MicroLogix 1500 Modbus RTU Master

[bobby_d 0]

Modbus Master RTU is a recent feature addition to the MicroLogix 1500 LRP Processor. I would like to use the MicroLogix 1500 Serial Modbus comm connection to pull floating point values out of a Eurotherm 5180V Video Recorder. We don't presently have the tech support contract, so I have only gotten limited info from A-B regarding Modbus. Our local distributor does not have experience with Modbus. However, a write-up in the A-B SLC comm's manual (MicroLogix is supposed to work the same) indicates that the decimal Modbus memory address entry in the A-B "MSG" instruction block is converted to a Hex address and is then sent to the Slave. Eurotherm tells me that the 5180V will only respond to decimal memory addresses. Hex addresses will get no response. Both A-B and Eurotherm claim that they comply with standard Modbus protocol. Can any shed any light on this situation? Thanks.

[Eddie Willers 2]

Strictly speaking, the "addresses" in a Modbus RTU frame are represented in "twos complement binary", not in "hex" or "decimal". 02 hex = 02 decimal = 00000010 binary, for example. 0C hex = 12 decimal = 00001100 binary, for another example. It doesn't really matter what form your programming software represents the value, as long as it comes out correctly in binary format. The only way a "hex address" and a "decimal address" could be different is if the Eurotherm device were using Modbus ASCII protocol, in which numbers are represented by their ASCII codes instead of in binary format. A-B supports only Modbus RTU format, and everything I read about the Eurotherm device says that it sues Modbus RTU format too. When you say "memory address", do you mean the Modbus register, or the network slave address ? You might have to get between the MicroLogix and the Eurotherm with a serial analyzer to figure this out for certain. Can you communicate with the Eurotherm via their own software ? Trapping and analyzing that traffic might give you a place to start.

[GALMA 0]

Hi, I must link MicroLogix1500 as Modbus RTU master to 5 inverter, but i don't know how use second port of 1764-lrp. I use this port always as 232 DF1. How connect 485 Modbus tu sub-d 9pin on LRP?? Help me please.

[Ken Roach 194]

I use the 1761-NET-AIC module to connect the MicroLogix Channel 1 RS232 port to a 3-wire RS485 network. I usually use the ordinary 1761-CBL-PM02 programming cable to connect the 1761-NET-AIC to the MicroLogix 1500 Channel 1 port, but you can also get a shorter 1761-CBL-AP00 cable that has the same pinout. Alternately, any null-modem DB9F/DB9F cable will connect the Net-AIC to the MicroLogix Channel 1 port. Which kind of inverters, exactly, are you using ?

[GALMA 0]

They are Commander SK of Control Techniques. They have a 485-Modbus pot on board of standard. When you use NET-AIC, how configure chanel 1 on RSLogix500? How DF1 or Modbus? When it's impossible linking directly to port without NET-AIC??

[Ken Roach 194]

Configure the MicroLogix 1500 Channel 1 port for Modbus RTU Master protocol. You will have to write logic in the controller with MSG instructions to send Modbus data to the inverters. The MicroLogix 1500 Channel 1 RS-232 port cannot be directly connected to the Control Techniques drives RS-485 port because the two signalling methods are electrically dissimilar. RS-232 RS-485

[GALMA 0]

Ok, I believe that port was 232 and 485 togheter. But, port is only 232. In rslogix, i can configure channel 485 too, but the elettrical link is always 232. I need of NET-AIC. Thank