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I. Introduction
The H-series PLC is a family of small, low cost, pure relay-ladder logic (i.e. No BASIC) based PLCs that are available since 1993. The family spans from the T22H with 12 inputs, 10 outputs to the T64H with 40 inputs and 24 outputs. Members of the H-series controller family differ mainly in the number of I/Os (inputs/outputs), and the type of I/Os they carry (relay or transistor outputs, PNP or NPN types, opto-isolator inputs or high voltage inputs, etc). Since each different PLC model has different wiring requirements, please refer to their respective Installation Guide on the wiring methods and installation details. The number of I/Os, timers, counters, internal relays as well as the maximum allowable program steps are also stated in the Installation Guide.
The H-series PLCs includes the T22H-npn, T28H-Relay, T40H-Relay, T44H-pnp, T44H-npn and the T64H-Relay. The following table summarizes the number of I/Os, timers, counters and internal relays for each existing model:
| Model Name | Inputs | Outputs | Internal Relays | Timers | Counters | Maximum Program Steps |
| T22H-npn | 12 | 10 | 128 | 20 | 20 | 400 |
| T28H-Relay | 12 | 8 | 128 | 20 | 20 | 400 |
| T40H-Relay | 12 | 8 | 128 | 20 | 20 | 400 |
| T44H-npn/pnp | 28 | 16 | 128 | 20 | 20 | 400 |
| T64H-Relay | 40 | 24 | 256 | 40 | 40 | 800 |