no modem, and the "null modem" or "crossed" RS-232 wiring will be needed. there is no DCE in "RS-232 terminology, i.e. Therefore you are connecting DTE device (this device you've mentioned) to DTE device (the PC) i.e. The PC's serial port will also be configured as DTE (unless it has a very unusual serial port - that won't apply here, as you would know if it did apply). a modem) the pin known as "Transmitted Data" (pin 2 on its 25 pin connector) is actually an input, which receives the signal from the DTE.) (And, as explained above, on a piece of DCE (e.g. On a piece of DTE, the pin labelled "Transmitted Data" (pin 2 on the 25 pin connector) does send data. How should the pins be defined? Are they relative to each device or relative to the "controller"?įrom the above background info, you can see that their labelling is correct if their device is acting as DTE (which most are, unless it's a modem or other WAN interface). In my mind this should be labeled RX because it is the pin the computer receives on. In their definitions they use to mean TX being the pin that sends data from the device. There has been some confusion as they have defined their TX and RX pins relative to the device. I have been working with a device that uses RS232 communication to a PC. This is where the use of "crossed" or "null modem" (i.e. Connecting pin 2 (which is an output) on one piece of DTE, to pin 2 (another output) on another piece of equipment configured as DTE, makes no sense (and when using logic level signals, could even cause hardware damage). However the pieces of equipment we are using these days (even when they don't use RS-232 and are instead using a TTL or other voltage UART protocol interface) are usually all effectively DTE (with one exception being modems). This all made sense when the typical connections were between a DCE and a DTE. Therefore you can see that the labelling of what was Transmitted and what was Received, was from the point of view of the DTE (i.e. Pin 3 "Received Data" was the data signal in the opposite direction - transmitted by the modem (DCE) on pin 3, and received by the terminal (DTE) on pin 3. The modem then sent that data out using the communications link, to whatever equipment was on the other end of that link. This meant that the cable linking a terminal and a modem was "straight through" - pin 2 on a terminal (DTE) at one end of the cable where data originated, was connected to pin 2 on a modem (or similar) at the other end of the cable (DCE) where that data was received. number 104") should be data from the DCE to the DTE. Pin 3 on the original 25-pin D connector (described in the standard as "Received Data", "Circuit BB", "V24. number 103") should be data from the DTE to the DCE. Pin 2 on the original 25-pin D connector (described in the standard as "Transmitted Data", "Circuit BA", "V24. Note: All pin numbers below refer to the original 25-pin D connector the numbering changed on the 9-pin connector used on later PCs.ĭTE = Data Terminal Equipment - in the old days, this would usually be a terminal or a printer, or equipment emulating those.ĭCE = Data Communications Equipment - in the old days, this would usually be a modem or other WAN interface. To disconnect from telnet issue Control-] Enter, then type quit at the telnet prompt.It might be easier to understand, if we quickly review how the RS-232 standard was originally used. Removing the loopback plug will not display data typed in the telnet session. Once connected, the word hello is being typed: For example, the following shows a telnet connection to the first serial port on a unit with the IP address of 192.168.1.1. Once CONNECTED, any data typed on the telnet session will be displayed twice. Telnet (IP_address_of_Digi_TransPort) 4000 By default 4000 is the first serial port, with 4001 being the second serial port, etc. Use telnet to connect directly to the serial port. RS-232 RJ-45 Loop Back connector consists of a single 8 pin RJ-45 plug wired as follows:Ģ. Pin 4 connected to pins 6 and 1 (DTR to DSR and DCD) Pin 8 connected to pins 7 and 9 (CTS to RTS and RI) RS-232 DB-9 Loop Back connector consists of a male DB-9 connector wired as follows: Pin 20 connected to pins 6 and 8 (DTR to DSR and DCD) Pin 4 connected to pins 22 and 5 (RTS to RI and CTS) RS-232 DB-25 Loop Back connector plug consists of a male DB-25 connector wired as follows: Insert a loopback plug into the serial port. Steps to perform serial loopback testing with Digi TransPort devices to check the integrity of the serial port:ġ.
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