Appendix B: Introduction to the RS232
The 'RS232' is a standard for bit serial asynchronous data communication. The standard defines the format for data transmission, the electrical specifications for the signal levels, and the mechanical dimensions of connectors.
Despite the definition of a standard, there are so many permutations of control lines, data formats, and transmission speeds, that getting two RS232 devices to communicate usually requires some work.
In this section, we will provide some basic information to aid you in connecting your RS232 device to the SR510 Computer Interface.
CASE 1 - The Simplest Configuration.
In this case, one wire is used to send data from device A to device B and another wire is used to send data from device B to device A. Notice that pin 2 is an output on device A and an input on device B. (It is good practice to run the ground, pin 7, between the devices as well). The RS232 defines two types of devices; DTE (Data Terminal Equipment) and DCE (Data Communications Equipment.) An RS232 port on a computer may be either a DTE or DCE but nearly every terminal with an RS232 port is a DTE. RS232 ports on a computer which are intended to connect to a modem, such as the COM1: port on the IBM PC, are DTE. The SR530 is configured as DCE, and so it may be directly connected to ASCII terminals and to the COM: ports on IBM PC's and compatibles.
As an example, consider connecting an RS232 ASCII computer terminal to the SR510 using a 2 wire link. The terminal is a DTE and the SR510 is a DCE. To operate correctly, the SR510 and the terminal must have the same settings for baud rate, parity, and number of stop bits. The control lines in the RS232 Standard, which are used to indicate that a device is ready to accept data, must also be connected correctly at the terminal end. If
the terminal responds to a control line, it will believe that the SR510 is not ready to accept data (because the line is not passed in this example) and will therefore not send any data.
CASE 2 - RS232 with Control Lines.
The data lines are the same as in Case 1. In addition to the data lines, there are two control lines used:
CTS - Pin 5
"Clear to send" is a signal asserted by the DCE to tell the DTE that the DCE is ready to receive data.
DTR - Pin 20
"Data Terminal Ready" is a signal asserted by the DTE to tell the DCE that the DTE is ready to receive data.
The SR510 responds to the control lines as follows:
1)If the lines are not connected, the SR510 assumes that you are ready to receive data.
2)Data will not be transmitted from the SR510 if the DTR line (pin 20) is low. This is useful in the case when your program is not yet ready to receive data. If data transmission is not suspended, then data may be overwritten in your computer's UART (as it is not being retrieved by the program and so will be lost.) When this happens, the
Baud Rate
The RS232 baud rate of the SR510 is switch selectable from 300 to 19.2K baud (see configuration switch setting in the front of this manual.) 19.2K baud means that data is transmitted at 19,200 bits/second. With one start bit, 2 stop bits, 8 data bits, and no parity bits, each ASCII character requires 573 ∝ sec to be
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