These links are to the media pages for the 11 I/O options available for the M‑Duino PLCs. Note that these same I/O options are currently available for the Raspberry PLCs (and soon to be released ESP32 series). At each of these pages are six documents – three datasheets and three user guides, for the Ethernet boards (base configuration), the GPRS / GSM option, and the WiFi & Bluetooth Low Energy (BLE) option. The datasheets each have a couple of pages that show the pinouts, list the available communications and I/O options, basic getting started, precautions, and contact info. The user guides help you understand the layout of the modules, and provide the details of how to set the switches, hook up the wires, program the controller, etc.
The PLC modules are available with two to four shields – one communications shield (Arduino Mega), and one, two, or three I/O shields. The I/O shields are either Analog (which have Analog, Digital, PWM, and Interrupt) inputs and outputs, or Relay (which also have Relay outputs). The numerical prefix in the identifiers is the nominal number of total Relay, Analog, Digital, PWM, and Interrupt ports available; the letters indicate the shield type (Relay or Analog) and their position in the module. For example, the 54ARA+ has 54 I/O’s, a communications board, an Analog shield, a Relay shield, and an Analog shield. Only one letter R is used when the module contains only relay boards (e.g. 38R+ has two relay boards and 57R+ has three), and no letters when the module contains only analog boards (e.g. 42+ has two analog boards and 58+ has three).
So, looking at the first image below, the 19R+ actually has 17 I/O ports, the communications board appears first (on the left side of picture), then the relay board is positioned next. That column of five green plastic connectors in the middle of the shield are for relays (three more are hidden on the other side). The second image is the 21+ (which has 21 ports) also has the controller on the left side, and the analog board is positioned next. That column of two green and orange plastic connectors in the middle of the module are for the analog and digital connections.