![\"James](\"https:\/\/scadametrics.com\/blog\/wp-content\/uploads\/2025\/07\/IMG_0987-1-150x150.jpeg\")
Jim ‘Slim’ Mimlitz, SCADAmetrics<\/p><\/div>The EtherMeter contains an RS485 port for Modbus\/RTU and DF1 communications in multi-drop applications.<\/p>\n
When using the RS485 port, the following EtherMeter terminals are used:<\/p>\n
- \n
- 19\u00a0 –\u00a0 RS485A (-)<\/li>\n
- 20\u00a0 –\u00a0 RS485B (+)<\/li>\n
- 21\u00a0 –\u00a0 Signal GND Reference (contains current-limiting resistor)<\/li>\n<\/ul>\n
In order to use RS485, attention\u00a0should be\u00a0given to the\u00a0EtherMeter’s dip switch positions:<\/p>\n
Latest EtherMeter:<\/strong><\/span><\/p>\n
- \n
- Switch 1\u00a0 – DOWN, RS485 Mode<\/strong>, (UP = RS232 Mode)<\/li>\n
- Switch 2\u00a0 – UP, RS485 Mode<\/strong>, (DOWN = RS232 Mode)<\/li>\n
- Switch 3\u00a0 –\u00a0 UP, Use 120 Ohm Terminator for RS485<\/strong>, (DOWN = No Terminator)<\/li>\n
- Switch 4\u00a0 –\u00a0 N\/C<\/li>\n
- Switch 5\u00a0 –\u00a0 N\/C<\/li>\n
- Switch 6\u00a0 –\u00a0 N\/C<\/li>\n
- Switch 7\u00a0 – UP (LCD Backlight ON), or DOWN (Power-Saver Mode)<\/li>\n
- Switch 8\u00a0 – DOWN (Run Mode)<\/strong>, (UP = Setup Mode, Modbus\/DF1 OFF)<\/li>\n<\/ul>\n
Legacy EtherMeter:<\/strong><\/span><\/p>\n
- \n
- Switch 1\u00a0 –\u00a0 DOWN, Run Mode, Modbus\/DF1=ON,<\/strong> (UP = Setup Mode)<\/li>\n
- Switch 2\u00a0 –\u00a0 DOWN, RS485 Mode<\/strong>, (UP = RS232 Mode)<\/li>\n
- Switch 3\u00a0 –\u00a0 UP, Use 120 Ohm Terminator for RS485<\/strong>, (DOWN = No Terminator)<\/li>\n
- Switch 4\u00a0 –\u00a0 UP (LCD Backlight ON), or DOWN (Power-Saver Mode)<\/li>\n<\/ul>\n
In my experience, it is preferable to use a single, centralized\u00a024VDC<\/sub> power supply to power the Modbus Master — along with all the remote RS485\/Modbus Slave devices.\u00a0 This ensures that signaling between all devices are referenced to a common ground potential.\u00a0 In order to most-easily achieve this, a single jacketed cable can be used to transmit both the 24VDC<\/sub> power and the RS485 Modbus signals.<\/p>\n
For the most demanding (higher baud rate and\/or long distance) applications, I generally recommend Belden 3084A cable<\/a>, which contains two shielded\/twisted pairs plus an overall copper braid.\u00a0 The\u00a024 gauge pair (Blue\/White) has a characteristic impedance of 120 ohms, which is optimal for RS485 transmission.\u00a0 The 22 gauge pair (Red\/Black)\u00a0is designated\u00a0for 24VDC<\/sub> and GND.\u00a0 The cost is generally around $1\/ft.\u00a0 And for especially long cable runs, a more stout version is available, the Belden 3082A cable<\/a>, whose power and data pairs are 15 gauge and 18 gauge respectively.\u00a0 The cost is generally around $3\/ft.<\/p>\n
For\u00a0more economical\u00a02-pair wiring, I recommend Belden 8723 cable<\/a>.\u00a0 It\u00a0consists of two\u00a022 gauge shielded\/twisted pairs (red\/black and green\/white).\u00a0\u00a0Although\u00a0it has a\u00a0characteristic impedance\u00a0of 52 ohms and\u00a0therefore does not conform exactly to the RS485 specification, it can work very well for\u00a0 shorter distances and\/or lower baud rates.\u00a0 The cost is generally around $0.40\/ft.<\/p>\n
In situations where it is not feasible to power the Modbus master and all remote slave devices from a single DC power supply, then all devices should be powered locally and individually using isolated DC power supplies.\u00a0 Furthermore,\u00a0the signal grounds of all devices should be tied together\u00a0to a common voltage reference.\u00a0 Each reference tie should\u00a0utilize a resistor to prevent large currents from traveling on the reference wire.\u00a0 For this purpose, Terminal 22 on the EtherMeter functions as a signal reference point with a built-in\u00a0100 ohm current-limiting resistor.<\/p>\n
![\"\"](\"https:\/\/scadametrics.com\/blog\/wp-content\/uploads\/2010\/10\/JACE_Tridium_Boston_001-300x225.jpg\" "\\"JACE_Tridium_Boston_001\\"")
