Using USS Protocol Instructions to Communicate with Drives
11-3
S7-200 Programmable Controller System Manual
A5E00066097-02
Name Address Comment
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Symbol Table
USS_LOW_V
USS_HIGH_V
USS_TIME_OUT
USS_I_LINE
USS_DAT_PTR0
USS_ACT_DRV0
USS_CUR_DRV0
USS_P0
USS_REQ_SNT0
USS_RETRY_P0
USS_CS_B0
USS_CS_D0
USS_CS_W0
USS_CS_W2
USS_ERR_CODE
USS_CS_B1
VD100
VD104
VW108
VW110
VD112
VD116
VD120
V124.0
V124.1
VB125
VB127
VD127
VW127
VW129
VB126
VB128
(DWORD) Enter the starting address for USS Protocol symbols here.
USR1 POU SymbolsUSS Protocol
Enter starting
address.
Figure 11-1 Symbol Table Configuration
Drive Communications Time
Communication with the drive is asynchronous to the CPU scan. Several CPU
scans will usually complete while one drive communication transaction completes.
This depends on the number of drives present, the baud rate, and the scan time of
the CPU. Table 11-1 shows communications transaction times. Once a USS_INIT
instruction assigns Port 0 to USS Protocol, the CPU regularly polls all active drives
at the intervals shown in Table 11-1. The timeout parameter of each drive must be
set to allow for this poll time.
Table 11-1 Drive Communications Times
Baud Rate
Time Between Polls
to the Active Drives (ms)
1200 (460 maximum/230 typical) * number of drives
2400 (240 maximum/120 typical) * number of drives
4800 (130 maximum/65 typical) * number of drives
9600 (80 maximum/40 typical) * number of drives
19200 (50 maximum/25 typical) * number of drives
Restriction
Only one READ_PM or WRITE_PM instruction can be active at a time. The Done
output of each instruction should signal completion before user logic initiates a new
instruction. Use only one DRV_CTRL instruction for each drive.
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