對用戶存儲容量只能作大略的預(yù)算。在僅對開關(guān)量進行操控的體系中，能夠用輸入總點數(shù)乘 10 字/ 點＋輸出總點數(shù)乘 5 字 / 點來預(yù)算；計數(shù)器 / 定時器按（ 3 ～ 5 ）字 / 個預(yù)算；有運算處理時按（ 5 ～ 10 ）字 / 量預(yù)算；在有模擬量輸入 / 輸出的體系中，能夠按每輸入 / （或輸出）一路模擬量約需（ 80 ～ 100 ）字左右的存儲容量來預(yù)算；有通訊處理時按每個接口 200 字以上的數(shù)量大略預(yù)算。終，一般按預(yù)算容量的 50 ～ 100 ％留有裕量。對缺乏閱歷的規(guī)劃者，挑選容量時留有裕量要大些。
（ 3 ）對 I/O 響應(yīng)時刻的挑選
PLC 的 I/O 響應(yīng)時刻包括輸入電路推遲、輸出電路推遲和掃描作業(yè)辦法引起的時刻推遲（一般在 2 ～ 3 個掃描周期）等。對開關(guān)量操控的體系， PLC 和 I/O 響應(yīng)時刻一般都能滿意實踐工程的要求，可不必考慮 I/O 響應(yīng)問題。但對模擬量操控的體系、特別是閉環(huán)體系就要考慮這個問題。

The user storage capacity can only be roughly budgeted. In the system that only controls the switching value, the total number of input points multiplied by 10 words / point + the total number of output points multiplied by 5 words / point can be used to budget; The counter / timer is budgeted by (3 ~ 5) words / piece; Budget according to (5 ~ 10) words / quantity when there is calculation processing; In the system with analog input / output, it can be budgeted according to the storage capacity of about (80 ~ 100) words for each input / (or output) analog; When there is communication processing, the rough budget is based on the quantity of more than 200 words per interface. Finally, there is usually a margin of 50 ~ 100% of the budgeted capacity. For inexperienced planners, it is better to leave more margin when selecting capacity.
(3) Selection of i/o response time
The i/o response time of PLC includes input circuit delay, output circuit delay and time delay caused by scanning operation method (generally in 2 ~ 3 scanning cycles), etc. For the switching value control system, the PLC and i/o response time can generally meet the requirements of practical engineering, so it is unnecessary to consider the i/o response problem. However, this problem should be considered for the analog control system, especially the closed-loop system.