From d3ac9c1696b4b8b9319019de9041ddcd1a1b2d26 Mon Sep 17 00:00:00 2001
From: Spencer Oliver <spen@spen-soft.co.uk>
Date: Sat, 10 May 2025 16:54:09 +0100
Subject: [PATCH] coils: ability to configure number of supported coils

No change to current default behaviour of allocating 2000 coils.
However memory usage can be reduced by defining NMBS_MAX_COILS.
---
 nanomodbus.c |  4 ++--
 nanomodbus.h | 12 +++++++++++-
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/nanomodbus.c b/nanomodbus.c
index d1f928f..5786301 100644
--- a/nanomodbus.c
+++ b/nanomodbus.c
@@ -546,7 +546,7 @@ static nmbs_error recv_read_discrete_res(nmbs_t* nmbs, nmbs_bitfield values) {
     uint8_t coils_bytes = get_1(nmbs);
     NMBS_DEBUG_PRINT("b %d\t", coils_bytes);
 
-    if (coils_bytes > 250) {
+    if (coils_bytes > NMBS_MAX_COIL_BYTES) {
         return NMBS_ERROR_INVALID_RESPONSE;
     }
 
@@ -1958,7 +1958,7 @@ nmbs_error nmbs_client_create(nmbs_t* nmbs, const nmbs_platform_conf* platform_c
 
 
 static nmbs_error read_discrete(nmbs_t* nmbs, uint8_t fc, uint16_t address, uint16_t quantity, nmbs_bitfield values) {
-    if (quantity < 1 || quantity > 2000)
+    if (quantity < 1 || quantity > NMBS_MAX_COILS)
         return NMBS_ERROR_INVALID_ARGUMENT;
 
     if ((uint32_t) address + (uint32_t) quantity > ((uint32_t) 0xFFFF) + 1)
diff --git a/nanomodbus.h b/nanomodbus.h
index b0c872d..50aa276 100644
--- a/nanomodbus.h
+++ b/nanomodbus.h
@@ -78,11 +78,21 @@ typedef enum nmbs_error {
  */
 #define nmbs_error_is_exception(e) ((e) > 0 && (e) < 5)
 
+#ifndef NMBS_MAX_COILS
+ #define NMBS_MAX_COILS 2000
+#endif
+
+/* check coil count divisible by 8 */
+#if ((NMBS_MAX_COILS & 7) > 0)
+ #error "NMBS_MAX_COILS must be disible by 8"
+#endif
+
+#define NMBS_MAX_COIL_BYTES (NMBS_MAX_COILS / 8)
 
 /**
  * Bitfield consisting of 2000 coils/discrete inputs
  */
-typedef uint8_t nmbs_bitfield[250];
+typedef uint8_t nmbs_bitfield[NMBS_MAX_COIL_BYTES];
 
 /**
  * Bitfield consisting of 256 values