<\/a>An EtherMeter Connected To A JACE-Tridium System Via Modbus\/RTU over RS485. The System Is Monitoring Total Consumption And Flow-Rate For Two Connected Water Meters (Sensus SR and Sensus Compound). This Facility Houses A Fortune-500 Software Company In The Boston Area. A Pair Of SCADAmetrics Touch-Read Filters Allow Both The Utility AMR System And The Building Automation System To Share The Meter Signals.<\/p><\/div>\n
I highly recommend that anyone using Modbus\/RTU over RS485 should browse the web for further information.\u00a0 Here are a few articles that I’ve found to be particularly useful and interesting:<\/p>\n
- \n
- B&B Electronics – Tips For Success In Using RS485 Data Communications<\/a><\/li>\n
- B&B Electronics – The RS485 Application “Cheat Sheet”<\/a><\/li>\n
- B&B Electronics – Troubleshooting Guide For RS485<\/a><\/li>\n
- Chipkin Automation Systems – <\/a>RS485 Cables \u2013 Why you need 3 wires for 2 (two) wire RS485<\/a><\/li>\n
- RS485 – Wikipedia<\/a><\/li>\n<\/ul>\n
![\"\"](\"https:\/\/scadametrics.com\/blog\/wp-content\/uploads\/2010\/10\/Williamsville_BPS_RS485-300x184.jpg\" "\\"Williamsville_BPS_RS485\\"")
<\/a>EtherMeter Connected To A Remote Controller Via Belden 8723 Cable. The EtherMeter Is A Modbus\/RTU Slave, And It Shares The Same 24VDC<\/sub> Power Supply As The Modbus\/RTU Master.<\/p><\/div>\n
![\"\"](\"https:\/\/scadametrics.com\/blog\/wp-content\/uploads\/2010\/10\/Williamsville_BPS_NeptuneEcoder-300x188.jpg\" "\\"Williamsville_BPS_NeptuneEcoder\\"")
<\/a>The Connected Meter: A Neptune 6″ HP Turbine, Outfitted With An E-Coder Register.<\/p><\/div>\n","protected":false},"excerpt":{"rendered":"
The EtherMeter contains an RS485 port for Modbus\/RTU and DF1 communications in multi-drop applications. When using the RS485 port, the following EtherMeter terminals are used: 19\u00a0 –\u00a0 RS485A (-) 20\u00a0 –\u00a0 RS485B (+) 21\u00a0 –\u00a0 Signal GND Reference (contains current-limiting resistor) In order to use RS485, attention\u00a0should be\u00a0given to the\u00a0EtherMeter’s dip switch positions: Latest EtherMeter: …<\/span> Read More →<\/span><\/a><\/span><\/p>\n","protected":false},"author":1,"featured_media":341,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3,8,5,6,7],"tags":[42,41,69,20,67,70,68],"class_list":["post-318","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-applications","category-general","category-modbusprotocol","category-rockwell-protocols","category-troubleshooting","tag-df1","tag-modbus","tag-modbusrtu","tag-rs-485","tag-rs485","tag-serial-communications","tag-twisted-pair"],"_links":{"self":[{"href":"https:\/\/scadametrics.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/318","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/scadametrics.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/scadametrics.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/scadametrics.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/scadametrics.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=318"}],"version-history":[{"count":43,"href":"https:\/\/scadametrics.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/318\/revisions"}],"predecessor-version":[{"id":2543,"href":"https:\/\/scadametrics.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/318\/revisions\/2543"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/scadametrics.com\/blog\/index.php?rest_route=\/wp\/v2\/media\/341"}],"wp:attachment":[{"href":"https:\/\/scadametrics.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=318"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/scadametrics.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=318"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/scadametrics.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=318"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
- B&B Electronics – The RS485 Application “Cheat Sheet”<\/a><\/li>\n
- Switch 2\u00a0 –\u00a0 DOWN, RS485 Mode<\/strong>, (UP = RS232 Mode)<\/li>\n
- Switch 2\u00a0 – UP, RS485 Mode<\/strong>, (DOWN = RS232 Mode)<\/li>\n
- Switch 1\u00a0 – DOWN, RS485 Mode<\/strong>, (UP = RS232 Mode)<\/li>\n