Micromod Micro-DCI: 53MC5000 MicroLink - Manual (Page 26)

Example calculation: The MicroLink network has 3 SUPERVISOR-PC units that will access via the
gateway (Datalink interrogator) instruments 9, 10, and 11, which are on the Datalink network oper-
ating at 28.8 Kbaud.
For this example, B05 and B08 must be calculated. B04, B06, and B07 are each set to 0 because
their corresponding instruments will not be accessed through the gateway or a SCADA system.
28.8 Kbaud Transaction Time . . . . . . .. . . 8.5 ms (Table 8-2)
Datalink Instrument and Gateway Data
Turn-around Time 3.0* ms
Total Transaction Time . . . . . . . . . 11.5 ms
*Turn-around Time is the time in milliseconds for data to pass through the gateway to the instru-
ment and back through the gateway again. This number is given as 3 ms and does not have to be
calculated. For SCADA applications, Turn-around Time is 2.0 ms for the gateway plus SCADA
time delays. See IB 53SU5000, Section 2, for SCADA time-out calculation procedures.
Time Delay B05 = (Transaction Time [see Table 8-2] + Instrument and Gateway Turnaround
Time)/10 [Rounded up]
Time Delay B05 = (8.5 ms + 3.0 ms)/10, = 11.5 ms/10 = 1.15 ms rounded up to 2.
(The Total Transaction Time of 11.5ms is greater than 10 ms; therefore, B05 is set to the next in-
crement which is a 20 ms timeout. A value of 2 will be entered for the Time Delay B05.)
B08 = [(10 X Time Delay Value) X (Number of MicroLink Supervisor PC units - 1)]
B08 = [(10 X 2) X (3 - 1)] = 20 X 2 = 40 ms.
In comparison to Table 8-1 at 3 MicroLink SUPERVISOR-PC units, values 2 and 39 would have
been obtained for B05 and B08; whereas the calculated values are 2 and 40 respectively. The ta-
ble and approximation calculation values differ slightly due to transaction time and turn-around
time refinements used to prepare the table data. The calculated value for B08 is adequate as a
close approximation (40 versus 39) because it may have to be altered to optimize network perform-
ance.
8.3.2 PCS EXTERNAL INPUT MODULE CONSIDERATIONS
Prerequisite is that the gateway functions in Datalink interrogator mode.
To minimize Datalink contention, the External Input Module (see IB 53MC5000, Section 4) of each
53MC5000 PCS on the MicroLink network must have task entries set to intersperse gateway with
non-gateway peer-to-peer accesses or skip entries in the module if only gateway instruments are
being accessed.
The entries in a 53MC5000 PCS External Input Module are processed sequentially, one every 5
ms. The completion of the previous module entry is not required before the next module entry gen-
erates a message. Therefore, a 53MC5000 PCS may have 24 active messages on the MicroLink
network at any given time. Gateway through-put (complete 1 message through the gateway) how-
ever, is 7.5 ms to 11.5 ms at 28.8 Kbaud, and 28 ms at 9.6 Kbaud Datalink data rates. At 28.8
Kbaud, if two messages were sent sequentially to destinations on the Datalink side of the gateway,
the second message would not be passed through the gateway because it can only process one
message at a time, which at 28.8 Kbaud requires up to 11.5 ms.
Section 8. Configuring the 53MC5000 PCS as a Gateway
27