: asynchronous, fullduplex Full duplex allows transmission on one pair of wires, SD (send data) while simultaneously allowing reception of data on the other pair of wires RD (receive data). RS-485 can be either a full duplex (4 wire) communications link or a half duplex (2 wire) communications link. I don't know what the CB1241 RS485 communication board is equipped with (2 or 4 wire RS-485 or something else) It's easy to convert from 4 wire to 2 wire for RS-485, - jumper the (+) to (+) to make a (+) connection and - jumper the (-) to (-) to make a (-) connection. but doing so converts the device from a 4 wire full duplex RS-485 to half duplex RS-485. If the load cell controller truly expects full duplex communications, it will easily make a mess of communications by sending, not listening, when the half duplex PLC is sending. You've asked, "can i connect as SD A- (Send Data) to TA , SD B+ (Send data) to TB, T/RA (Receive Data) to RD A- and T/RB (Receive Data) to RD B+ respectively. will it be same?" I don't know what TA or TB is, probably the A and B lines for 2 wire RS-485 on the CD1241, so it is unlikely that half of a receive or transmit line will operate properly (otherwise the complementary wire wouldn't be there, would it?) The redeeming virtue of RS-485 is that if you connect it backwards, it doesn't die. It won't work, but it won't damage the drivers. Try it and see. I doubt it will work, but you can try it. When it doesn't work, try jumpering (+) to (+) and (-) to (-), connect one to A, the other to B, and see if works. If it doesn't, keep the jumpering, but swap the connections to A and B. Again, it's not clear to me that a device spec'd as full duplex will ever operate properly in half duplex mode.

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12/29/2014 9:59 AM	Rate (0)
subin Posts: 84 Rating: (20)	Sir, Thanks for your reply. From your reply i get clear about full duplex and half duplex. I here by attach the pin detail of the CB1241. please let me know if it is workable as mentiond before or else i will try just by directly jumping the both send and receive + and -. Attachment cb1241.jpg (676 Downloads)
subin Posts: 84 Rating: (20)
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12/29/2014 10:04 AM	Rate (0)
subin Posts: 84 Rating: (20)	Now the next i need is how to do the communication program. I saw the Functions SEND_PTP and RCV_PTP but not understand how to enter the string command in the buffer. From the manual i see to address the slave we have to send a start commend S01CR. and to read gross weight we have to send read command RACR. Please help me understand on PTP program. will be helpful if a can get a sample program. Attachment unipulse_communication.jpg (591 Downloads)
subin Posts: 84 Rating: (20)
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12/30/2014 12:06 PM	Rate (0)
Marko Bursic Platinum Member Joined: 9/8/2009 Last visit: 7/17/2023 Posts: 1410 Rating: (154)	For proper termination and bias you need to connect TA<->TRA, TB<->TRB. Why don't you use CM1241 RS485, instead? It has the possibility to connect with 4 wires and full duplex. It would be easier for you to get a PC<->RS485 converter, for testing and tracing with PC. You won't throw your money, if you think to work in this sector also in the future.Converters with galvanic isolation are more expense, but essential if working in the field, frankly I don't buy if it is not isolated. As for ptp comm. program, I recomend you to open a new thread, once you will be surethe hardware connection is working.
	Last edited by: Marko Bursic at: 12/30/2014 12:25 PM
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