GoPLC HTTP Client Guide

James M. Belcher Founder, JMB Technical Services LLC April 2026 | GoPLC v1.0.535

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1. Overview

GoPLC provides 16 built-in functions for making HTTP requests from Structured Text. Call REST APIs, fetch data from web services, post telemetry to cloud platforms, and integrate with any HTTP-based system — all from your PLC program.

(* Simple GET — one line *)
body := HTTP_GET_BODY('http://api.example.com/status');

(* Full control *)
resp := HTTP_GET('http://api.example.com/data');
IF HTTP_OK(resp) THEN
    data := HTTP_BODY(resp);
END_IF;

Response Pattern

Most HTTP functions return a response map with these fields:

Field	Type	Description
status	INT	HTTP status code (200, 404, 500, etc.)
body	STRING	Response body
headers	MAP	Response headers (when available)
error	STRING	Error message (empty on success)

Use the helper functions HTTP_STATUS, HTTP_BODY, HTTP_OK, and HTTP_ERROR to extract fields from the response.

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2. Simple Request Functions

HTTP_GET — GET Request

resp := HTTP_GET('http://10.0.0.144:8086/health');

IF HTTP_OK(resp) THEN
    body := HTTP_BODY(resp);
END_IF;

Param	Type	Required	Description
url	STRING	Yes	Request URL
timeout_sec	INT	No	Timeout in seconds (default: 5)

Returns: response map.

HTTP_GET_BODY — GET, Returns Body Directly

The simplest form — returns the response body as a string, or empty string on error.

body := HTTP_GET_BODY('http://10.0.0.50/api/info');
IF LEN(body) > 0 THEN
    (* Parse body... *)
END_IF;

No response map, no status code — just the body. Use this for quick reads where you don’t need error details.

HTTP_POST — POST Request

resp := HTTP_POST('http://10.0.0.144:8086/api/v2/write',
                  'temperature,site=Plant1 value=72.5',
                  'text/plain');

Param	Type	Required	Description
url	STRING	Yes	Request URL
body	STRING	Yes	Request body
content_type	STRING	No	Content-Type header (default: application/json)
timeout_sec	INT	No	Timeout in seconds (default: 5)

HTTP_POST_JSON — POST JSON, Parse Response

Posts JSON and attempts to parse the response as JSON automatically.

resp := HTTP_POST_JSON('http://api.example.com/devices',
                       '{"name": "Pump-1", "type": "VFD"}');
(* If response is JSON, resp is a parsed JSON object *)
(* If not, resp is a standard {status, body, error} map *)

HTTP_PUT — PUT Request

resp := HTTP_PUT('http://10.0.0.50/api/config',
                 '{"scan_time_ms": 100}');

Same parameters as HTTP_POST.

HTTP_PATCH — PATCH Request

resp := HTTP_PATCH('http://10.0.0.50/api/config',
                   '{"log_level": "debug"}');

Same parameters as HTTP_POST.

HTTP_DELETE — DELETE Request

resp := HTTP_DELETE('http://10.0.0.50/api/programs/old_program');

Param	Type	Required	Description
url	STRING	Yes	Request URL
timeout_sec	INT	No	Timeout in seconds (default: 5)

HTTP_HEAD — HEAD Request

Returns status and headers only (no body). Useful for checking if a resource exists.

resp := HTTP_HEAD('http://10.0.0.50/api/info');
code := HTTP_STATUS(resp);
IF code = 200 THEN
    (* Resource exists *)
END_IF;

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3. Advanced: HTTP_REQUEST

Full control over method, headers, and body.

(* Build custom headers *)
hdrs := HTTP_SET_HEADER('', 'Authorization', 'Bearer my-token');
hdrs := HTTP_SET_HEADER(hdrs, 'X-Custom-Header', 'GoPLC');

(* Make request *)
resp := HTTP_REQUEST('POST',
                     'http://api.example.com/data',
                     '{"temp": 72.5}',
                     hdrs,
                     10);     (* 10 second timeout *)

Param	Type	Required	Description
method	STRING	Yes	HTTP method (GET, POST, PUT, DELETE, PATCH, etc.)
url	STRING	Yes	Request URL
body	STRING	No	Request body
headers	MAP/STRING	No	Headers map or JSON handle
timeout_sec	INT	No	Timeout in seconds (default: 5)

Returns: response map with status, body, headers, and error.

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4. Response Helper Functions

HTTP_STATUS — Get Status Code

code := HTTP_STATUS(resp);    (* 200, 404, 500, etc. *)

HTTP_BODY — Get Response Body

body := HTTP_BODY(resp);

HTTP_OK — Check for Success (2xx)

IF HTTP_OK(resp) THEN
    (* Status is 200-299 *)
END_IF;

HTTP_ERROR — Get Error Message

err := HTTP_ERROR(resp);
IF LEN(err) > 0 THEN
    DEBUG_ERROR('http', CONCAT('Request failed: ', err));
END_IF;

HTTP_HEADERS — Get All Response Headers

hdrs := HTTP_HEADERS(resp);

HTTP_GET_HEADER — Get Single Header

Case-insensitive header name lookup.

content_type := HTTP_GET_HEADER(resp, 'Content-Type');
server := HTTP_GET_HEADER(resp, 'Server');

HTTP_SET_HEADER — Build Request Headers

Creates or adds to a headers map for use with HTTP_REQUEST.

hdrs := HTTP_SET_HEADER('', 'Authorization', 'Bearer token123');
hdrs := HTTP_SET_HEADER(hdrs, 'Content-Type', 'application/json');
hdrs := HTTP_SET_HEADER(hdrs, 'Accept', 'application/json');

Pass empty string as the first argument to create a new headers map.

