GE GFK0579B user manual Timing, Estimating Bus Scan Time for Remote I/O Drops

1

Timing

A fully±loaded (128 input bytes and 128 output bytes) remote drop adds at least 20mS to the bus scan. The table below shows the bus scan time contribution, at each baud rate, for remote drops with a total of 16, 32, 64, 128, and 256 bytes (fully±loaded), when the Remote I/O Scanner receives outputs from only one bus controller at a time.

Total Amount of Input and	Contribution time in mS at each baud rate *
Output Data for
	153.6 Kb	153.6 Kb	76.8 Kb	38.4 Kb
Remote Drop
	std	ext
16 bytes	2.09	2.16	3.83	7.16
32 bytes	3.24	3.31	6.12	11.74
64 bytes	5.52	5.60	10.69	20.89
128 bytes	10.10	10.17	19.85	39.20
256 bytes (fully±loaded)	19.25	19.32	38.15	75.80

* See below to calculate the exact scan time contribution for the application.

Estimating Bus Scan Time for Remote I/O Drops

The scan time contribution for a remote drop depends on its I/O data usage. Follow the procedure below to estimate the the scan time contribution of a remote drop. (Each analog channels is 2 bytes. Eight discrete points are one byte).

1.Find the total number of input bytes and output bytes.

number of input bytes	=	________
number of output bytes	=	________
total bytes	=	________

2.With this total, calculate a scan time contribution using the formula below that corresponds to the Genius bus baud rate.

Formula for 153.6 Kbaud Standard:

0.943mS + (0.0715 x total bytes) = ________ mS

Formula for 153.6 Kbaud Extended:

1.015mS + (0.0715 x total bytes) = ________ mS

Formula for 76.8 Kbaud:

1.538mS + (0.143 x total bytes) = ________ mS

Formula for 38.4 Kbaud:

2.583mS + (0.286 x total bytes) = ________ mS

Other devices on the bus, including the Bus Controller, Hand±held Monitor, and I/O blocks also add to the bus scan time. The Genius I/O System Manual (Volume 1) shows how to calculate bus scan time based on the devices actually present.

12 Series 90±70 Remote I/O Scanner User's Manual ± July 1992