Micromod Micro-DCI: 53MC5000 PLC AND PRINTER INTERFACES - Manual (Page 34)

Each character has 11 bits (start, 8 data bits, parity, and stop bit) and at 9600 baud, 1 character
time is 1.15 ms (9600/11 = 872 characters; 1 second/872 = 1.15 ms per character). It is true that
the 1.15 ms character time used in the overhead calculation is also affected by the baud rate; how-
ever, for conservative calculations this number should remain constant for all baud rates at 9600 or
above. Baud rates lower than 9600 should double the 1.15 ms character time for each half reduc-
tion in the baud rate (e.g., 9600/2 = 4800 baud = 2 x 1.15 = 2.3 ms per character time, etc.).
Given the information above, a formula that can be used to calculated Scan Time intervals for baud
rates at 9600 or higher is as follows:
Scan Time
=
( [ 2 (L
rd
+ C
rd
+ L
wrt
+ C
wrt
) + 52 ] Ч 11 Ч 1.15 ) + 1920
Baud
Formula constants:
2 = 2 eleven bit characters per word (should not be used if L
rd
, etc.,
are already given in bytes)
52 = 52 minimum character overhead
11 = 11 bits per character
1.15 = character time in ms
1920 = minimum PCS-PLC turnaround time in ms
Example: Using the data transfers of Figure 1-2, the read phase has 20 L-words and 8 C-words;
the write phase has 4 L-words and 3 C-words. At 9600 baud, the Scan Time would be calculated
as follows:
Scan Time = ( [2 (20 + 8 + 4 + 3) + 52] x 11 x 1.15 ) + 1920/ 9600
= ( [2 (35) + 52] x 11 x 1.15 ) + 1920/ 9600
= (122 x 11 x 1.15 ) + 1920 / 9600
= 3463/9600 = 0.361 seconds or 361 ms
= 4 for 400 ms (361 ms is rounded to next higher increment of 100)
The applicable DDI port datapoint, B294 [DDI-A] or B460 [DDI-B] is configured with a 4.
NOTE
When specifying low scan times for large data transfers with some PLC
types, persistent timeout errors (error code 255) or command errors
(e.g. checksum error, etc.) may occur because the PLC may not always
be able to respond within the allowed time period. If this happens, the
scan time should be increased until the errors stop.
53MC9015 53MC5000 PLC and Printer Interfaces
3-10
DESCRP