The remote I/O cards provide a means to serially connect the host computer to external devices ( switches, photoeyes, limit switches, relays, etc.). A simple 4-wire RS422 connection is used for serial communication.
Three different messages are available to the computer:
The remote I/O card is replaceable without removing wires or setting jumpers. The card plugs into a wiring base (p.n. 1593) mounted on a standard din rail. The base contains screw terminals for the wires and contains jumpers for selecting the card address and RS422 terminators. Two sets of power and RS422 connections make daisy-chaining easier. Two captive screws hold the card to the base.
- Who are you? message. The card returns its address and what type of card it is. This makes it easy to see which addresses are being used and what type of cards are being used.
- Send output data message. The card updates the outputs and returns the address, outputs, and inputs.
- Read data message. The card returns the address, outputs, and inputs.
The I/O cards only respond to messages sent by the computer. Three messages are available (request card type, send new output, request current data). Each computer message has the following format:
- preamble - 3 bytes of all ones ( FFh, FFh, FFh ).
- code/address - 1 byte ( msb c,c, a,a,a,a,a,a lsb ) .
- data - 1 byte ( if needed ).
- Communication is 8 bit, no parity, 1 stop bit, 115.2 kBaud.
Request Card Type message (4 bytes).
The computer requests the type of card for this address. If it is a 1592B I/O card, it responds with the code/address and the type number 9.
- From computer (4 bytes) -- FFh, FFh, FFh, ( 1 0 a a a a a a ).
- Card response (2 bytes) -- ( 1 0 a a a a a a ), 09h.
Send New Output Message (5 bytes).
The computer sends new output data to the card. The card updates the output relays and responds with the code/address and the current output data and the current input data.
- From computer (5 bytes) -- FFh, FFh, FFh, ( 0 0 a a a a a a ), output.
- Card response (3 bytes) -- ( 0 0 a a a a a a ),output, input .
Request Current Data Message(4 bytes).
The computer requests the current data from the card. The card responds with the code/address and the current output data and current input data. No change to the output relays will occur.
Communication from the computer can be continuous. The next card request message can be tacked onto the end of the current request message and so on. In this manner, data can be sent and received at a rate of one card every 434 micro seconds. The response from each card begins as soon as the address is received.
- From computer (4 bytes) -- FFh, FFh, FFh, ( 0 1 a a a a a a ).
- Card response (3 bytes) -- ( 0 1 a a a a a a ),output, input .