5. Gateway Initialization and Diagnostics
14FB_HS_SEND: New command from the Profibus-DP master
Before changing the value of FB_DU, the Profibus-DP master must compare the values of FB_HS_SEND and ABC_HS_CONFIRM (bit 14 of the gateway’s status word). If these two values are different, this means that the gateway has not yet acknowledged the previous Profibus-DP master command. Else, the Profibus-DP master can issue a new command, updating the FB_DU bit according to the nature of its command (shutdown or activation of Modbus exchanges), then toggling the value of the FB_HS_SEND bit to inform the gateway that it has sent it a new command.
Summary:
•If ( FB_HS_SEND ≠ ABC_HS_CONFIRM ) Æ The Profibus-DP master command word still contains a command which has not yet been acknowledged by the gateway. So the Profibus-DP master cannot use this word to place a new command in it.
•Else Æ The previous command of the Profibus-DP master has been acknowledged by the gateway, which allows it to transmit a new command. In this case, it changes the value of the FB_DU bit, then toggles the value of the FB_HS_SEND bit.
13FB_DU: Modbus exchange startup
The setting of this bit to one by the Profibus-DP master allows communications between the gateway and the Modbus slaves. Resetting it to zero is used to inhibit them.
When the Profibus-DP master sets this bit to one, it is preferable for all of the output data it has placed in the gateway’s output memory to be up-to-date (“FB_DU” means “FieldBus – Data Updated”). If they are not, these data will be transmitted to the Modbus slaves “as it.”
0-12 Reserved.
The correct use of this command word by the Profibus-DP master, to transmit a new command to the gateway, goes through the following steps:
•Checking of (FB_HS_SEND = ABC_HS_CONFIRM).
• The command, that is to say the value of the FB_DU bit, is updated.
• The value of the FB_HS_SEND bit is inverted.
N.B. It is possible to simplify this use as follows:
• The FB_DU and FB_HS_SEND bits are set to one to activate the Modbus communications.
• The FB_DU and FB_HS_SEND bits are reset to halt Modbus communications.
On the other hand, do not write directly in 16-bit format in the Profibus-DP master command word, because this would disrupt the operation of the transfer of the gateway diagnostics (undesired change to FB_HS_CONFIRM). However, during some debug or test phase, you could, for instance, write 16#6000 in the Profibus-DP master command word (that is to say 16#6000 in the %QW4.0 output word) in order to activate the Modbus communications, and 16#0000 to stop them.