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5. Complete Example: REST API Integration

Poll a weather API and publish results to MQTT:

PROGRAM POU_WeatherPoll
VAR
    state : INT := 0;
    scan_count : DINT := 0;
    resp : STRING;
    body : STRING;
    doc : STRING;
    temp_f : REAL;
    humidity : INT;
    ok : BOOL;
END_VAR

scan_count := scan_count + 1;

CASE state OF
    0: (* Initialize MQTT *)
        ok := MQTT_CLIENT_CREATE('weather', 'tcp://10.0.0.144:1883', 'goplc-weather');
        ok := MQTT_CLIENT_CONNECT('weather');
        state := 10;

    10: (* Poll weather every 600 scans (60s at 100ms) *)
        IF (scan_count MOD 600) = 0 THEN
            resp := HTTP_GET('http://api.weather.local/current');

            IF HTTP_OK(resp) THEN
                body := HTTP_BODY(resp);
                doc := JSON_PARSE(body);
                temp_f := JSON_GET_REAL(doc, 'temp_f');
                humidity := JSON_GET_INT(doc, 'humidity');

                (* Publish to MQTT *)
                MQTT_PUBLISH('weather', 'plant/weather/temp', REAL_TO_STRING(temp_f));
                MQTT_PUBLISH('weather', 'plant/weather/humidity', INT_TO_STRING(humidity));
            ELSE
                DEBUG_WARN('weather', CONCAT('API error: ', HTTP_ERROR(resp)));
            END_IF;
        END_IF;
END_CASE;
END_PROGRAM

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6. Complete Example: Webhook Notifications

Send alarm notifications to a webhook endpoint:

PROGRAM POU_AlarmWebhook
VAR
    temperature : REAL;
    alarm_active : BOOL := FALSE;
    alarm_sent : BOOL := FALSE;
    resp : STRING;
    payload : STRING;
    hdrs : STRING;
END_VAR

(* Check alarm condition *)
alarm_active := temperature > 180.0;

(* Send webhook on alarm rising edge *)
IF alarm_active AND NOT alarm_sent THEN
    payload := JSON_OBJECT(
        'event', 'HIGH_TEMP_ALARM',
        'value', temperature,
        'threshold', 180.0,
        'source', 'GoPLC-Plant1'
    );

    hdrs := HTTP_SET_HEADER('', 'Authorization', 'Bearer webhook-secret');
    resp := HTTP_REQUEST('POST',
                         'https://hooks.example.com/alerts',
                         JSON_STRINGIFY(payload),
                         hdrs);

    IF HTTP_OK(resp) THEN
        alarm_sent := TRUE;
    END_IF;
END_IF;

(* Reset when alarm clears *)
IF NOT alarm_active THEN
    alarm_sent := FALSE;
END_IF;

END_PROGRAM

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7. Complete Example: Cross-PLC Communication

Read variables from another GoPLC instance via its REST API:

PROGRAM POU_RemotePLC
VAR
    scan_count : DINT := 0;
    body : STRING;
    doc : STRING;
    remote_temp : REAL;
    remote_running : BOOL;
END_VAR

scan_count := scan_count + 1;

(* Poll remote PLC every 50 scans (5s at 100ms) *)
IF (scan_count MOD 50) = 0 THEN
    body := HTTP_GET_BODY('http://10.0.0.51:8082/api/variables');

    IF LEN(body) > 0 THEN
        doc := JSON_PARSE(body);

        IF JSON_HAS(doc, 'temperature') THEN
            remote_temp := JSON_GET_REAL(doc, 'temperature.value');
        END_IF;

        IF JSON_HAS(doc, 'motor_running') THEN
            remote_running := JSON_GET_BOOL(doc, 'motor_running.value');
        END_IF;
    END_IF;
END_IF;

END_PROGRAM

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Appendix A: Quick Reference

Function	Parameters	Returns	Description
HTTP_GET	(url [, timeout])	Response map	GET request
HTTP_GET_BODY	(url [, timeout])	STRING	GET, returns body directly
HTTP_POST	(url, body [, content_type] [, timeout])	Response map	POST request
HTTP_POST_JSON	(url, json_body [, timeout])	Parsed JSON or map	POST JSON, auto-parse response
HTTP_PUT	(url, body [, content_type] [, timeout])	Response map	PUT request
HTTP_PATCH	(url, body [, content_type] [, timeout])	Response map	PATCH request
HTTP_DELETE	(url [, timeout])	Response map	DELETE request
HTTP_HEAD	(url [, timeout])	Response map	HEAD (status + headers only)
HTTP_REQUEST	(method, url [, body] [, headers] [, timeout])	Response map	Full control request
HTTP_STATUS	(response)	INT	Extract status code
HTTP_BODY	(response)	STRING	Extract body
HTTP_OK	(response)	BOOL	TRUE if status 200-299
HTTP_ERROR	(response)	STRING	Extract error message
HTTP_HEADERS	(response)	MAP	Extract all headers
HTTP_GET_HEADER	(response, name)	STRING	Get single header (case-insensitive)
HTTP_SET_HEADER	(headers, name, value)	MAP	Build/add to headers map

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GoPLC v1.0.535 | 16 HTTP Client Functions | REST Integration from ST

© 2026 JMB Technical Services LLC. All rights reserved.