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QUESTION:
What are the recommendations for reliable RS485 communication between S7-200 and MICROMASTER4?
ANSWER:
General
Unlike PROFIBUS, the RS485 com port is not optically isolated, so care has to be taken that the installation practices do not lead to communication failures or damage to RS485 drivers.
EMC correct installation
This is very important to prevent noise causing communications failures, and, in extreme cases, damage to RS485 drivers.
The minimum measures that should be adopted are:
1) Motor cables to be screened - the screen to be correctly earthed at both ends. If possible, breaks in the cable should be avoided - if this is not possible then care must be taken to ensure EMC correct continuity of the screen at the join.
2) All nodes are to be well earthed (EMC earth)
3) All relay coils etc to be suppressed
4) Pay attention to cable segregation - keep RS485 cables away from other cables, particularly motor cables.
5) Ensure the screens of RS485 cables are correctly earthed.
Further guidance can be found in "EMC correct installation of MASTERDRIVES" on the Intranet / Internet (ID: 1157657).
Bus termination and biasing
Correct biasing is required to allow the start of the telegram to be identified. The bus idle state should have P 0.5 to 1V positive with respect to N. The S7-200 has a 5V supply for biasing (D connector pins 5 and 6) - the same resistor values as for PROFIBUS can be used. The 220R bus termination matches PROFIBUS cable. MICROMASTER has a 10V supply for biasing - the pull up / pull down resistors shown below have been tested for optimum performance, and also leave enough capacity in the 10V supply for a 4K7 potentiometer.
On small systems with less than 2m of RS485 cable and all nodes on a common ground plane, acceptable performance may be obtained if the termination / biasing is at one end only, e.g. the PLC.
( 180 KB )
(For larger version click on image)
0V interconnections
Since the RS485 is referenced to 0V (M on the S7-200), optimum performance is usually obtained when 0V lines (MM terminal 2, S7-200 "M" terminal) on all nodes are connected together. This could be done as an additional core in the RS485 cable, or a separate wire close to it. Measures should be taken to prevent noise being coupled to this. However, if the 0V is connected to earth at one or more points, this could provide a path for additional noise. This is particularly likely at the PLC if the 24V supply (L+, M) is used for supplying other devices and/or the I/O. In such cases better performance may result if the PLC M terminal is not connected to the MICROMASTERS.
What to do if you have problems
(1) Look at the diagnostic parameters r2024 - r2031:
r2024 and r2025:- If telegrams are being received by the inverter, then you should see r2024 being incremented. If it is not, then no telegrams are being received by the inverter. In a reliable system r2024 (received telegrams) should be incrementing at least fifty times faster than r2025 (rejected telegrams). If r2025 is increasing rapidly then it is likely that either the telegram structure is incorrect or that the telegrams are being corrupted (e.g. by noise interference).
r2026 - r2031:- If only one of these values is being incremented, then you should look to the specific one of these in relation to the structure. In particular, if r2030 or r2031 is incrementing, this is usually a sign that communication is good but there is an error in the telegram itself.
r2026 - r2029:- If these are all increasing, this is a sign of emc or biasing problems. See above and (2) below for what to do. When there is inadequate bias, the network is highly susceptible to noise between messages. Any negative-going spike on the network may overcome the bias and be received as logic "0", which normally signifies the start of a message. The high occurrence of false start bit errors (counted by P2029) confirms this.
(2) Look at the bus voltage on an oscilloscope. Check that the idle state bias is 0.5 - 1V; an example of a good voltage is shown below. The differential signal (P-N, top trace in the example) should be substantially free from noise. Some noise on the common mode signal (P or N to earth, bottom trace) is acceptable, but noise spikes should not exceed 5V as this may cause RS485 driver damage. Check all of the above points.
Bus voltage, 19.2 kbaud
Ref A = P to N
Ref B = N to earth
Expanded version of above including start of telegram
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