In the programmable mask logic, the states of all relay elements are collected into a single
group of binary digits called the Relay Word. Each bit position reports the state of one relay
element. 0 indicates the element is not picked up; 1 indicates the element is picked up.
Figure 2.9 shows a three-bit Relay Word with elements X, Y, and Z. Each bit corresponds
to one relay element contact in the contact logic equivalent. The operator sets or clears bits
in the mask for the A1 output rather than using switches to select relay elements which
control the A1 output (see Section 3: COMMUNICATIONS, LOGIC command). In the
figure, the operator sets the logic mask to bits (1,1,0), selecting only assertion of the X and Y
elements.
The Z element is not selected, so its assertion cannot close the A1 output contact due to an
open path from the positive to negative bus. The computer ANDs each bit in the Relay Word
with the corresponding bit the operator set in the mask. Next it ORs all three outputs
together, forming the condition which drives the output relay A1. A convenient shorthand
expression for this bitwise AND followed by an OR operation is:
where R is the Relay Word (X,Y,Z), MA1 is the mask (1,1,0), "*" indicates the bitwise
AND, and "+" indicates the OR operation.
While the mask elements are fixed, the Relay Word is updated each quarter-cycle. In this
example, if the X or Y element is set to (1) in the Relay Word, the A1 contact closes. The
A1 contact state is independent of the Z element state in the Relay Word because the corr-
esponding Z element in the mask equals zero.
The user programmable logic masks in this relay control the TRIP and programmable output
contacts. The logic masks are saved in nonvolatile memory with the other settings and
retained through loss of control power.
The masking concept provides more flexibility than switch selectable logic, is more conve-
nient than making wiring changes to hard wired discrete relay systems, and provides
noticeable benefits during commissioning and routine testing.
Figure 2.9: Programmable Logic Mask Analogy
Date Code 920508
Specifications
2-37
SEL-221F, -1 Instruction Manual
Содержание SEL-221F
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Страница 174: ...Figure 6 8 Dc External Connection Diagram Typical Date Code 920508 Installation SEL 221F 1 Instruction Manual 6 11 ...
Страница 228: ...APPENDICES TABLE OF CONTENTS Parts Placement Diagram Date Code 920508 Table of Contents SEL 221F 1 Instruction Manual i ...
Страница 229: ...SEL 200 Series Relay Main Board Troubleshooting Test Points and Jumper Locations ...