Chapter 6: Instructions 71
Instruction
Mnemonic Up/Down Counter Range
■ Bit
Byte
UDC Up/Down counter
Word
Ladder
Description
1. When the up count input (U input) turns On, the Present Value (PV) increases by 1. When the
down count input (D input) turns On, PV decreases by 1. When PV is greater than or equal to
the Set Value (SV) or is reduced to 0, the output done contact turns On.
2. In the following cases, the output done contact changes from On to Off:
When the reset input is turned On.
When the PV is decreased below the SV by the down count pulse input.
When the PV increases from 0 to 1 by the up count pulse input.
3. If the reset input (R input) is On, the output is Off. In this state, the up/down counter input
pulses are ignored and the Present Value stays reset to 0.
4. When the up count input pulse and the down count input pulse occur at the same time, the PV
does not change.
5. When the PV is 0, if the down count pulse is input, the Present Value does not change, and
the output is On. When the Present Value is 65,535, if the up-counter pulse is input, the
Present Value is reset to 0.
6. When the counter Set Value is 0, if the reset input is On then the output is Off. If up or down
is input while the reset input is Off, the output changes to On.
7. The timer/counter channel can only be used once. It cannot be reused by other timer or
counter instructions (TIM, SST, UC, DC). The number of channels available is 256 (Ch 0
through Ch 255).
8. The SV can be set to a maximum value of 65,535.
condition
1
condition
2
condition
3
UDC
U CH =
SV =
S
R
U input
(condition 1)
D input
(condition 2)
Reset
(condition 3)
Present
Value (PV)
Output (TC)
Set Value
(SV)
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